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Abstract
In this paper we give an outlook on the basis of recently published results, of the complementarity of these light sources for double core-hole spectroscopy. Some new developments and results are also presented with a prospective of what could be done in the future with the technical breakthrough that will be necessary to improve our further understanding of chemical analysis.
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... The study of thin films on surfaces and the determination of the thickness via the overlayer technique has been widely used in Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) [20]. Earlier research, it was assumed that the signal decayed exponentially, however this neglected the impact of elastic scattering [21]. In addition, the overlayer technique measures the exponential decay of a peak (Auger or photoelectron) and this provides a measure of the Effective Attenuation Length (EAL) (i.e. the distance travelled for a loss of 1/e in the signal), which is not the same as the Inelastic Mean Free Path (IMFP) (which is defined as the mean distance between inelastic events) [21]. ...
... Earlier research, it was assumed that the signal decayed exponentially, however this neglected the impact of elastic scattering [21]. In addition, the overlayer technique measures the exponential decay of a peak (Auger or photoelectron) and this provides a measure of the Effective Attenuation Length (EAL) (i.e. the distance travelled for a loss of 1/e in the signal), which is not the same as the Inelastic Mean Free Path (IMFP) (which is defined as the mean distance between inelastic events) [21]. In TEM, no distinction is made between EAL and IMFP due to the high beam energies used. ...
... The results of the present MC simulation are considerably lower than those in the literature. The influence of elastic scattering causes the path length traversed to be lower than a straight line path [21]. This effect needs to be accounted for if one is to determine the correct film thickness using low-energy TEM experiments. ...
The electron energy spectra of transmitted scattered electrons from free-standing films are simulated using a Monte Carlo computational approach. Elastic scattering is simulated using Mott cross-sections and inelastic scattering via discrete processes determined from dielectric function data. This allows one to simulate the secondary electrons as well as the loss peaks near the elastic (zero loss) peak. The current study suggested a directed approach for determining the electron inelastic mean free path (IMFP) of materials at low primary electron energies. The IMFP of the reference material are not necessary for the suggested technique. The suggested technique makes use of the ratio between the transmitted elastic peak intensity and the background intensity of backscattered electrons. Free standing films of Si, Cu and Au were studied with thicknesses varying from 2 to 12 nm. Primary electron energies of 1 keV, 3 keV, and 5 keV were applied. The results appeared very good with the percentage error range was being between 5% and 25 %. We also investigate the proportion of the first and second plasmon peaks intensities to the elastic peak's intensity. We believe that the latter could provide a directed method of measuring the IMFP of materials.
... Сравнительный анализ рис. 3 и рис. 4 указывает на обстоятельство, которое не раз подчеркивалось в работах [3,13]: влияние послойного состава исследуемого образца на энергетический спектр РФЭС в широком интервале потерь энергии (PES) более выражено, чем на интенсивность no-loss XPS пиков. В то же время, если не принимать во внимание наряду с no-loss пиком энергетический спектр потерь, то РФЭС-сигнал не позволит определить истинный послойный состав анализируемого элемента [3,13]. ...
... Сравнительный анализ рис. 3 и рис. 4 указывает на обстоятельство, которое не раз подчеркивалось в работах [3,13]: влияние послойного состава исследуемого образца на энергетический спектр РФЭС в широком интервале потерь энергии (PES) более выражено, чем на интенсивность no-loss XPS пиков. В то же время, если не принимать во внимание наряду с no-loss пиком энергетический спектр потерь, то РФЭС-сигнал не позволит определить истинный послойный состав анализируемого элемента [3,13]. На это обстоятельство ссылаются и в учебниках по РФЭС [2]. ...
... Рис. 4 иллюстрирует ошибку в определении спектральной интенсивности, возникающую в пренебрежении только одним из эффектов упругого рассеяния фотоэлектронов, величина которой составляет сотни процентов. Поэтому для определения послойного состава необходимо, как отмечено в работах [3,13], рассматривать спектр РФЭС в широком интервале потерь энергии, но расчет этого спектра необходимо выполнять последовательно, учитывая процессы многократного упругого рассеяния фотоэлектронов. ...
Граничная задача для уравнения переноса, как с внутренними источниками, так и без них, решается методом инвариантного погружения. Анализируется влияние процессов упругого рассеяния фотоэлектронов на энергетические спектры фотоэлектронной эмиссии. Наибольшее внимание сосредоточено на анализе изменения интенсивности и спектрального состава восходящего потока фотоэлектронной эмиссии слоя при появлении нижележащих слоев различного состава. Продемонстрированы физические основы влияния упругого рассеяния на формирование рентгеновских фотоэлектронных спектров. Выполнен расчет интенсивностей пиков однородных мишеней из золота Au 4s1/2 и кремния Si 2s1/2 с учетом и без учета процессов отражения от нижележащих слоев. Определена погрешность, возникающая в результате пренебрежения процессами упругого рассеяния, переводящими нисходящее движение фотоэлектрона в восходящее.
... In this review we will summarize the progress recently achieved in these different subjects, and extend the previous reviews that have already been published on the subject [5][6][7][8][9]. ...
... This is either because the two K-shell holes are on atoms of a different nature, implying different energy ranges for the two emitted Auger electrons in reaction (2), such as in CO [14], or because the dead time of the detector was reduced. A value of 6 ns, corresponding to a to a dead zone in energy of ∼12 eV for electrons of 240 eV, made it possible to observe K −1 K −1 signals in C 2 H 4 and propylene molecules [9]. A subsequent advantage is that the background is much reduced in 4-fold coincidences. ...
... (b) Associated K −2 spectrum showing the composite structure of the main peak due to the three different C atoms. Reprinted from [9], Copyright (2015), with permission from Elsevier. demonstrated with the HAXPES spectrometer at the GALAXIES beamline in SOLEIL to study Ar + (1s −1 2p −1 nl) satellite states [70]. ...
The interest of molecular double core holes was predicted in 1986 by Cederbaum et al who showed that their spectroscopy can be more informative than that of single core holes, especially when the holes are located at different sites in the molecule (Cederbaum et al 1986 J. Chem. Phys. 85 6513). Their experimental study of single photon formation had to wait until 2009-2010 with progress in synchrotron sources and the development of efficient multi-electron coincidence experiments based on a magnetic bottle time-of-flight spectrometer. At the same time the advent of x-ray free electron lasers opened the possibilty of creating them in a two-photon process, and motivated new theoretical studies of their properties. We will illustrate here the progress made recently in the field with a few examples, including the formation of double core holes by double core photoionization, their spectroscopy and decay paths, and the related process of simultaneous core ionization and core excitation.
... The abundance of energy states in many-electron systems and the inherent weakness of the O(α 2 S ) scattering signal imply that photon scattering in materials is always dominated by nearly resonant transitions through intermediate virtual states. This explains why the Kramers-Heisenberg approximation is ubiquitous in spectroscopy with synchrotron radiation [12,[15][16][17][18][19][20][21]. The Kramers-Heisenberg formula has been successfully applied e.g. to CaF 2 [22], lanthanum and lanthanum compounds [23][24][25] as well as compounds of other rare-earth elements [26][27][28][29], titanium and titanium compounds [30], cobalt compounds [31,32], lithium fluoride [33], silicon and aluminum and their compounds [34][35][36], zinc oxide [37], aequous solutions of transition metals [38,39], and N 2 [40]. ...
... and the assumption of approximately constant dispersion factor relates this again to the density of states (41), This applies e.g. to scattering between band states |i = |n, k e → |f = |n ′ , k e , where dipole approximation yields again direct interband transitions k ′ e = k e . Adapting the Kramers-Heisenberg relation to this situation became important with the availability of synchrotron light sources for inelastic X-ray scattering between energy bands in materials [15][16][17][18]. ...
We provide a compendium of the quantum mechanical equations for photon emission rates, photon absorption cross sections, and photon scattering cross sections. For each case, the different equations that apply for discrete or continuous quantum states of the emitting, absorbing, or scattering material are given.
... Также была применена низковольтная электронная пушка для нейтрализации зарядки образца, которая происходит в диэлектриках из-за медленного возмещения потери части электронов вызываемой рентгеновскими лучами. Спектры записаны при давлении не ниже 10 -8 Па с предварительно дегазированных в сверхвысоком вакууме образцов [10][11][12]. ...
... Используя уравнения (6-8) и учитывая уравнения (9, 10), выражения (11,12) были получены для незаполненных фракций центров s и S*: ...
В данной статье представлены данные по физико-химическому исследованию гетерогенного рутений содержащего катализатора Ru/СПС MN 100. Представлена важность таких исследование для изучения каталитических реакций, для установления возможного механизма реакции гидрирования, а так же как дополнения при кинетических исследованиях. В статье катализатор исследован методом низкотемпературной адсорбции азота, хемосорбции водорода, просвечивающей электронной микроскопии (ПЭМ) и рентгенофотоэлектронной спектроскопии (РФЭС). Метод низкотемпературной адсорбции азота позволил установить, что катализатор характеризуется развитой внутренней удельной поверхностью (726 м/г по модели БЭТ) и характеризуется значительной мезопористостью, при этом наибольший диаметр пор составляет около 3.6 нм. Удельная площадь поверхности активного металла - Ru, по данным метода хемосорбции водорода, составляет 1 м/г. Рутений содержащие частицы распределены по всему объему носителя, при этом они способны образовывать небольшие агрегаты и характеризуются различной степенью кристалличности. Установлен элементный состав поверхности катализатора; Ru имеет различные степени окисления. На основании полученной ранее математическая модель процесса и проведенных физико-химических исследований катализатора предположена модель Ленгмюра-Хиншельвуда для описания механизма реакции жидкофазного каталитического гидрирования моно- и дисахаридов.
This article presents data on the physical and chemical study of heterogeneous ruthenium-containing catalyst Ru/SPS MN 100. The importance of such studies for the study of catalytic reactions, for establishing the possible mechanism of the hydrogenation reaction, as well as additions in kinetic studies is presented. In this paper, the catalyst was studied by low-temperature nitrogen adsorption, hydrogen chemisorption, transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). The method of low-temperature nitrogen adsorption allowed us to establish that the catalyst is characterized by a developed internal specific surface (726 m/g according to the BET model) and is characterized by significant mesoporicity, with the largest pore diameter of about 3.6 nm. The specific surface area of the active metal - Ru, according to the method of hydrogen chemisorption, is 1 m/g. Ruthenium containing particles are distributed over the entire volume of the carrier, while they are able to form small aggregates and are characterized by different degrees of crystallinity. The elemental composition of the catalyst surface has been determined; Ru has different oxidation States. Based on the previously obtained mathematical model of the process and physical and chemical studies of the catalyst, the Langmuir-Hinshelwood model is proposed to describe the reaction mechanism of liquid-phase catalytic hydrogenation of mono - and disaccharides.
... 23 Studies of the Auger decay of DCHs greatly accelerated after it became apparent that these states are also created in ion-atom collisions; the term hypersatellites was soon also used to designate the energy-shifted Auger lines emerging from DCHs. 24 In recent years, strong experimental and theoretical activities have been devoted to the study of inner-shell double photoionization, 25 prompted both by the perspective to create such states using x-ray free-electron lasers (XFELs) with very high brightness 26,27 as well as by the availability of sophisticated coincidence detection schemes that greatly enhance the amount of information on DCHs compared to using more conventional synchrotron light sources (see, e.g., Refs. [28][29][30][31][32][33][34][35][36]. ...
We present a combined experimental and theoretical investigation of the radiationless decay spectrum of an O 1s double core hole in liquid water. Our experiments were carried out using liquid-jet electron spectroscopy from cylindrical microjets of normal and deuterated water. The signal of the double-core-hole spectral fingerprints (hypersatellites) of liquid water is clearly identified, with an intensity ratio to Auger decay of singly charged O 1s of 0.0014(5). We observe a significant isotope effect between liquid H2O and D2O. For theoretical modeling, the Auger electron spectrum of the central water molecule in a water pentamer was calculated using an electronic-structure toolkit combined with molecular-dynamics simulations to capture the influence of molecular rearrangement within the ultrashort lifetime of the double core hole. We obtained the static and dynamic Auger spectra for H2O, (H2O)5, D2O, and (D2O)5, instantaneous Auger spectra at selected times after core-level ionization, and the symmetrized oxygen-hydrogen distance as a function of time after double core ionization for all four prototypical systems. We consider this observation of liquid-water double core holes as a new tool to study ultrafast nuclear dynamics.
... where p corresponds to a two-dimensional Gaussian probe centered at (t, t ′ ) = (τ, τ ). This review does not encompass theoretical challenges related to the interpretation of the TR-ARPES signal and we instead refer the reader to the relevant literature De Giovannini et al., 2022;Eckstein, 2021;Freericks et al., 2009a,b;Kemper et al., 2014Kemper et al., , 2017Marini et al., 2022;Perfetto et al., 2020;Schwarz et al., 2020;Sentef et al., 2013;Tao and Zhu, 2010;Xu et al., 2019a). However, it is important to recognize that the similarities between equilibrium ARPES and TR-ARPES discussed in the previous section may be deceptive for time delays comparable to the system's characteristic dephasing times, or when new electonic states with no equilibrium counterpart (e.g., Floquet-Bloch states) emerge. ...
Angle-resolved photoemission spectroscopy (ARPES) -- with its exceptional sensitivity to both the binding energy and momentum of valence electrons in solids -- provides unparalleled insights into the electronic structure of quantum materials. Over the last two decades, the advent of femtosecond lasers, which can deliver ultrashort and coherent light pulses, has ushered the ARPES technique into the time domain. Now, time-resolved ARPES (TR-ARPES) can probe ultrafast electron dynamics and the out-of-equilibrium electronic structure, providing a wealth of information otherwise unattainable in conventional ARPES experiments. This paper begins with an introduction to the theoretical underpinnings of TR-ARPES followed by a description of recent advances in state-of-the-art ultrafast sources and optical excitation schemes. It then reviews paradigmatic phenomena investigated by TR-ARPES thus far, such as out-of-equilibrium electronic states and their spin dynamics, Floquet-Volkov states, photoinduced phase transitions, electron-phonon coupling, and surface photovoltage effects. Each section highlights TR-ARPES data from diverse classes of quantum materials, including semiconductors, charge-ordered systems, topological materials, excitonic insulators, van der Waals materials, and unconventional superconductors. These examples demonstrate how TR-ARPES has played a critical role in unraveling the complex dynamical properties of quantum materials. The conclusion outlines possible future directions and opportunities for this powerful technique.
... The inelastic background is modelled at a given energy, ( ), by the integrated area between the intensity at ( ) and the lower binding energy limit. Although Shirley did not make clear the physical meaning of this background model, 38 good agreement has been found between experiment and theory and it remains one of the most popular methods used to date. 39 Raman Spectroscopy. ...
Quantifying the crystallographic phases present at a surface is an important challenge in fields such as functional materials and surface science. X-ray photoelectron spectroscopy (XPS) is routinely employed in surface characterisation to identify and quantify chemical species through core line analysis. Valence band (VB) spectra contain characteristic but complex features that provide information on the electronic density of states (DoS) and thus can be understood theoretically using density functional theory (DFT). Here we present a method of fitting experimental photoemission spectra with DFT models for quantitative analysis of heterogeneous systems, specifically, mapping the anatase to rutile ratio across the surface of mixed-phase TiO2 thin films. The results were correlated with mapped photocatalytic activity measured using a resazurin based smart ink. This method allows large-scale functional and surface composition mapping in heterogeneous systems, and demonstrates the unique insights gained from DFT-simulated spectra on the electronic structure origins of complex VB spectral features.
... In principle, these changes may be observed in a photoelectron spectroscopy study. In particular, the photoeffect cross section for the Ar 3p-shell excitation by X-ray AIK, band is three times higher than that for the Si 3s-shell [12], and that provides necessary conditions (in case of sufficient concentration of noble gas) for the identification of Ar 3p-states involved in the formation of It, MO. Unfavourable for such an analysis may be a Si 3s-band splitting due to the deformation of Si-Xi bonds. ...
... We can adjust the resolution and transmission by changing the pass energy and/or width of the entrance slit according to the experiment's requirement. When using the electron spectrometer in the coincidence mode, the communication between the electron and TOF spectrometers should be fast, which is achieved by using micro channel plates (MCPs) and a resistive anode [79]. The electron output signal has a much shorter delay than the actual electron arrival time with the resistive anode detector (RAD). ...
Understanding the molecular radiation damage is crucial in radiobiology,
molecular physics, and atmospheric science. In this thesis, the initial steps of
radiation damage of anhydrous gas-phase molecules and hydrated nanoparticles
were studied using synchrotron radiation-based electron-ion coincidence
spectroscopy and X-ray absorption spectroscopy under vacuum conditions.
Electron-ion coincidence spectroscopy was used to study the photofragmentation
and molecular dynamics of the isolated gas-phase molecules. In
addition to the photofragmentation of the gas-phase molecules, the effect of
the initial ionization site, initial molecular geometry, and the intramolecular
chemical environment has been studied. In avobenzone, core ionization
leads to massive fragmentation, with a slight site-selectivity concerning fragment
production. In ortho-aminobenzoic acid, core ionization leads to the
production of a hydronium ion, indicating that the importance of functional
group's position for double intramolecular hydrogen transfer. X-ray absorption
spectroscopy was used to probe hydrated nanoparticles prepared at different
relative humidities. In hydrated inorganic and mixed inorganic-organic
nanoparticles, water is present in a liquid-like state. With different ranges of
relative humidity, the primary hydration layers of the hydrated nanoparticles
stays the same. In mixed nanoparticles, there is evidence for interaction
between the included organic biomolecule with the inorganic and/or water
molecules.
... ts DCH states [17]. The complementary approach at XFELs relies on the very large number of x-ray photons available in durations on par with the decay lifetime of SCH states (a few femtoseconds for light elements [18]). ...
Formamide, a simple model bio-molecule (HCONH2), is irradiated with high intensity, ultrashort pulses from an x-ray free electron laser. Ionic fragments resulting from photoionization and subsequent decay processes are recorded, as well as the electronic signature of the different inner shell ionization events that can take place during the x-ray pulses. The formation of double-core-hole states, where a second inner shell electron is removed before the first core hole has been refilled is observed in the electron spectra, recorded at all three sites (C, N, O) of the molecule. The individual ionization potentials are compared with results of ab initio calculations at different level of theory. Based on our results, future opportunities for advanced studies of inner-shell-induced electronic and nuclear dynamics are explored.
... The removal of the oxide layer in antimonene (8h) was increased as the ultrasonication period increases and thereby O 1s peak is decreased for antimonene(8h). [62][63][64][65] Electrochemical Performance Analysis. The cyclic voltammetry (CV), chrono potentiometry (CP) studies give insight into charge storage kinetics of surface redox reaction in pseudocapacitance and ion adsorption-desorption kinetics in double layer capacitance energy storage materials. ...
The antimonene is an exfoliated 2D nanomaterial of bulk antimony. It is a new class of 2D material for energy storage applications. In the present work, the antimonene was synthesized by the high energy ballmilling-sonochemical method. The structural, morphological, thermal and electrochemical properties of antimonene were comparatively analyzed with bulk antimony. The XRD analysis confirms the crystal structure and 2D structure of the antimonene as the peak shift was observed. The Raman spectra show the peak shift for E g and A 1g modes of vibration of antimony, which confirms the formation of antimonene. The SEM and HRTEM images depict the exfoliation of antimonene from bulk antimony. The thermal analysis unveiled the thermal stability of antimonene up to 400 o C with only 3 % of weight loss. The XPS analysis reveals the formation of antimonene which is free from contamination. The electrochemical properties of antimony and antimonene were investigated by cyclic voltammetery (CV) and chronopotentiometric (CP) analysis using 2M KOH as an electrolyte. The antimonene exhibited the relatively high specific capacitance of 597 Fg-1 compared to ball-milled antimony (101 Fg-1) at the scan rate of 10 mVs-1. Moreover, the electrochemical impedance spectroscopic (EIS) analysis revealed that the antimonene has relatively low equivalence series resistance (RESR) and low charge transfer resistance (RCT) compared to bulk antimony which favors high electrochemical performance. The cyclic stability of the antimonene was studied for 3000 cycles and the result shows high cyclic stability. The electrochemical results demonstrated that antimonene is a promising material for energy storage applications.
... The effects of correlation can be attributed to Fig. 7 b) and the dynamic correlation effects are well isolated by our calculations, which use a localised core hole, kept frozen during the optimization. This prevents fractional occupation about the degenerate core holes and mitigates the effect of the delocalization error in DFT 52,55 . With further bench-marking, plotting the difference in coherent scattering may provide useful for evaluating electron correlation in ab-initio methods, requiring evaluation of the density from high-level multiconfigurational approaches. ...
We examine x-ray scattering from an isolated organic molecule from the linear to nonlinear absorptiveregime. In the nonlinear regime, we explore the importance of both the elastic and inelastic channelsand observe the onset of nonlinear behavior as a function of pulse duration and energy. In the linearregime, we test the sensitivity of the scattering signal to molecular bonding and electronic correlationvia calculations using the independent atom model (IAM), Hartree-Fock (HF) and density functionaltheory (DFT). Finally, we describe how coherent x-ray scattering can be used to directly visualizefemtosecond charge transfer and dissociation within a single molecule undergoing x-ray multiphotonabsorption.
... In order to have sufficient statistics of the number of electron pairs, the energy spectra presented here are integrated over the complete acceptance angular range in Fig. 3. Since the energy of each of the two electrons in a pair is resolved, the DPE energy spectra are two-dimensional (2D) with two energy axes (E 1 , E 2 ) for the two electrons in each detected photoelectron pair. As has been shown by previous DPE experiments at surfaces [12,22,23 [234][235][236][237], the signatures of electron pairs can be more clearly identified by the total energy of a pair in the 2D energy phase space. This aspect will be emphasized by the following DPE spectra on metals and oxides. ...
The recent development of double photoemission (DPE) spectroscopy at surfaces using laser-based high-order harmonic generation in combination with time-of-flight electron spectroscopy is reviewed. Relevant experimental conditions including the solid angle for collecting photoelectron pairs, the energy and angular resolutions, as well as the repetition rate and the photon energy range of light sources are introduced. As examples, we provide an overview of laser-based DPE results on the noble metals Ag and Cu as well as transition metal oxides NiO and CoO. The DPE energy and angular distributions of photoelectron pairs are compared with emphasis on the possible indications of electron-electron interaction. Potential further developments including femtosecond time-resolved DPE experiments are outlined.
... Most neutrally dissociating potential energy surfaces of molecules evolve in the direction of higher electron binding energy. [37][38][39] The rising kinetic energy observed in the experimental spectrum indicates that the ionization energy is dropping during the dissociation. Since the observed change is too large to be explained by dissociation of any neutral ammonia fragment, we deduce that the precursor is a positively charged molecule. ...
High intensity XUV radiation from a free-electron (FEL) was used to create a nanoplasma inside ammonia clusters with the intent of studying the resulting electron-ion interactions and their interplay with plasma evolution. In a plasma-like state, electrons with kinetic energy lower than the local collective Coulomb potential of the positive ionic core are trapped in the cluster and take part in secondary processes (e.g. electron-impact excitation/ionization and electron-ion recombination) which lead to subsequent excited and neutral molecular fragmentation. Using a time-delayed UV laser, the dynamics of the excited atomic and molecular states are probed from -0.1 ps to 18 ps. We identify three different phases of molecular fragmentation that are clearly distinguished by the effect of the probe laser on the ionic and electronic yield. We propose a simple model to rationalize our data and further identify two separate channels leading to the formation of excited hydrogen.
... The neodymium spectrum was slightly hampered by the presence of two 3d 5/2 peaks overlapping with the oxygen KLL Auger peak. However, due to the conductivity of the samples, it was probable that some Nd was also successfully doped into ZnO.[238] 19 shows the Al 2p (a), Ga 2p (c), and Si 2p (e) regions. ...
This thesis focuses on the synthesis of a variety of different transparent conducting oxide (TCO) nanomaterials using a continuous hydrothermal flow synthesis process, wherein aqueous solutions of chemical precursors were mixed with heated, pressurised water to facilitate nanoparticle formation. In Chapter 3, a screening investigation was carried out by doping zinc oxide with a number of different elements in order to highlight the most promising systems with regards to electronic conductivity. Of the twenty-four materials tested, zinc oxides doped with aluminium (AZO), gallium (GZO), and silicon (SiZO), Chapters 4 and 5, respectively, were selected for compositional optimisation and further testing. Aluminium and gallium doping and co-doping (AGZO) optimisation resulted in materials of similar conductivity to indium tin oxide (ITO), the industry standard TCO material. Upon completion of compositional optimisation, ITO and AGZO were synthesised with a citrate coating added in-process (Chapter 6). This aided in the dispersion of the nanoparticles for deposition into thin films by inkjet printing and spin coating; the latter was also carried out with un-coated GZO, AGZO, and SiZO. Preliminary inkjet printed films demonstrated very high conductivity (ITO) or very high transparency (AGZO), but never both in the same film, indicating the promise of the deposition method while requiring further investigation to be carried out. The spin coated films of all four materials were highly transparent and conductive, competitive with the best performing materials so far reported in literature. The AGZO spin coated films in particular, were the most conductive ever reported, superior even to those deposited by the sputtering methods currently used in industry.
... The first layer frequency of the breathing mode of pyridine is 990 cm -1 (see Figure 10.) This Raman wavenumber indicates clearly that pyridine adsorbed on the (111) surface is observed, in agreement with Campion's results on pyridine adsorbed on Ag(111) 22 . The 990 cm -1 was also observed for pyridine adsorbed on smooth silver films with (111) orientation, deposited at room temperature (figure 1 in 23 about 1000cm -1. ...
This article is a revue of surface enhanced Raman scattering (SERS) of molecules adsorbed on roughened silver electrodes and on well-characterized,
clean Ag-surfaces in ultra high vacuum (UHV). The resonances of the SERS intensities of pyridine, pyrazine and CN - adsorbed on roughened silver
electrodes shift when using different Laser wavelengths. SERS is quenched by pulling the electrodes out of the electrolyte, thus exposing them to oxygen. The resonances are assigned to transient electron transfer within surface complexes, which arise from roughening silver electrodes. For pyridine and pyrazine an electron is ransferred from the Fermi level E F of the silver electrode to the lowest unoccupied orbital (LUMO). In the case of CN- the lectron is transiently transfered the highest occupied orbital (HOMO) to E F . The energy difference between E F and the LUMO’s of pyridine and pyrazine has been measured by inverse photoemission, in good
agreement with the results of SERS. The distance dependence of SERS at well-characterized surface s in UHV shows an enhancement of about 100 restricted to the first adsorbed monolayer, on top of the long range electromagnetic (EM) enhancement. electron energy loss spectroscopy (EELS) of pyrazine and pyridine on Ag(111) has detected weak structures assigned to electron transfer. Charge transfer reactions induce surface resistance.
... The first layer frequency of the breathing mode of pyridine is 990 cm -1 (see Figure 10.) This Raman wavenumber indicates clearly that pyridine adsorbed on the (111) surface is observed, in agreement with Campion's results on pyridine adsorbed on Ag(111) 22 . The 990 cm -1 was also observed for pyridine adsorbed on smooth silver films with (111) orientation, deposited at room temperature ( figure 1 in 23 ). ...
This article is a review of surface-enhanced Raman scattering (SERS) of molecules adsorbed on roughened silver electrodes and on well characterized, clean Ag-surfaces in ultra high vacuum (UHV). The resonances of the SERS intensities of pyridine, pyrazine and CN-adsorbed on roughened silver electrodes shift when using different Laser wavelengths. SERS is quenched by pulling the electrodes out of the electrolyte, thus exposing them to oxygen. The resonances are assigned to transient electron transfer within surface complexes, which arise from roughening silver electrodes. For pyridine and pyrazine an electron is transferred from the Fermi level E F of the silver electrode to the lowest unoccupied orbital (LUMO). In the case of CN-, the electron is transiently transferred from the highest occupied orbital (HOMO) to E F. The energy difference between E F and the LUMO's of pyridine and pyrazine has been measured by inverse photoemission, in good agreement with the results of SERS. The distance dependence of SERS at well characterized surface s in UHV shows an enhancement of about 100, restricted to the first adsorbed monolayer, on top of the long range electromagnetic (EM) enhancement. Electron energy loss spectroscopy (EELS) of pyrazine and pyridine on Ag(111) has detected weak structures assigned to electron transfer. Charge transfer reactions induce surface resistance.
... It is based on a magnetic bottle time-of-flight spectrometer developed by Eland et al [2] and has been combined with synchrotron radiation [3,4] to study multiple photoionization processes in atoms and molecules [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Detailed information on single core hole and, more recently, on double core hole ionization has been obtained by this technique [11,[19][20][21][22][23]. In addition, precise information on ions can be obtained. ...
A magnetic bottle time-of-flight spectrometer has been used to perform spectroscopy of Kⁿ⁺ and Rbⁿ⁺ states with ionization degrees n of 2, 3 and 4. Energy levels are directly measured by detecting in coincidence the n electrons that are emitted as a result of single photon absorption. Experimental results are compared with the energies from the NIST atomic database and ab initio multiconfiguration Dirac–Fock calculations. Previously unidentified 3p ⁴(³P)3d ¹⁴D energy levels of K²⁺ are assigned.
... Photoionization of a core electron produces, in addition to the main core-hole state, shake-up states that are formed via core ionization accompanied by the promotion of a valence electron into an unoccupied orbital. Despite numerous experimental and theoretical studies on the properties and dynamics of the Auger decay processes from the main core-hole states, [2][3][4] Auger decay from shake-up states has been scarcely studied until recently, although full interpretation of the Auger spectra requires identification of the whole Auger structures associated with the individual shake-up states. The lack of experimental study is because the formation of shake-up states is unfavorable compared with that of the main core-hole state, and furthermore, the weak Auger transitions from the shake-up states usually overlap, even in a high-resolution Auger spectrum, with intense Auger lines associated with the main corehole state. ...
The single, double, and triple Auger decays from the 1s shake-up states of O2 have been studied using a multi-electron coincidence method. Efficient populations of two-hole final states are observed in single Auger decays of the π-π* shake-up states, which is understood as a characteristic property of the Auger transitions from shake-up states of an open-shell molecule. The O2³⁺ populations formed by double Auger decays show similar profiles for both the O1s⁻¹ and shake-up states, which is due to the contributions from cascade double Auger processes. While the cascade contributions to the double Auger decays increase with the initial shake-up energy, the probability of direct double Auger processes remains unchanged between the O1s⁻¹ and shake-up states, which implies a weak influence of the excited electron on the double Auger emission that originates from the electron correlation effect.
... They are known to undergo UFD within the Cl 2p −1 lifetime of ∼8 fs [22]. The former double-core-hole Cl 2p −2 σ Ã states are yet exotic and can be also created as so-called "super"shake-up satellites of direct 2p −2 double-core-hole ionization [30][31][32]. The lifetimes of the double-core-hole states (τ DCH ) are predicted to be more than two times shorter than that of corresponding single-core-hole states (τ SCH ) [33]. ...
Creation of deep core holes with very short (τ≤1 fs) lifetimes triggers a chain of relaxation events leading to extensive nuclear dynamics on a few-femtosecond time scale. Here we demonstrate a general multistep ultrafast dissociation on an example of HCl following Cl 1s→σ∗ excitation. Intermediate states with one or multiple holes in the shallower core electron shells are generated in the course of the decay cascades. The repulsive character and large gradients of the potential energy surfaces of these intermediates enable ultrafast fragmentation after the absorption of a hard x-ray photon.
In general, ZnTe is an intrinsic p-type semiconductor, probably arising from cation (Zn) vacancies (Zn1-δTe). For the first time, n-type ZnTe alloy is prepared via facile top-down physical process: melting-rotation-quenching-sintering. The XRD and Raman analyses showed an n-type ZnTe formation at 850 °C through an unidentified intermediate phase formed at 450 °C. It is verified that the n-type conduction of ZnTe by EPMS-WDS and XPS analyses showed anion (Te) vacancies in ZnTe alloy (ZnTe1-δ). The relative density of the sintered pellet is ~92%. The optical band gap calculated from the UV-Visible spectra is 1.60 eV. Hall measurement performed at room temperature revealed n-type conduction for ZnTe alloy. The thermoelectric properties such as electrical conductivity, Seebeck coefficient and thermal conductivity are measured for n-type ZnTe in the temperature range of 300 K – 423 K. The carrier transport in n-type ZnTe is described on the basis of Mott’s VRH transport mechanism. The maximum power factor ~0.125 μW/mK2 is obtained at 423 K. The thermal gravimetric analysis (TGA) study confirmed that the ZnTe alloy is thermally stable up to 1305 K. The remarkable low thermal conductivity ~0.85 W/mK achieved at 623 K for n-type ZnTe alloy, which is lower than that of previous reports suggests that n-ZnTe alloy can be a good thermoelectric material for energy harvesting applications.
Extending the lifetime of electrocatalytic materials is a major challenge in electrocatalysis. Here, we employ atomic layer deposition (ALD) to coat the surface of carbon black supported platinum nanoparticles (Pt/CB)...
The dominant decay pathways of argon 2p−2 double-core-hole states have been investigated using synchrotron radiation and a magnetic-bottle-type spectrometer coupled with an ion time-of-flight spectrometer. This experiment allows for efficient multi-electron-ion coincidence measurements, and thus for following the Auger cascade step by step in detail. Dominant decay pathways leading to Ar4+ final states via Ar3+ intermediate states have been assigned with the help of theoretical ab initio calculations. The weak correlated decay of the two core holes by emission of a single Auger electron, leading to Ar3+ final states, has been observed at 458.5-eV kinetic energy. Compared to the total decay of the 2p−2 double core vacancies, this two-electron–one-electron process was measured to have a branching ratio of 1.9×10−3±1.0×10−3. Furthermore, the remaining decay paths of the Ar1+(1s−1) core hole to higher charge states and their respective contributions to the total yield have been analyzed and show very good agreement with theoretical results.
Single-photon multiple photoionization results from electron correlations that make this process possible beyond the independent electron approximation. To study this phenomenon experimentally, the detection in coincidence of all emitted electrons is the most direct approach. It provides the relative contribution of all possible multiple ionization processes, the energy distribution between electrons that can reveal simultaneous or sequential mechanisms, and, if possible, the angular correlations between electrons. In the present work, we present a new magnet design of our magnetic bottle electron spectrometer that allows the detection of multiply charged Xen+ ions in coincidence with n electrons. This new coincidence detection allows more efficient extraction of minor channels that are otherwise masked by random coincidences. The proof of principle is provided for xenon triple ionization.
Using a magnetic bottle multi-electron time-of-flight spectrometer in combination with synchrotron radiation, double-core-hole pre-edge and continuum states involving the K-shell of the carbon atoms in n-butane (n-C4H10) have been identified, where the ejected core electron(s) and the emitted Auger electrons from the decay of such states have been detected in coincidence. An assignment of the main observed spectral features is based on the results of multi-configurational self-consistent field (MCSCF) calculations for the excitation energies and static exchange (STEX) calculations for energies and intensities. MCSCF results have been analyzed in terms of static and dynamic electron relaxation as well as electron correlation contributions to double-core-hole state ionization potentials. The analysis of applicability of the STEX method, which implements the one-particle picture toward the complete basis set limit, is motivated by the fact that it scales well toward large species. We find that combining the MCSCF and STEX techniques is a viable approach to analyze double-core-hole spectra.
In machines that work with technological material, which consists of individual particles, there is an interaction of these particles with the rough surfaces of the working bodies. The working bodies can be stationary or perform various movements. The determination of the kinematic characteristics of particle movement has its own characteristics, which depend on the shape, design parameters of the working bodies, the nature of the interaction with the technological material, the properties of the material and so on. It is important to know the patterns of this interaction, as it helps to improve the design of the executive bodies of machines. The paper considers the relative movement of a particle on the outer rough surface of a cone rotating around a vertical axis with a given angular velocity. The formula for finding the limiting value of the angular velocity, which depends on the angle of inclination of the generatrices, the coefficient of friction and the distance from the top of the cone to the particle, is found. It is also valid for a flat disk, for the case when the angle of inclination of the generatrices is equal to zero. Differential equations of movement of particles in projections on the axis of a fixed coordinate system are compiled, which are solved by numerical methods. The initial velocity of the particle at the moment of hitting the surface of the cone after falling from a certain height is taken into account. The relative trajectories of the particle sliding along the surface of the cone are constructed, as well as the absolute trajectories of its movement with respect to the fixed coordinate system. Visualization of kinematic characteristics is presented. The material considered in the article takes place in sowing machines, in which the seeds fall on a rotating cone.
Using synchrotron radiation in the tender X-ray regime, a photoelectron spectrum showing the formation of single site double-core-hole pre-edge states, involving the K shell of the O atom in CO, has been recorded by means of high-resolution electron spectroscopy. The experimentally observed structures have been simulated, interpreted and assigned, employing state-of-the-art ab initio quantum chemical calculations, on the basis of a theoretical model, accounting for their so-called direct or conjugate character. Features appearing above the double ionization threshold have been reproduced by taking into account the strong mixing between multi-excited and continuum states. The shift of the σ* resonance below the double ionization threshold, in combination with the non-negligible contributions of multi-excited configurations in the final states reached, gives rise to a series of avoided crossings between the different potential energy curves.
The metal centres in metalloenzymes and molecular catalysts are responsible for the rearrangement of atoms and electrons during complex chemical reactions, and they enable selective pathways of charge and spin transfer, bond breaking/making and the formation of new molecules. Mapping the electronic structural changes at the metal sites during the reactions gives a unique mechanistic insight that has been difficult to obtain to date. The development of X-ray free-electron lasers (XFELs) enables powerful new probes of electronic structure dynamics to advance our understanding of metalloenzymes. The ultrashort, intense and tunable XFEL pulses enable X-ray spectroscopic studies of metalloenzymes, molecular catalysts and chemical reactions, under functional conditions and in real time. In this Technical Review, we describe the current state of the art of X-ray spectroscopy studies at XFELs and highlight some new techniques currently under development. With more XFEL facilities starting operation and more in the planning or construction phase, new capabilities are expected, including high repetition rate, better XFEL pulse control and advanced instrumentation. For the first time, it will be possible to make real-time molecular movies of metalloenzymes and catalysts in solution, while chemical reactions are taking place.
The availability of X‐ray light sources with increased resolution and intensity has provided a foundation for increasingly sophisticated experimental studies exploiting the spectroscopy of core electrons to probe fundamental chemical, physical, and biological processes. Quantum chemical calculations can play a critical role in the analysis of these experimental measurements. The relatively low computational cost of density functional theory (DFT) and time‐dependent density functional theory (TDDFT) make them attractive choices for simulating the spectroscopy of core electrons. An overview of current developments to apply DFT and TDDFT to study the key techniques of X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, X‐ray emission spectroscopy, and resonant inelastic X‐ray scattering is presented. Insight into the accuracy that can be achieved, in conjunction with an examination of the limitations and challenges to modeling the spectroscopy of core electrons with DFT is provided.
This article is categorized under:
• Electronic Structure Theory > Ab Initio Electronic Structure Methods
• Theoretical and Physical Chemistry > Spectroscopy
• Electronic Structure Theory > Density Functional Theory
Abstract
An overview of the capabilities and challenges of modeling the spectroscopy of core electrons with density functional theory.
Over the past decades, the field of mineral dissolution kinetics has undergone a spectacular evolution, with an increasingly detailed description of the atomic scale mechanisms of fluid-solid interactions. The development of probabilistic dissolution models has played a prominent role in this evolution, as they allow for bridging the outputs of ab initio calculations to macroscopic observables such as dissolution rates and nanoscale surface features. It is however admitted that these models cannot be easily adapted to simulate natural systems at large space and time scales due to the restricted dimensions and durations that can be simulated numerically. In the present study, we demonstrate that the steady-state outputs of the face-specific stochastic treatment of enstatite dissolution, which was experimentally validated in a previous paper, can be boiled down to a single analytical expression under the form: r_bulk^((hfl))=kP_(Mg-O-Mg)^(α)P_(Mg-O-Si)^(β)P_(Si-O-Si)^(γ) where r_bulk^((hkl)) is the steady-state dissolution rate of a defect-free (hkl) face [Å/iter], PM-O-M’ stands for the bond-breaking probability of the M-O-M’ bond, and k, α, β and γ are fitting parameters adjusted following the outputs of the stochastic simulations. When dislocations outcrop at the surface of a given (hkl) face of enstatite, the relation then becomes: r^((hkl))=r_bulk^((hkl))+r_dislocation^((hkl))(P_(Mg-O-Mg),P_(Mg-O-Si),P_(Si-O-Si)) where r_dislocation^((hkl)) stands for the contribution of the dislocations to the overall dissolution rate. The derivation of simple analytical expressions to get steady-state rate data that are similar to those obtained using stochastic dissolution models, may contribute to parametrize efficiently the bond-breaking probability of various atoms for pyroxene solid solutions, and raises the question of the extension of such surrogate expressions to other silicate structures. Finally, the development of surrogate models such as those reported here represents one of the possible strategies for upscaling dissolution processes from the atomic scale to the micron scale.
Benchmark scalar-relativistic delta-coupled-cluster calculations of hetero-site double core ionization energies of small molecules containing second-row elements are reported. The present study has focused on the high-spin triplet components of two-site double core-ionized states, which are single reference in character and consistent with the use of standard coupled-cluster methods. Contributions to computed double core ionization energies from electron-correlation and basis-set effects as well as corrections to the core-valence separation approximation have been analyzed. Based on systematic convergence of computational results with respect to these effects, delta-coupled-cluster calculations have been shown to be capable of providing accurate double core ionization energies with remaining errors estimated to be below 0.3 eV, and thus are recommended for use to facilitate experimental studies of two-site double core-ionized states that are involved in x-ray pump/x-ray probe studies of electronic and molecular dynamics following inner shell ionization or excitation.
We demonstrate how the near-edge X-ray absorption ne structure (NEXAFS) spectroscopy of single and double core-hole states created by the ionization of a het- eroatom can be used to probe subtle changes in intramolecular chemical environments that are nearly indistinguishable by conventional NEXAFS spectroscopy. Using pro- totypical organic molecules (2/3-pentanone and pentanal), we show how new spectral features emerge in the C K-edge NEXAFS spectra, when creating single and double core-holes at the oxygen heteroatom site. The eect on the lowest unoccupied molecular orbitals is analyzed by studying the double-core-hole-induced ultrafast valence electron dynamics of the three molecules. The predicted changes from our simulations should be observable with state-of-the-art experiments at X-ray free-electron lasers (XFELs).
In this work, we revisited the idea of using the coupled-cluster (CC) ground state formalism to target excited states. Our main focus was targeting doubly excited states and double core hole states. Typical equation-of-motion (EOM) approaches for obtaining these states struggle without higher-order excitations than doubles. We showed that by using a non-Aufbau determinant optimized via the maximum overlap method, the CC ground state solver can target higher energy states. Furthermore, just with singles and doubles (i.e., CCSD), we demonstrated that the accuracy of ΔCCSD and ΔCCSD(T) (triples) far surpasses that of EOM-CCSD for doubly excited states. The accuracy of ΔCCSD(T) is nearly exact for doubly excited states considered in this work. For double core hole states, we used an improved ansatz for greater numerical stability by freezing core hole orbitals. The improved methods, core valence separation (CVS)-ΔCCSD and CVS-ΔCCSD(T), were applied to the calculation of the double ionization potential of small molecules. Even without relativistic corrections, we observed qualitatively accurate results with CVS-ΔCCSD and CVS-ΔCCSD(T). Remaining challenges in ΔCC include the description of open-shell singlet excited states with the single-reference CC ground state formalism as well as excited states with genuine multireference character. The tools and intuition developed in this work may serve as a stepping stone toward directly targeting arbitrary excited states using ground state CC methods.
Single-site Double-Core Hole (ss-DCH or K⁻²) and two-site Double-Core Hole (ts-DCH or K⁻¹K⁻¹) photoelectron spectra including satellite lines were experimentally recorded for the aromatic C6H6 molecule using the synchrotron radiation and multielectron coincidence technique. Density functional theory and post-Hartree-Fock simulations providing binding energies and relative intensities allow us to clearly assign the main K⁻² line and its satellites. K⁻¹K⁻¹ states’ positions and assignments are further identified using a core-equivalent model. We predict that, contrary to what has been observed in the C2H2n series of molecules, the K⁻¹K⁻¹ energy-level ordering in C6H6 does not reflect the core-hole distances between the two holes.
With the help of newly developed X-ray free-electron laser (XFEL) sources, creating double core holes (DCHs) simultaneously at the same or different atomic sites in a molecule has now become possible. DCH X-ray emission is a new form of X-ray nonlinear spectroscopy that can be studied with a XFEL. Here, we computationally explore the metal K-edge valence-to-core (VtC) X-ray emission spectroscopy (XES) of metal/metal and metal/ligand DCH states in a series of transition metal complexes with time-dependent density functional theory. The simulated DCH VtC-XES signals are compared with conventional single core hole (SCH) XES signals. The energy shifts and intensity changes of the DCH emission lines with respect to the corresponding SCH-XES features are fingerprints of the coupling between the second core hole and the occupied orbitals around the DCHs that contain important chemical bonding information of the complex. The difference between delocalized/localized core hole models on DCH VtC-XES is also briefly discussed. We theoretically demonstrate that DCH XES provides subtle information on the local electronic structure around metal centers in transition metal complexes beyond conventional linear XES. Our predicted changes from calculations between SCH-XES and DCH-XES features should be detectable with modern XFEL sources.
Two samples of manganese oxide nanoparticles (NPs) were synthesized by a simple solution-combustion method at ignited temperature 250 0 C (C250) and 300 0 C (C300). The structural and optical properties have been characterized by Xray diffraction (XRD), Fourier transform infrared spectrum (FTIR), UV-Vis absorption and Photo-luminescence (PL) spectroscopy. The XRD patterns of C300 showed a tetragonal hausmannite structure of Mn 3 O 4 NPs, where the XRD patterns of C250 revealed a mix phase structure of the manganese oxides NPs. The obtained PL emission illustrated two broad bands. The obtained FT-IR results coincided with the XRD results. The calculated average crystallite size (D) was 27 and 26 nm for C250 and C300, respectively. The electrochemical performance of the prepared electrodes based on the synthesized samples C250 and C300 were examined in 1 M NaSO 4 electrolyte solution using cyclic voltammetry. The examined electrodes based on sample C250 presented a relatively large specific capacitance 128 F.g −1 . As a result, manganese oxide NPs could be a promising candidate to be used as an electrode material for supercapacitors.
This paper investigates the first satellite band in the outer valence photoelectron spectra (PES) for a set of isoelectronic diatomic molecules: carbon monoxide, carbon monosulfide, carbon monoselenide, silicon monoxide and boron monofluoride. In particular, we analyze the effect of the electronic structure, with the change of the atomic pair along the row and column of the periodic table and of the bond distance, on the position of the satellite peak as well as on the related dynamical observables profiles. For this investigation, highly correlated calculations have been performed on the primary ionic states and the satellite band for all the molecules considered. Cross sections for the primary ionic states, calculated using Dyson orbitals, have been compared with those obtained with Hartree-Fock and Density Functional Theory to probe the impact of the correlation in the bound states on the photoionization observables. Limitations of a simple intensity borrowing mechanism clearly result from the analysis of the satellite state, characterized by different features with respect to the relevant primary states.
We present the results of a combined study by band theory and angle resolved photoemission spectroscopy (ARPES) of the purple bronze, LiMoO. Structural and electronic origins of its unusually robust quasi-one dimensional (quasi-1D) behavior are investigated in detail. The band structure, in a large energy window around the Fermi energy, is basically 2D and formed by three Mo -like extended Wannier orbitals, each one giving rise to a 1D band running at a 120 angle to the two others. A structural "dimerization" from to gaps the xz and yz bands while leaving the xy bands metallic in the gap, but resonantly coupled to the gap edges and, hence, to the other directions. The resulting complex shape of the quasi-1D Fermi surface (FS), verified by our ARPES, thus depends strongly on the Fermi energy position in the gap, implying a great sensitivity to Li stoichiometry of properties dependent on the FS, such as FS nesting or superconductivity. The strong resonances prevent either a two-band tight-binding model or a related real-space ladder picture from giving a valid description of the low-energy electronic structure. We use our extended knowledge of the electronic structure to newly advocate for framing LiMoO as a weak-coupling material and in that framework can rationalize both the robustness of its quasi-1D behavior and the rather large value of its Luttinger liquid (LL) exponent . Down to a temperature of 6K we find no evidence for a theoretically expected downward renormalization of perpendicular single particle hopping due to LL fluctuations in the quasi-1D chains.
Extremely exotic dense matter states can be produced in the interaction of a relativistic femtosecond optical laser with a solid density matter. Here we theoretically investigate triple-core-hole (TCH) states produced by an intense polychromatic x-ray field formed by hot electrons in the interaction of a relativistic femtosecond optical laser with a thin silver foil. X-ray emission spectra of solid-density silver plasmas show unambiguously the production of TCH states at an electron temperature of a few hundreds of eV and radiative temperature of 1-3 keV of the polychromatic x-ray field. Practical calculations show that the emissivity originating from the TCH states exceeds that from the single- and double-core-hole states in Ne-like Ag37+ at electron temperature of ~500 eV and radiative temperature of ~1500 eV. For the neighbouring ionization stages of Ag36+ and Ag38+, TCH emissivity is roughly equivalent or comparable to that from the single- and double-core-hole states. Present work deepens our insight into investigation of the properties of extremely exotic states, which is important in high energy density physics, astrophysics and laser physics.
Double-core-hole potentials of cytosine tautomers were calculated within the framework of the density functional theory. The differences between the relaxation energies of these tautomers were visualized using a Wagner plot. Relationship of molecular structure with relaxation energies among tautomers was discussed.
The formation of double core hole pre-edge states of the form 1s^{-1}2p^{-1}(^{1,3}\text{P})\sigmaup^*,n\ell for HCl, located on the binding energy scale as deep as 3 keV, has been investigated by means of a high resolution single channel electron spectroscopy technique recently developed for the hard x-ray region. A detailed spectroscopic assignment is performed based on \textit{ab initio} quantum chemical calculations and by using a sophisticated fit model comprising regular Rydberg series. Quantum defects for the different Rydberg series are extracted and the energies for the associated double core hole ionization continua are extrapolated. Dynamical information such as the lifetime width of these double-core-hole pre-edge states and the slope of the related dissociative potential energy curves are also obtained. In addition, 1s^{-1}2p^{-1}V^{-1}n\ell\lambdaup n^{\prime}\ell^{\prime}\lambdaup^{\prime} double shake-up transitions and double core hole states of the form 1s^{-1}2s^{-1}(^{1,3}\text{S})\sigmaup^*,4s are observed.
Este texto describe el proceso de adsorción el cual se fundamenta de la instalación de filtros industriales, para eliminar las partículas de polvos contenido en el humo de las chimeneas, o sustancias químicas indeseables en líquidos.
A highly uniform porous film of MnO2 was deposited on carbon fiber by anodic electrodeposition method for the fabrication of high-performance electrodes in wearable supercapacitors (SCs) application. The effects of potentiostatic and galvanostatic electrodeposition and the deposition time were investigated. The morphology, crystalline structure and chemical composition of the obtained fiber-shaped samples were analyzed by field-emission scanning electron microscopy (FESEM), X-ray Diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The charge storage performance of the carbon fibers@MnO2 composite electrode coupled to a gel-like polymeric electrolyte was investigated by cyclic voltammetry and galvanostatic charge-discharge measurements. The specific capacitance of the optimized carbon fiber@MnO2 composite electrodes could reach up to 62 Fg-1 corresponding to 23 mFcm-1 in PVA/NaCl gel-polymer electrolyte, i.e. the highest capacitance value ever reported for fiber-shaped SCs. Finally, the stability and the flexibility of the device were studied and the results indicate exceptional capacitance retention and superior stability of the device subjected to bending even at high angles up to 150 0.
The detailed study of single photon multiple ionization processes requires the detection in coincidence of all the electrons emitted from the same ionization event. The advent of magnetic bottle experiments made possible the development of this multi-electron coincidence spectroscopy. First we will briefly review the achievements of this technique. Then we will illustrate more specifically its high sensitivity taking as an example the decay by emission of three Auger electrons of 2p and 2s holes in argon. Our results show that the processes are completely different depending on the initial core hole: the three Auger electrons are emitted dominantly in a simultaneous path for the 2p case, but in cascade for the 2s one.
Relativistic corrections with single- and double-core vacancies at the K-, L1-, and L23-edges from Li to Kr were estimated using the restricted active space self-consistent field method with fairly large basis sets based on the spin-free part of the sixth-order Douglas-Kroll-Hess Hamiltonian. Our estimations for Ne, Ar, and Kr are consistent with the previously reported values. Estimated corrections for all edges were fitted by a power law and are consistent with previous reports. The basis set dependence for the corrections is also discussed. The relativistic corrections for nine small molecules are estimated, and we found that the relativistic corrections for atoms are a good approximation for molecules. The present results are useful for the future study to predict the single and double core-electron binding energies correctly.
We report here the realization of ferroelectricity, ferromagnetism and magnetocapacitance effect in SrFe12O19 ceramics at room temperature. The ceramics demonstrate a saturated polarization hysteresis loop, two I-V peaks and large anomaly of dielectric constant near Curie temperature. These evidences confirmed the ferroelectricity of SrFe12O19 ceramics after annealing in O2 atmosphere. The remnant polarization of the SrFe12O19 ceramic is 103 {\mu}C/cm2. The material also exhibits strong ferromagnetic characterization, the coercive field and remnant magnetic moment are 6192Oe and 35.8emu/g, respectively. Subsequent annealing SrFe12O19 ceramics in O2 not only reveals its innate ferroelectricity but also improves the ferromagnetic properties through transforming Fe2+ into Fe3+. By applying a magnetic field, the capacitance demonstrates remarkable change along with B field, the maximum relative change of dielectric constant is 1174%, which reflects a giant magnetocapacitance effect in SrFe12O19. These combined functional responses in SrFe12O19 ceramics opens substantial possibilities for applications in novel electric devices.
The multi-configurational self-consistent field method is employed to simulate the two-dimensional all-X-ray double-quantum-coherence (XDQC) spectroscopy, a four-wave mixing signal that provides direct signatures of double core hole (DCH) states. The valence electronic structure is probed by capturing the correlation between the single (SCH) and double core hole states. The state-averaged restricted-active-space self-consistent field (SA-RASSCF) approach is used which can treat the valence, SCH, and DCH states at the same theoretical level, and applies to all types of DCHs (located on one or two atoms, K-edge or L-edge), with both accuracy and efficiency. Orbital relaxation introduced by the core hole(s) and the static electron correlation is properly accounted for. The XDQC process can take place via different intermediate DCH state channels by tuning the pulse frequencies. We simulate the XDQC signals for the three isomers of aminophenol at 8 pulse frequency configurations, covering all DCH pathways involving the N1s and O1s core hole (N1sN1s, O1sO1s and N1sO1s), which reveal different patterns of valence excitations.
We present in detail a theoretical model that provides absolute cross sections for simultaneous core-ionization core-excitation (K(-2)V ) and compare its predictions with experimental results obtained on the water molecule after photoionization by synchrotron radiation. Two resonances of different symmetries are assigned in the main K(-2)V peak and comparable contributions from monopolar (direct shake-up) and dipolar (conjugate shake-up) core-valence excitations are identified. The main peak is observed with a much greater width than the total experimental resolution. This broadening is the signature of nuclear dynamics.
The formalism developed in the companion Paper I is used here for the interpretation of spectra obtained recently on the nitrogen molecule. Double core-hole ionization K(-2) and core ionization-core excitation K(-2)V processes have been observed by coincidence electron spectroscopy after ionization by synchrotron radiation at different photon energies. Theoretical and experimental cross sections reported on an absolute scale are in satisfactory agreement. The evolution with photon energy of the relative contribution of shake-up and conjugate shake-up processes is discussed. The first main resonance in the K(-2)V spectrum is assigned to a K(-2)π(∗) state mainly populated by the 1s→ lowest unoccupied molecular orbital dipolar excitation, as it is in the K(-1)V NEXAFS (Near-Edge X-ray Absorption Fine Structure) signals. Closer to the K(-2) threshold Rydberg resonances have been also identified, and among them a K(-2)σ(∗) resonance characterized by a large amount of 2s/2p hybridization, and double K(-2)(2σ(∗)/1π/3σ)(-1)1π(∗2) shake-up states. These resonances correspond in NEXAFS spectra to, respectively, the well-known σ(∗) shape resonance and double excitation K(-1)(2σ(∗)/1π/3σ)(-1)1π(∗2) resonances, all being positioned above the threshold.
Generalized atomic processes are proposed to establish a consistent description from the free-atom approach to the heated and even up to the cold solid. It is based on a rigorous introduction of the Fermi-Dirac statistics, Pauli blocking factors and on the respect of the principle of detailed balance via the introduction of direct and inverse processes. A probability formalism driven by the degeneracy of the free electrons enables to establish a link of atomic rates valid from the heated atom up to the cold solid. This allows to describe photoionization processes in atomic population kinetics and subsequent solid matter heating on a femtosecond time scale. The Auger effect is linked to the 3-body recombination via a generalized 3-body recombination that is identified as a key mechanism, along with the collisional ionization, that follows energy deposition by photoionization of inner shells when short, intense and high-energy radiation interacts with matter. Detailed simulations are carried out for aluminum that highlight the importance of the generalized approach.
Seeded free electron lasers theoretically have the intensity, tunability, and resolution required for
multiphoton spectroscopy of atomic and molecular species. Using the seeded free electron laser FERMI
and a novel detection scheme, we have revealed the two-photon excitation spectra of dipole-forbidden
doubly excited states in helium. The spectral profiles of the lowest ð−1; 0Þþ1 1Se and ð0; 1Þ0 1De
resonances display energy shifts in the meV range that depend on the pulse intensity. The results are
explained by an effective two-level model based on calculated Rabi frequencies and decay rates.
We report on a detailed investigation into the electron emission processes of Ne atoms exposed to intense femtosecond x-ray pulses, provided by the Linac Coherent Light Source Free Electron Laser (FEL) at Stanford. The covariance mapping technique is applied to analyse the data, and the capability of this approach to disentangle both linear and nonlinear correlation features which may be hidden on coincidence maps of the same data set is demonstrated. Different correction techniques which enable improvements on the quality of the spectral features extracted from the covariance maps are explored. Finally, a method for deriving characteristics of the x-ray FEL pulses based on covariance mapping in combination with model simulations is presented.
We have investigated nonlinear processes in small molecules by x-ray photoelectron spectroscopy using the Linac Coherent Light Source free electron laser, and by simulations. The main focus of the experiments was the formation of the two-site double core-hole (tsDCH) states in the molecules CO2, N2O and N2. These experiments are described in detail and the results are compared with simulations of the photoelectron spectra. The double core-hole states, and in particular the tsDCH states, have been predicted to be highly sensitive to the chemical environment. The theory behind this chemical sensitivity is validated by the experiments. Furthermore, our simulations of the relative integrated intensities of the peaks associated with the nonlinear processes show that this type of simulation, in combination with experimental data, provides a useful tool for estimating the duration of ultra-short x-ray pulses.
SEXTANTS is a new SOLEIL beamline dedicated to soft X-ray scattering techniques. The beamline, covering the 50-1700 eV energy range, features two Apple-II undulators for polarization control and a fixed-deviation monochromator. Two branch-lines host three end-stations for elastic, inelastic and coherent scattering experiments.
The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in Ne_{2} is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive Ne^{+}(2p^{-1})-Ne^{+}(2p^{-1}) state, which is probed with a second pulse, removing a further electron. The yield of coincident Ne^{+}-Ne^{2+} pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the Ne_{2}^{+}(2s^{-1}) state to be (150±50) fs, in agreement with quantum calculations.
When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called "partial covariance mapping" to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.
We have observed single photon double K-shell photoionization in the C_{2}H_{2n} (n=1-3) hydrocarbon sequence and in N_{2} and CO, using synchrotron radiation and electron coincidence spectroscopy. Our previous observations of the K^{-2} process in these molecules are extended by the observations of a single photon double photoionization with one core hole created at each of the two neighboring atoms in the molecule (K^{-1}K^{-1} process). In the C_{2}H_{2n} sequence, the spectroscopy of K^{-1}K^{-1} states is much more sensitive to the bond length than conventional electron spectroscopy for chemical analysis spectroscopy based on single K-shell ionization. The cross section variation for single photon K^{-1}K^{-1} double core ionization in the C_{2}H_{2n} sequence and in the isoelectronic C_{2}H_{2}, N_{2} and CO molecules validates a knock-out mechanism in which a primary ionized 1s photoelectron ejects another 1s electron of the neighbor atom. The specific Auger decay from such states is clearly observed in the CO case.
The C 1s photoelectron spectrum of ethyl trifluoroacetate (CF3COOCH2CH3), also known as the ‘ESCA molecule’, is the most illustrative showcase of chemical shifts in photoelectron spectroscopy. The binding energies of the four carbon atoms of this molecule spread over more than 8 eV with energy separations ranging from 1.7 to 3.1 eV owing to different chemical environments and hence different charge states of these atoms. The paper discusses history and importance of this spectrum in the field of photoelectron spectroscopy starting from the time of invention of the ESCA technique. The main focus of the paper is a ‘revisit’ of this spectrum using the most modern experimental and computational tools. Large geometrical changes, different for each ionization site, and the presence of two conformers of ethyl trifluoroacetate influence the spectral lineshapes of all four C 1s lines. These effects are carefully modeled by theory and investigated in the experimental spectrum.
The free-electron laser, first proposed by Madey(1) in 1971, has significantly reduced laser wavelengths to the vacuum ultraviolet(2,3) and soft X-ray regions(4). Recently, an X-ray free-electron laser (XFEL) was operated at 1.2 angstrom at the Linac Coherent Light Source (LCLS)(5). Here, we report the successful generation of sub-angstrom laser light using a compact XFEL source, combining a short-period undulator with an 8 GeVelectron beam. The shortest wavelength attained-0.634 angstrom (63.4 pm)-is four orders of magnitude smaller than the 694 nm generated by Maiman's first laser(6). The maximum power exceeded 10 GW with a pulse duration of 10(-14) s. This achievement will contribute to the widespread use of XFEL sources and provide broad opportunities for exploring new fields in science.
The recently commissioned Linac Coherent Light Source is an X-ray free-electron laser at the SLAC National Accelerator Laboratory. It produces coherent soft and hard X-rays with peak brightness nearly ten orders of magnitude beyond conventional synchrotron sources and a range of pulse durations from 500 to <10 fs (10−15 s). With these beam characteristics this light source is capable of imaging the structure and dynamics of matter at atomic size and timescales. The facility is now operating at X-ray wavelengths from 22 to 1.2 Å and is presently delivering this high-brilliance beam to a growing array of scientific researchers. We describe the operation and performance of this new ‘fourth-generation light source’.
Cascade Auger electron emission following Xe 3d photoionization has been investigated using a multi-electron coincidence technique, which utilizes an electron spectrometer of magnetic bottle type. It has been found that the Xe2+ states of the 4p(-1)4d(-1) configuration, formed by the Auger decay of the Xe+ 3d(3/2,5/2)(-1) states, dominantly turn into triply charged states of the 4d(-2)5p(-1)/4d(-2)5s(-1) configurations. The Xe2+ 4s(-1)4d(-1) states, formed by the 3d Auger decay, yield the 4p(-1)4d(-1)5p(-1) states as well as the 4d(-3) states. From the coincidence spectrum among three Auger electrons, it is suggested that the Xe2+ 4p(-1)4d(-1) states give rise to the following cascade processes: 4p(-1)4d(-1) -> 4d(-2)5p(-1) -> 4d(-1)5p(-3).
The emission of one or two Auger electrons, following Kr 3d inner-shell ionization by synchrotron light, has been investigated both experimentally and theoretically. All electrons emitted in the process are detected in coincidence and analyzed in energy thanks to a magnetic-bottle electron time-of-flight spectrometer. In addition, noncoincident high-resolution electron spectra have been measured to characterize the cascade double-Auger paths more fully. Combination of the two experimental approaches and of our calculations allows a full determination of the decay pathways and branching ratios in the case of Kr 3d single-and double-Auger decays. The Kr(3+) threshold is found at 74.197 +/- 0.020 eV.
The word 'attosecond' (1 as = 10 −18 s) officially entered the vocabulary of physics when sub-femtosecond pulses of UV/XUV light produced either by nonlinear frequency conversion of a ultra-short infrared pump pulse or Fourier synthesis of broad bandwidth radiation were established. The physics of these pulses is based on nonlinear, nonperturbative laser–atom interaction: stimulated Raman scattering or high harmonic generation (HHG) is used to generate the necessary bandwidth, which naturally encompasses the visible and UV/XUV spectral range. However, the crucial element for attosecond pulse generation is the control of the spectral phase. New methods of temporal characterization at frequencies lying in the UV/XUV had to be elaborated. These methods rely on the energy/momentum analysis of photoelectrons produced by XUV attosecond flashes in the presence of an intense infrared field whose optical cycle itself becomes the basic clock. Single 650 as pulses have been produced and applied to trace the dynamics of electrons inside atoms following the creation of an inner-shell hole. Periodic combs of 250 as pulses have been synthesized by superposing just four harmonics and applying to the attosecond timing of the electron motion in HHG. Although it is easy to increase the bandwidth by coupling more harmonics, a fundamental limit to the duration of the light bursts produced has been discovered. It is imposed by the lack of synchronization of the different harmonic orders. The current limit is estimated to be 130 as. The latest advances include a direct autocorrelation of an attosecond pulse train and the production of a single 250 as soft x-ray pulse. This paper offers a snapshot of the state-of-the-art in the production and characterization of attosecond light pulses, with a glimpse at the first steps in attophysics.
We have performed x-ray two-photon photoelectron spectroscopy using the Linac Coherent Light Source x-ray free-electron laser in order to study double core-hole (DCH) states of CO2, N2O, and N2. The experiment verifies the theory behind the chemical sensitivity of two-site DCH states by comparing a set of small molecules with respect to the energy shift of the two-site DCH state and by extracting the relevant parameters from this shift.
We observe the formation in a single-photon transition of two core holes, each at a different carbon atom of the molecule. At a photon energy of 770.5 eV, the probability of this 2-site core double ionization amounts to 1.6\ifmmode\pm\else\textpm\fi{}0.4% of the 1-site core double ionization. A simple theoretical model based on the knockout mechanism gives reasonable agreement with experiment. Spectroscopy and Auger decays of the associated double core hole states are also investigated.
Theory predicts that double-core-hole (DCH) spectroscopy can provide a new powerful means of differentiating between similar chemical systems with a sensitivity not hitherto possible. Although DCH ionization on a single site in molecules was recently measured with double- and single-photon absorption, double-core holes with single vacancies on two different sites, allowing unambiguous chemical analysis, have remained elusive. Here we report that direct observation of double-core holes with single vacancies on two different sites produced via sequential two-photon absorption, using short, intense X-ray pulses from the Linac Coherent Light Source free-electron laser and compare it with theoretical modeling. The observation of DCH states, which exhibit a unique signature, and agreement with theory proves the feasibility of the method. Our findings exploit the ultrashort pulse duration of the free-electron laser to eject two core electrons on a time scale comparable to that of Auger decay and demonstrate possible future X-ray control of physical inner-shell processes.
The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne(8+) ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.
The formation of hollow molecules (with a completely empty K shell in one constituent atom) through single-photon core double ionization has been demonstrated using a sensitive magnetic bottle experimental technique combined with synchrotron radiation. Detailed properties are presented such as the spectroscopy, formation, and decay dynamics of the N(2)(2+) K(-2) main and satellite states and the strong chemical shifts of double K holes on an oxygen atom in CO, CO2, and O2 molecules.
We investigate the creation of double K-shell holes in N2 molecules via
sequential absorption of two photons on a timescale shorter than the core-hole
lifetime by using intense x-ray pulses from the Linac Coherent Light Source
free electron laser. The production and decay of these states is characterized
by photoelectron spectroscopy and Auger electron spectroscopy. In molecules,
two types of double core holes are expected, the ?rst with two core holes on
the same N atom, and the second with one core hole on each N atom. We report
the ?rst direct observations of the former type of core hole in a molecule, in
good agreement with theory, and provide an experimental upper bound for the
relative contribution of the latter type.
The photodetachment of hydrogen negative ion in the magnetic field near a metal surface is studied using a semiclassical theory of photodetachment microscopy. During photodetachment of H- in the presence of magnetic field near a metal surface, the trajectories of the detached electron emitting from different directions may intersect at a large distance from H-, thus creating an interference pattern in the electron flux distributions. For a given ion-surface distance, the electron flux distributions are calculated at various magnetic field strength. The results show that with the increase in magnetic field strength the interference pattern in the flux distributions becomes much more complicated because the number of the classical trajectories of the detached electrons contributing to the electron flux distributions increases. In addition, we find as the detached electron’s energy changes, the detached-electron flux distributions change accordingly. Therefore, the interference pattern in the detached-electron flux distributions can be controlled by adjusting the magnetic field strength and the detached electron’s energy. We hope that our studies may guide future experimental research in the photodetachment microscopy.
The mechanisms leading to the production of hollow K shell atoms via single photon impact were investigated for a variety of light elements with 12 ≤ Z ≤ 23. The double 1s vacancy states were produced by irradiating the samples with intense monoenergetic synchrotron radiation beams. The double-to-single K-shell photoionization probabilities PKK and the absolute double K-shell photoionization cross sections σ2+ were determined by measuring with a high-resolution bent von Hamos crystal spectrometer the Kαh hypersatellite X-ray emission of the samples. The measurements were performed over a wide range of incoming photon energies from threshold up to energies beyond the broad maximum of the double-to-single photoionization cross section ratios. The PKK and σ2+ were determined from the relative yields of the resolved Kαh hypersatellite lines. For Mg, Al and Si, the two-electron one-photon (TEOP) Kααh transitions which represent an alternative but much weaker decay channel for double 1s vacancy states could be also observed, using a highly efficient flat crystal wavelength dispersive spectrometer. This observation of single photon-induced TEOP transitions has shown that the I(Kαh)/I(Kααh) branching ratios are very poorly reproduced by most of existing theoretical models. Besides the relative yields of the hypersatellite and TEOP transitions, the energies and natural linewidths of the Kαh and Kααh X-ray lines were also determined. The energies are found to be in good agreement with different theoretical predictions, whereas the linewidths are significantly underestimated by the calculations, except if non-lifetime broadening effects such as the outer-shell ionization and the open valence configuration are taken into consideration.
Multidimensional covariance analysis and its validity for correlation of processes leading to multiple products are investigated from a theoretical point of view. The need to correct for false correlations induced by experimental parameters which fluctuate from shot to shot, such as the intensity of self-amplified spontaneous emission x-ray free-electron laser pulses, is emphasized. Threefold covariance analysis based on simple extension of the two-variable formulation is shown to be valid for variables exhibiting Poisson statistics. In this case, false correlations arising from fluctuations in an unstable experimental parameter that scale linearly with signals can be eliminated by threefold partial covariance analysis, as defined here. Fourfold covariance based on the same simple extension is found to be invalid in general. Where fluctuations in an unstable parameter induce nonlinear signal variations, a technique of contingent covariance analysis is proposed here to suppress false correlations. In this paper we also show a method to eliminate false correlations associated with fluctuations of several unstable experimental parameters.
Self-organization phenomena such as rate oscillations, chemical wave patterns, and precipitation of nanoparticles can be observed in the catalytic H2 + O2 reaction on a Rh(111) surface after alloying with Ni. The bimetallic Rh(111)/Ni surface has been studied in the 10–6–10–4 mbar range using PEEM (photoemission electron microscopy) and LEEM/SPELEEM (low energy electron microscopy and its spectroscopic variant) as the main analytical methods. The Rh(111)/Ni catalysts are prepared by thermal decomposition of Ni(CO)4 on Rh(111), resulting in an alloyed surface with about 25% Ni in the topmost layers. One finds rate oscillations and chemical wave patterns comprising target patterns, pulse trains, and rotating spiral waves. The oscillatory behavior is attributed to periodic changes in the composition of the bimetallic surface alloy causing concomitant variations in catalytic activity. Under pattern-forming reaction conditions, three-dimensional NiO particles develop on top of the alloyed Rh/Ni surface, with dimensions ranging from <1 μm up to 50 μm. Their size which depends on the total pressure controls the Ni content in the surface alloy.
A great deal of attention has been devoted in the last few years to photoionization processes in isolated molecules leading to the formation of double core-hole (DCH) states. There are two main experimental avenues to induce such processes, namely single-photon absorption followed by the simultaneous ejection of two core electrons, and x-ray-induced multiphoton processes leading to the production of DCH states via the sequential absorption of two soft x- ray photons on a time scale on the order of the molecular Auger lifetime (4–8 femtoseconds for light elements). The formation of molecular two-site (ts) DCH states, in particular, shows great potential as a powerful tool for chemical analysis. A compelling motivation for the study of ts-DCH states is their ability to probe the local chemical environment more sensitively than either single core-hole (SCH) or single-site (ss) DCH states. The enhanced sensitivity originates from the fact that the double ionization potential (DIP) of ts-DCH states is directly coupled to induced changes in the valence charge distribution at the two different atomic sites. Here a review of the recent literature is presented on both types of experiments, and on the related theoretical work.
Single-site N1s and O1s double core ionisation of the NO and N2O molecules has been studied using a magnetic bottle many-electron coincidence time-of-flight spectrometer at photon energies of 1100 eV and 1300 eV. The double core hole energies obtained for NO are 904.8 eV (N1s-2) and 1179.4 eV (O1s-2). The corresponding energies obtained for N2O are 896.9 eV (terminal N1s-2), 906.5 eV (central N1s-2), and 1174.1 eV (O1s-2). The ratio between the double and single ionisation energies are in all cases close or equal to 2.20. Large chemical shifts are observed in some cases which suggest that reorganisation of the electrons upon the double ionization is significant. Δ-self-consistent field and complete active space self-consistent field (CASSCF) calculations were performed for both molecules and they are in good agreement with these results. Auger spectra of N2O, associated with the decay of the terminal and central N1s-2 as well as with the O1s-2 dicationic states, were extracted showing the two electrons emitted as a result of filling the double core holes. The spectra, which are interpreted using CASSCF and complete active space configuration interaction calculations, show atomic-like character. The cross section ratio between double and single core hole creation was estimated as 1.6 × 10-3 for nitrogen at 1100 eV and as 1.3 × 10-3 for oxygen at 1300 eV.
We have developed a tapered microchannel plate with a large open-area ratio (90%). The absolute detection efficiency of the tapered-MCP for xenon ions exceeded that of a normal MCP roughly in proportion to the open-area ratio.
We have studied single photon double K-shell ionization of small molecules (N2, CO, C2H2n (n = 1–3), …) and the Auger decay of the resulting double core hole (DCH) molecular ions thanks to multi-electron coincidence spectroscopy using a magnetic bottle time-of-flight spectrometer. The relative cross-sections for single-site (K−2) and two-site (K−1K−1) double K-shell ionization with respect to single K-shell (K−1) ionization have been measured that gives important information on the mechanisms of single photon double ionization. The spectroscopy of two-site (K−1K−1) DCH states in the C2H2n (n = 1–3) series shows important chemical shifts due to a strong dependence on the C
−2
Simultaneous core ionization and core excitation have been observed in the C_{2}H_{2n} (n=1, 2, 3) molecular series using synchrotron radiation and a magnetic bottle time-of-flight electron spectrometer. Rich satellite patterns corresponding to (K^{-2}V) core excited states of the K^{-1} molecular ions have been identified by detecting in coincidence the photoelectron with the two Auger electrons resulting from the double core hole relaxation. A theoretical model is proposed providing absolute photoionization cross sections and revealing clear signatures of direct (monopolar) and conjugate (dipolar near-edge x-ray absorption fine structure) shakeup lines of comparable magnitude.
Primary steps in the interaction of high energy photons with water creating multiply ionised products are examined experimentally and theoretically. Double Auger decay from a 1s-hole state populates triply ionised states between 80 and 140 eV binding energy. Ejection of one 1s electron and one valence electron gives states around 570 eV which decay to triply ionised states between 75 and 110 eV. Nuclear motion in these states competes with Auger decay and substantially modifies the final state spectra. The double core–hole state from ionisation of both 1s electrons is found at 1171 ± 1 eV and calculated at 1170.85 eV.
Because of the six equivalent carbon atoms in benzene, there is only one type of single core vacancy, but four different types of double core vacancies. The binding energies of single and double core vacancies are calculated and analyzed. Particular attention is paid thereby to reveal the relation between the single and double hole states. The π and σ electrons are found to contribute differently to the different types of double vacancies. It is generally stressed that double core ionization probes the bonding properties much more sensitively than single core ionization.
The energies needed to create different types of double core vacancies as well as the resulting redistribution of the valence electrons are analyzed in comparison with single core vacancies. Numerical results are presented for CH4 and in particular for the molecules C2H2, C2H4, and C2H6. A detailed perturbation theory analysis of the relaxation energies in terms of localized and delocalized molecular orbital pictures is presented. It is shown that the binding energies associated with double core vacancies where each of the two core holes is at a different atomic site sensitively probe the chemical environment of the atoms.