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Soft x‐ray emission spectroscopy using monochromatized synchrotron radiation (invited)

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Abstract

Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.

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... This results in a need for brilliant light sources. Soft X-ray spectrometers based on gratings in Rowland circle design or varied line spacing gratings are well-established instrumentation around the world [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Depending on the experimental requirements, some reach theoretically high resolution up to E/ΔE = 10 5 . ...
... Depending on the experimental requirements, some reach theoretically high resolution up to E/ΔE = 10 5 . However, these are very large instruments with up to 15 m arm length [14,19,20,[22][23][24][25][26]. Other spectrometer designs use additional optical elements to increase the solid angle, e.g. a Wolter type I or a set of collecting mirrors before the diffraction optics [27,28]. ...
... The spectrometer is motorized to achieve the optimal position and high reproducibility of the optical element for a high efficiency with sufficient energy resolution of 0.3 eV at 395 eV, which is comparable to existing grating spectrometers [15,17,22]. ...
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... A grating spectrometer, on the other hand, would be expected to have the same signal-to-background level displayed in Fig. 2(a) but would have to spend 10-100 times as long as our TES to collect that signal, due to the lower throughput of most grating systems. 26,27,29,38 Figure 3(a) shows the Fe L 3 -L 2 region of the 3d2p and 3s2p PFY-XAS spectra for the 500 mM K 3 [Fe iii (CN) 6 ] sample. Our 3s2p PFY-XAS spectrum is the first reported of this compound. ...
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... It is selectively sensitive to different molecular orbitals, specific elements at various sites, and probes truly bulk properties [4]. Limitations are the low radiative decay yield after core-excitation for biologically and chemically relevant systems due to the fact that they contain mostly light elements, as well as the low detection efficiency of typical instruments [5,6]. Consequently, high brilliant x-ray sources are required to perform such kinds of experiments. ...
... This leads to an energy resolution of the spectrometer of ∼0.46 at 532 eV, corresponding to a resolving power of E∕ΔE ∼ 1200. This is comparable with conventional x-ray optical elements [5], though at a much higher detection efficiency, when taking into consideration the pure optical element (grating) without efficiency-enhancing treatments or covers. Usually, a good-quality spectrum of liquid water requires tens of minutes [26,27]. ...
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... The commercial spectrometer (XES 355) employed at FERMI presents one order of magnitude worse resolution than the MERIXS custom spectrometer we used at ALS. Taking into account the spectrometer settings for the measurements, we actually expected an energy resolution of ~170 meV. We suppose the improved resolution observed at FERMI depends on the fact that the FEL spot on the sample dispersive direction was significantly smaller than the spectrometer entrance slit (~50 μ m) 19 . Contributions to the energy resolution broadening are due to the FEL pulse energy bandwidth, FEL beam mode mixing, and the XES 355 spectrometer resolution associated to the reduced FEL beam size with respect to the slit aperture. ...
... The X-rays scattered from the sample were energy dispersed and recorded using the XES 355 spectrometer (VG Scienta) mounted in the vertical geometry (π -polarization). The 300 lines/mm spectrometer grating operated at the 1 st diffraction order was chosen for analyzing the low energy X-ray inelastic emission 19 . The spectrometer detector unit consists of an MCP stack, a phosphor screen, and a CCD. ...
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... Current typical RIXS analyzers are based on reflecting even line spacing or variable line spacing (VLS) gratings, which collect the emitted light and disperse it across a 2D detector 19,[22][23][24][25][26][27][28][29][30][31][32] . In such an implementation, no information is conveyed along the non-dispersive direction (E out ) of the detector (see Fig. 1a), and the dispersed signal is simply integrated (projected) along this direction. ...
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... In the hard X-ray regime, RIXS has been applied as well to study electron-phonon interactions of cuprates by focusing on the phonon progression [59], and in essence, the same technique can be performed in soft X-ray RIXS with high-energy resolution as long as there are not too many different phonon modes interfering. In the soft X-ray regime, electron-phonon scattering properties have been analyzed by measuring both RIXS and X-ray emission spectroscopy (XES) with a relatively low-energy resolution RIXS spectrometer [60] compared to the current standard as function of temperature for silicon and silicon carbide. ...
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... The x-rays scattered from the sample were energy dispersed and recorded using the XES 355 spectrometer (VG Scienta) mounted in the vertical geometry (π-polarization). The 300 lines mm −1 spectrometer grating operated at the 1st diffraction order was chosen for analyzing the low energy x-ray inelastic emission [124]. The spectrometer detector unit consists of an micro-channel plate (MCP) stack, a phosphor screen and a CCD. ...
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... On the other hand, optics in the soft X-ray region rely on very shallow incidence angles and are limited in size. While RIXS signals are emitted rather isotropically, typical spectrometers can only accept fractions on the order of 10 −6 to 10 −4 of the full emission angle [15][16][17]. New methods to improve signal levels in RIXS while keeping similar information content are thus desirable. ...
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Article
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Preprint
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... As mentioned earlier, the spectrometer is designed based on the principle of the Rowland Circle (RC) geometry. 28 In such geometry, a curved, concave grating (cylindrical or spherical) fulfills the requirements of both dispersing and focusing radiations with different energies. Thereby, it allows for a rather simple design with only one reflection behind the sample. ...
... The grating inside the spectrometer disperses the X-rays with respect to their energies onto the imaging detector, while in the transverse direction the X-rays can either be collimated or focused to increase the angular acceptance of the instrument. These spectrometers, whether using spherical or plane gratings with constant or varied line spacing (VLS) rulings, can have disparate size and resolving power optimized for their science missions (Nordgren et al., 1989;Hague et al., 2005;Hatsui et al., 2005;Chuang et al., 2006Chuang et al., , 2017Ghiringhelli et al., 2006;Agå ker et al., 2009;Fuchs et al., 2009;Strocov et al., 2010;Harada et al., 2012;Yamane et al., 2013;Chiuzbȃ ian et al., 2014;Lai et al., 2014;Warwick et al., 2014;Yin et al., 2015;Dvorak et al., 2016;Brookes et al., 2018). However, using them to study the electronic structures of materials that are expected to exhibit strong spatial inhomogeneity can be challenging. ...
Article
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The optical design of a Hettrick–Underwood-style soft X-ray spectrometer with Wolter type 1 mirrors is presented. The spectrometer with a nominal length of 3.1 m can achieve a high resolving power (resolving power higher than 10000) in the soft X-ray regime when a small source beam (<3 µm in the grating dispersion direction) and small pixel detector (5 µm effective pixel size) are used. Adding Wolter mirrors to the spectrometer before its dispersive elements can realize the spatial imaging capability, which finds applications in the spectroscopic studies of spatially dependent electronic structures in tandem catalysts, heterostructures, etc . In the pump–probe experiments where the pump beam perturbs the materials followed by the time-delayed probe beam to reveal the transient evolution of electronic structures, the imaging capability of the Wolter mirrors can offer the pixel-equivalent femtosecond time delay between the pump and probe beams when their wavefronts are not collinear. In combination with some special sample handing systems, such as liquid jets and droplets, the imaging capability can also be used to study the time-dependent electronic structure of chemical transformation spanning multiple time domains from microseconds to nanoseconds. The proposed Wolter mirrors can also be adopted to the existing soft X-ray spectrometers that use the Hettrick–Underwood optical scheme, expanding their capabilities in materials research.
... The two spectrometers are mounted horizontally, facing each other, perpendicular to the photon beam (see Fig. 1). The low-energy region of the beamline, 27-200 eV, is covered by a plane-grating spectrometer, PGS (Agå ker et al., 2009), whereas a modified grazing-incidence Rowland circle spherical-grating spectrometer, GRACE (Nordgren et al., 1989), covers the energy range 50-1500 eV. In addition to the two spectrometers, a retractable detector for NEXAFS measurements is also available at the endstation. ...
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SPECIES is an undulator-based soft X-ray beamline that replaced the old I511 beamline at the MAX II storage ring. SPECIES is aimed at high-resolution ambient-pressure X-ray photoelectron spectroscopy (APXPS), near-edge X-ray absorption fine-structure (NEXAFS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) experiments. The beamline has two branches that use a common elliptically polarizing undulator and monochromator. The beam is switched between the two branches by changing the focusing optics after the monochromator. Both branches have separate exit slits, refocusing optics and dedicated permanent endstations. This allows very fast switching between two types of experiments and offers a unique combination of the surface-sensitive XPS and bulk-sensitive RIXS techniques both in UHV and at elevated ambient-pressure conditions on a single beamline. Another unique property of the beamline is that it reaches energies down to approximately 27 eV, which is not obtainable on other current APXPS beamlines. This allows, for instance, valence band studies under ambient-pressure conditions. In this article the main properties and performance of the beamline are presented, together with selected showcase experiments performed on the new setup.
... The monochromator energy calibration was checked against the N 1s XAS of h-BN. The RIXS and XAS measurements were carried out on the liquid-jet setup of the FlexRIXS end-station, 22 equipped with a Gammadata Scienta XES300 Nordgren-type soft X-ray grazing incidence grating spectrometer, 23 which was positioned in the horizontal plane at normal angle to the incident light beam. The entrance slit of the spectrometer was vertical, parallel to the liquid jet. ...
Article
In this paper we report an experimental and computational study of liquid acetonitrile (H3C-CN) by resonant inelastic X-ray scattering (RIXS) at the N K-edge. The experimental spectra exhibit clear signatures of the electronic structure of the valence states at the N site and incident-beam-polarization dependence is observed as well. Moreover, we find fine structure in the quasielastic line that is assigned to finite scattering duration and nuclear relaxation. We present a simple and light-to-evaluate model for the RIXS maps and analyze the experimental data using this model combined with ab initio molecular dynamics simulations. In addition to polarization-dependence and scattering-duration effects, we pinpoint the effects of different types of chemical bonding to the RIXS spectrum and conclude that the H2C-CNH isomer, suggested in the literature, does not exist in detectable quantities. We study solution effects on the scattering spectra with simulations in liquid and in vacuum. The presented model for RIXS proved to be light enough to allow phase-space-sampling and still accurate enough for identification of transition lines in physical chemistry research by RIXS.
... The diameter of the liquid jet around 20 μm is similar to the comparatively small focus size of the incoming X-rays, so that a maximum number of photons is absorbed by the sample, while the spot size is small enough to serve as the source point for the high-transmission, medium-resolution spectrometer 105 . ...
Thesis
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In this thesis, the dependencies of charge localization and itinerance in two classes of aromatic molecules are accessed: pyridones and porphyrins. The focus lies on the effects of isomerism, complexation, solvation, and optical excitation, which are concomitant with different crucial biological applications of specific members of these groups of compounds. Several porphyrins play key roles in the metabolism of plants and animals. The nucleobases, which store the genetic information in the DNA and RNA are pyridone derivatives. Additionally, a number of vitamins are based on these two groups of substances. This thesis aims to answer the question of how the electronic structure of these classes of molecules is modified, enabling the versatile natural functionality. The resulting insights into the effect of constitutional and external factors are expected to facilitate the design of new processes for medicine, light-harvesting, catalysis, and environmental remediation. The common denominator of pyridones and porphyrins is their aromatic character. As aromaticity was an early-on topic in chemical physics, the overview of relevant theoretical models in this work also mirrors the development of this scientific field in the 20th century. The spectroscopic investigation of these compounds has long been centered on their global, optical transition between frontier orbitals. The utilization and advancement of X-ray spectroscopic methods characterizing the local electronic structure of molecular samples form the core of this thesis. The element selectivity of the near-edge X-ray absorption fine structure (NEXAFS) is employed to probe the unoccupied density of states at the nitrogen site, which is key for the chemical reactivity of pyridones and porphyrins. The results contribute to the growing database of NEXAFS features and their interpretation, e.g., by advancing the debate on the porphyrin N K-edge through systematic experimental and theoretical arguments. Further, a state-of-the-art laser pump – NEXAFS probe scheme is used to characterize the relaxation pathway of a photoexcited porphyrin on the atomic level. Resonant inelastic X-ray scattering (RIXS) provides complementary results by accessing the highest occupied valence levels including symmetry information. It is shown that RIXS is an effective experimental tool to gain detailed information on charge densities of individual species in tautomeric mixtures. Additionally, the hRIXS and METRIXS high-resolution RIXS spectrometers, which have been in part commissioned in the course of this thesis, will gain access to the ultra-fast and thermal chemistry of pyridones, porphyrins, and many other compounds. With respect to both classes of bio-inspired aromatic molecules, this thesis establishes that even though pyridones and porphyrins differ largely by their optical absorption bands and hydrogen bonding abilities, they all share a global stabilization of local constitutional changes and relevant external perturbation. It is because of this wide-ranging response that pyridones and porphyrins can be applied in a manifold of biological and technical processes.
... As core electrons of the same elements in different chemical environments within an organic molecule (e.g. C atoms) have different binding energies, their soft XES spectra can more easily be separated [3] than is possible in x-ray absorption spectroscopy (XAS). In XAS, the contributions from atoms in non-equivalent environments can overlap each other, giving rise to spectral congestion at the edge due to core-transitions from a given inner shell. ...
Article
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We report on the C K-edge x-ray absorption spectra and the resonant (RXES) and non-resonant (NXES) x-ray emission spectra of ethylene, allene and butadiene in the gas phase. The RXES and NXES show clear differences for the different molecules. Overall both types of spectra are more structured for ethylene and allene, than for butadiene. Using density functional theory–restricted open shell configuration interaction single calculations, we simulate the spectra with remarkable agreement with the experiment. We identify the spectral features as being due to transitions involving localised 1s orbitals. For allene, there are distinct spectral bands that reflect transitions predominantly from either the central or terminal carbon atoms. These results are discussed in the context of ultrafast x-ray studies aimed at detecting the passage through conical intersections in polyatomic molecules.
... In order to measure such a broad range of lasing wavelengths and due to the space requirements at the end of the photon diagnostic section of FLASH2, a new compact wide-spectral-range XUV spectrometer was designed and constructed. This type of spectrometer based on the spherical-variable-line-space (SVLS) grating is a popular tool to characterize the photon properties of photon sources (for example, [9][10][11]). The spectrometer was used during the commissioning phase of FLASH2 including characterization of the SASE radiation and is also needed for future FEL seeding and optional spectral monitoring for users. ...
... Pump and probe spectra were recorded for every shot using a downstream x-ray spectrometer and used to establish the central photon energies, bandwidth, and relative pulse energies of the pump and probe pulses. The spectrometer was an adapted version of the instrument described in Ref. [51], with a 1200 l/mm grating operated in the Rowland geometry imaging a mechanical slit onto a microchannel plate coupled to a phosphor screen. A gas detector was used to extract the absolute pulse energies. ...
Article
Full-text available
The possibility of suddenly ionized molecules undergoing extremely fast electron hole (or hole) dynamics prior to significant structural change was first recognized more than 20 years ago and termed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specific hole within the molecule. We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump–x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment. We record the signature of transient hole dynamics and make the first tentative observation of dynamics driven by frustrated Auger-Meitner transitions. We verify that the effective hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localized in space and time in molecules and thus to charge transfer phenomena that are fundamental in chemical and material physics.
... A CO background pressure of 1.0 × 10 −8 torr was used to ensure a fully recovered surface adlayer once the same spot was revisited after scanning the complete sample. XAS was recorded by detecting x-ray fluorescence, using a previously described detector setup [24], and the incident energy was stepwise changed across the carbon 1s → 2π Ã resonance edge in the range of 285-290 eV. The XAS was thus plotted by averaging the total number of emitted photons recorded as a function of the incident energy for every desired pump-probe delay. ...
Article
We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.
... A CO background pressure of 1.0 × 10 −8 torr was used to ensure a fully recovered surface adlayer once the same spot was revisited after scanning the complete sample. XAS was recorded by detecting x-ray fluorescence, using a previously described detector setup [24], and the incident energy was stepwise changed across the carbon 1s → 2π Ã resonance edge in the range of 285-290 eV. The XAS was thus plotted by averaging the total number of emitted photons recorded as a function of the incident energy for every desired pump-probe delay. ...
Preprint
We use a pump-probe scheme to measure the time evolution of the C K-edge X-ray absorption spectrum (XAS) from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Due to the short duration of the X-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first ps after the pump can be resolved with unprecedented time resolution. By comparing with theoretical (DFT) spectrum calculations we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the ps regime. The ~100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e. g. electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to non-thermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.
... Pump and probe spectra were recorded for every shot using a downstream x-ray spectrometer and used to establish the central photon energies, bandwidth, and relative pulse energies of the pump and probe pulses. The spectrometer was an adapted version of the instrument described in [52], with a 1200 l/mm grating operated in the Rowland geometry imaging a mechanical slit onto a microchannel plate coupled to a phosphor screen. A gas-detector was used to extract the absolute pulse energies. ...
Preprint
Full-text available
The possibility of suddenly ionized molecules undergoing extremely fast electron hole dynamics prior to significant structural change was first recognized more than 20 years ago and termed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specific hole within the molecule. We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump/x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment. We record the signature of transient hole dynamics and make the first observation of dynamics driven by frustrated Auger-Meitner transitions. We verify that the hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localized in space and time in molecules and thus to charge transfer phenomena that are fundamental in chemical and material physics.
... Emission spectra were recorded using a Nordgren-type grazing-incidence spherical grating spectrometer with a resolution of 100 meV. 39 The energy scale was calibrated relative to Zn Lα 1,2 and Lβ 1 emission lines of Zn metal. ...
Article
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Ternary lanthanide indium oxides LnInO3 (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by X-ray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO3 materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO6 octahedra and highly distorted LnO12 polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO3 compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4f n compounds PrInO3, NdInO3, and SmInO3, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4f n-1 terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series Pr-Nd-Sm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO3 is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO3 and SmInO3, but a lower gap of 3.6 eV for PrInO3 was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.
... The endstation currently houses two spectrometers both mounted perpendicularly with respect to the incident photon beam and opposite to each other. The first spectrometer is a modified Scienta XES350 (Grace) operated in slitless mode (Nordgren et al., 1989). The Grace spectrometer houses three spherical gratings with line densities of 300 lines mm À1 (3 m radius), 400 lines mm À1 (5 m radius) and 1200 lines mm À1 (5 m radius). ...
Article
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The SPECIES beamline has been transferred to the new 1.5 GeV storage ring at the MAX IV Laboratory. Several improvements have been made to the beamline and its endstations during the transfer. Together the Ambient Pressure X-ray Photoelectron Spectroscopy and Resonant Inelastic X-ray Scattering endstations are capable of conducting photoelectron spectroscopy in elevated pressure regimes with enhanced time-resolution and flux and X-ray scattering experiments with improved resolution and flux. Both endstations offer a unique capability for experiments at low photon energies in the vacuum ultraviolet and soft X-ray range. In this paper, the upgrades on the endstations and current performance of the beamline are reported.
... After passing through the sample, the x-ray spectrum is analysed with an inline grating-based x-ray spectrometer [21]. The spectrometer was provided by Uppsala University as an adapted version of the spectrometer described in reference [22], featuring a constant line spaced grating (1200 lines mm −1 ) in the Rowland geometry, which images a mechanical slit onto a MCP detector coupled to a phosphor screen. This phosphor screen is also imaged with a single-shot CCD camera. ...
Article
Full-text available
X-ray photoelectron spectroscopy (XPS) measures the binding energy of core-level electrons, which are well-localised to specific atomic sites in a molecular system, providing valuable information on the local chemical environment. The technique relies on measuring the photoelectron spectrum upon x-ray photoionisation, and the resolution is often limited by the bandwidth of the ionising x-ray pulse. This is particularly problematic for time-resolved XPS, where the desired time resolution enforces a fundamental lower limit on the bandwidth of the x-ray source. In this work, we report a novel correlation analysis which exploits the correlation between the x-ray and photoelectron spectra to improve the resolution of XPS measurements. We show that with this correlation-based spectral-domain ghost imaging method we can achieve sub-bandwidth resolution in XPS measurements. This analysis method enables XPS for sources with large bandwidth or spectral jitter, previously considered unfeasible for XPS measurements.
... The different non-linear processes can typically be ordered by the number of involved photons: sum-frequency generation, two-photon absorption and stimulated emission with two photons, followed by other non-linear X-ray phenomena, time resolved transient gratings (XUV-TG) or four wave mixing (XUV-FWM) spectroscopy (see [8] and references therein). Fluorescent decays in the soft X-ray region allow unique access to the structure of the occupied valence states, while keeping the element selectivity and chemical state specificity of soft Xray spectroscopies [9]. Unfortunately, the probability for a fluorescent decay in the soft X-ray range is below 1%. ...
Article
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We present an overview of the FERMI (acronym of Free Electron laser Radiation for Multidisciplinary Investigations) seeded free electron laser (FEL) facility located at the Elettra laboratory in Trieste. FERMI is now in user operation with both the FEL lines FEL-1 and FEL-2, covering the wavelength range between 100 nm and 4 nm. The seeding scheme adopted for photon pulse production makes FERMI unique worldwide and allows the extension of table top laser experiments in the extreme ultraviolet/soft X-ray region. In this paper, we discuss how advances in the performance of the FELs, with respect to coherent control and multi-colour pulse production, may push the development of original experimental strategies to study non-equilibrium behaviour of matter at the attosecond-nanometer time-length scales. This will have a tremendous impact as an experimental tool to investigate a large array of phenomena ranging from nano-dynamics in complex materials to phenomena that are at the heart of the conversion of light into other forms of energy.
Chapter
The decay of the X-ray absorption spectroscopy (XAS) excited state back to the ground state is in most cases a cascade decay with several intermediate states. This chapter considers the first decay and describes x-ray emission spectroscopy (XES) as a second-order optical process which includes resonant inelastic x-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) or partial fluorescence yield (PFY) XAS. It provides an introduction to theoretical and experimental aspects with some references to applications to illustrate the possibilities of this technique. The chapter also mentions x-ray Raman scattering which is not a second-order process but requires similar instrumentation and is therefore often performed at XES experimental stations. It summarizes some important points about the resonant Kramers-Heisenberg equation. The chapter mainly reviews the instrumental developments and the overall hard x-ray emission advances boosted, realized and established due to the synchrotron radiation availability.
Article
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A novel experimental setup is presented for resonant inelastic X-ray scattering investigations of solid and liquid samples in the soft X-ray region for studying the complex electronic configuration of (bio)chemical systems. The uniqueness of the apparatus is its high flexibility combined with optimal energy resolution and energy range ratio. The apparatus enables investigation of chemical analyses, which reflects the chemical imprints. The endstation is composed of a main sample chamber, a sample holder for either solid or liquid jet delivery system, and a soft X-ray grating spectrometer for 210–1250 eV with a resolving power of ∼1000. It combines for the first time liquid jet technology with a soft X-ray spectrometer based on the variable line spacing principle. This setup was commissioned at the soft X-ray beamline P04 at PETRA III of the Deutsches Elektronen-Synchrotron in Hamburg which is currently the most brilliant storage-ring-based X-ray radiation source in the world. The first results of liquid and solid samples show that this setup allows the detection of photons across an energy range of ∼300 eV. This covers simultaneously the emission lines of life-important elements like carbon, nitrogen and oxygen in a shot-based procedure.
Article
Organic semiconductors (OSCs) are at the center of attention in a wide range of research fields since their unique advantages meet the requirements for next-generation optoelectronics applications. Since OSCs are lacking intrinsic carriers, charges for device operation have to be injected through organic/electrode and organic/organic interfaces. Therefore, the charge injection efficiency, which is determined by the energy level alignments at those interfaces, governs the device performance. In other words, high performance organic devices cannot be achieved without facilitating proper energy level alignments. Thus, the interfacial electronic structure, which should be determined from accurate measurements of the charge transport level, must be understood to establish the design strategy for high performance organic devices. In this review, various spectroscopic methods to investigate the surface and interface electronic structures, including direct photoelectron spectroscopy, inverse photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray emission spectroscopy, are discussed along with their fundamental principles. Examples of device performance enhancements with modification of the interfacial electronic structure in organic photovoltaics and organic light-emitting diodes are presented.
Article
Nanocrystalline tungsten trioxide (WO3) thin films prepared by DC magnetron sputtering have been studied using soft x-ray spectroscopy and optical spectrophotometry. Resonant inelastic x-ray scattering (RIXS) measurements reveal band gap states in sub-stoichiometric γ-WO3−x with x = 0.001–0.005. The energy positions of these states are in good agreement with recently reported density functional calculations. The results were compared with optical absorption measurements in the near infrared spectral region. An optical absorption peak at 0.74 eV is assigned to intervalence transfer of polarons between W sites. A less prominent peak at energies between 0.96 and 1.16 eV is assigned to electron excitation of oxygen vacancies. The latter results are supported by RIXS measurements, where an energy loss in this energy range was observed, and this suggests that electron transfer processes involving transitions from oxygen vacancy states can be observed in RIXS. Our results have implications for the interpretation of optical properties of WO3, and the optical transitions close to the band gap, which are important in photocatalytic and photoelectrochemical applications.
Chapter
Resonant inelastic X-ray scattering (RIXS) has emerged as a powerful method for the study of the electronic structure of matter in various phases and ambient conditions. After some historical remarks, an overview over recent achievements in studies of molecular systems and processes is given. Prospects associated with new radiation sources are discussed, especially concerning the exploitation of high energy resolution, short pulse duration, high intensity, controllable polarisation, and coherence. The concept of a novel type of imaging grating spectrometer is presented.
Article
In this paper, we present a spectrometer that is designed for element-specific and time-resolved transverse magneto-optic Kerr effect experiments at the high-harmonic generation pump-probe facility High Energy Laser Induced Overtone Source (HELIOS) laboratory. HELIOS delivers photons with energies between 30 eV and 72 eV with an overall time resolution of less than 40 fs. The spectrometer is based on a Rowland-circle geometry and allows for simultaneous measurements of all magnetic transition-metal elements. The setup also features easy sample transfer and alignment, and it combines high photon throughput, optimized data acquisition, and a fast switching of the magnetic field at the sample. The spectrometer performance is demonstrated by measuring the ultrafast demagnetization of permalloy. Our data are, for all practical purposes, identical to what have been reported in the earlier high-order harmonic generation work of a similar sample by Mathias et al. [Proc. Natl. Acad. Sci. U. S. A. 109, 4792–4797 (2012)], however, obtained within 15% of the acquisition time compared to their study. Furthermore, our data show a shift of the demagnetization curve of Ni relative to Fe, which has previously been interpreted as a delay of the Nidemagnetization to that of Fe [S. Mathias et al., Proc. Natl. Acad. Sci. U. S. A. 109, 4792–4797 (2012)].
Article
We have implemented and successfully tested an off-axis transmission Fresnel zone plate as a novel type of analyzer optics for resonant inelastic x-ray scattering (RIXS). We achieved a spectral resolution of 64 meV at the nitrogen K-edge (E/dE = 6200), closely matching theoretical predictions. The fundamental advantage of transmission optics is the fact that it can provide stigmatic imaging properties. This opens up a variety of advanced RIXS configurations, such as efficient scanning RIXS, parallel detection for varying incident energy and time-resolved measurements.
Article
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The valence orbitals of aqueous histidine under basic, neutral and acidic conditions and their X-ray induced transformations have been monitored through N 1s resonant inelastic X-ray scattering. Using density functional ab initio molecular dynamics simulations in the core-hole state within the Z + 1 approximation, core-excitation-induced molecular transformations are quantified. Spectroscopic evidence for a highly directional X-ray-induced local N-H dissociation within the scattering duration is presented for acidic histidine. Our report demonstrates a protonation-state and chemical-environment dependent propensity for a molecular dissociation, which is induced by the absorption of high energy photons. This case study indicates that structural deformations in biomolecules under exposure to ionizing radiation, yielding possible alteration or loss of function, is highly dependent on the physiological state of the molecule upon irradiation.
Article
The comparability and reliability of the analysis of the electronic structure of selected cyanates and thiocyanates at the nitrogen K edge based on BSE calculations have been investigated in this work. Using high-resolution x-ray spectroscopy with calibrated instrumentation reliable and reproducible experimental results for x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) were achieved. These results were used to validate theoretical modeling with first-principles calculations based on the Bethe-Salpeter equation approach for the excited-state interactions. Furthermore, radiation-induced damages occurring in the samples are correlated with absolute doses.
Book
Cambridge Core - Materials Science - X-ray Microscopy - by Chris Jacobsen
Article
We present a high energy resolution x-ray spectrometer for the tender x-ray regime (1.6–5.0 keV) that was designed and operated at Stanford Synchrotron Radiation Lightsource. The instrument is developed on a Rowland geometry (500 mm of radius) using cylindrically bent Johansson analyzers and a position sensitive detector. By placing the sample inside the Rowland circle, the spectrometer operates in an energy-dispersive mode with a subnatural line-width energy resolution (∼0.32 eV at 2400 eV), even when an extended incident x-ray beam is used across a wide range of diffraction angles (∼30° to 65°). The spectrometer is enclosed in a vacuum chamber, and a sample chamber with independent ambient conditions is introduced to enable a versatile and fast-access sample environment (e.g., solid/gas/liquid samples, in situ cells, and radioactive materials). The design, capabilities, and performance are presented and discussed.
Article
A number of iterative and perturbative approximations to the full equation-of-motion coupled cluster method with single, double, and triple excitations (EOM-CCSDT) are evaluated in the context of calculating the K-edge core-excitation and core-ionisation energies of several small molecules. Several of these methods are found to accurately reproduce the full EOM-CCSDT energies well, in particular the EOM-CCSD* method which, when using the core-valence separation (CVS) ansatz, scales rigorously with the sixth power of molecular size. These results highlight that very accurate core excitation and ionisation energies for molecules with first-row atoms can be computed at a cost not much larger than that for EOM-CCSD.
Article
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An atomic-level picture of molecular and bulk processes, such as chemical bonding and charge transfer, necessitatesanunderstandingofthedynamicalevolutionofthesesystems. Ontheultrafasttimescalesassociatedwithnuclearandelectronicmotion,thetemporalbehaviourofasystem is often interrogated in a ‘pump-probe’ scheme. Here, an initial ‘pump’ pulse triggers dynamics through photoexcitation, and after a carefully controlled delay a ‘probe’ pulse initiates projection of the instantaneous state of the evolving system onto an informative measurable quantity, such as electron binding energy. In this paper, we apply spectral ghost imaging to a pump-probe time-resolved experiment at an X-ray free-electron laser (XFEL) facility, where the observable is spectral absorption in the X-ray regime. By exploiting the correlation present in the shot-to-shot fluctuations in the incoming X-ray pulses and measured electron kinetic energies, we show that spectral ghost imaging can be applied to time-resolved pump-probe measurements. In the experiment presented, interpretation of the measurement is simplified because spectral ghost imaging separates the overlapping contributions to the photoelectron spectrum from the pump and probe pulse.
Chapter
This chapter introduces in detail the different elements assembled and designed to get the soft X-ray transient absorption experiment ready. Some maintenance routines and operational specifications naturally stem from the laser source’s reliance on a commercial system. It describes and details every pulse involved in the experiment, as well as the optical post-treatments applied to the pump pulse and the high harmonic generation driver to increase the system overall performances. The condition of soft X-ray generation in helium with long wavelength lasers is discussed and compared to the most advanced similar systems, providing an overview of the state of the art for tabletop time-resolved absorption spectroscopy in the water window. The characteristics of the flat-field vacuum spectrometers that were used to measure the signals over the full water window are also presented and discussed in detail. Finally, the data acquisition and data analysis methods are provided.
Article
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The I21 beamline at Diamond Light Source is dedicated to advanced resonant inelastic X-ray scattering (RIXS) for probing charge, orbital, spin and lattice excitations in materials across condensed matter physics, applied sciences and chemistry. Both the beamline and the RIXS spectrometer employ divergent variable-line-spacing gratings covering a broad energy range of 280–3000 eV. A combined energy resolution of ∼35 meV (16 meV) is readily achieved at 930 eV (530 eV) owing to the optimized optics and the mechanics. Considerable efforts have been paid to the design of the entire beamline, particularly the implementation of the collection mirrors, to maximize the X-ray photon throughput. The continuous rotation of the spectrometer over 150° under ultra high vacuum and a cryogenic manipulator with six degrees of freedom allow accurate mappings of low-energy excitations from solid state materials in momentum space. Most importantly, the facility features a unique combination of the high energy resolution and the high photon throughput vital for advanced RIXS applications. Together with its stability and user friendliness, I21 has become one of the most sought after RIXS beamlines in the world.
Article
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Resonant inelastic X-ray scattering in the XUV-regime has been implemented at BESSY II, pushing for a few-meV bandwidth in inelastic X-ray scattering at transition metal M -edges, rare earth N -edges and the K -edges of light elements up to carbon with full polarization control. The new dedicated low-energy beamline UE112-PGM1 has been designed to provide 1 µm vertical and 20 µm horizontal beam dimensions that serve together with sub-micrometre solid-state sample positioning as the source point for a high-resolution plane grating spectrometer and a high-transmission Rowland spectrometer for rapid overview spectra. The design and commissioning results of the beamline and high-resolution spectrometer are presented. Helium autoionization spectra demonstrate a resolving power of the beamline better than 10 000 at 64 eV with a 300 lines mm ⁻¹ grating while the measured resolving power of the spectrometer in the relevant energy range is 3000 to 6000.
Article
The electronic structures of carbon nanocages (CNCs) and nitrogen/phosphorus doped carbon nanocages (N-CNCs/P-CNCs) have been studied by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant X-ray emission spectroscopy (RXES). The doping configurations for N/P dopants are identified from the experiments. The results have shown that there are three major doping configurations for nitrogen but only one doping configuration for phosphorus. The nitrogen doping reveals the complex coexistence of graphite-like, pyridine-like and pyrrole-like configurations that are proved by density functional theory (DFT) simulations, while the phosphorus doping presents only the “graphite-like” configuration. The different configuration profiles result in less atomic structure ordering of N-CNCs than that of P-CNCs. XAS spectra obtained from both surface and bulk sensitive detection suggest different chemical environments between the interior and shell for all types of nanocages. The electronic structure modifications show significant difference between nitrogen and phosphorus doping from the DOS calculations.
Article
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We report dependence of the extreme ultraviolet quantum efficiency (QE) of a microchannel plate (MCP) detector upon the electric field strength above its input face. Using an uncoated plate, we measured increases up to 80% as the field was raised from 0 V/μ to between 0.01 and 0.1 V/μ. Further increases in electric field resulted in a monotonic decrease in QE. Detector spatial resolution was found to degrade for these small field values but could be recovered, while maintaining most of the QE increase, by operating with fields in excess of 0.3 V/μ. Other detector parameters such as modal gain and output charge pulse‐height distribution were not significantly affected by the applied electric field. We explain the QE and resolution variations in terms of photoelectrons ejected from the interchannel web and subsequently returned to the input face of the MCP by the applied electric field. We present a model and a computer simulation which quantitatively reproduce our experimental results. Applying our model to MCPs coated with photocathode materials, we conclude that the maximum QE is obtained by optimizing the combined contributions from the web area and open area of the MCP, rather than by maximizing the open area alone. In this case, most of the QE can arise from the interchannel web rather than from the directly illuminated channels.
Article
An instrument has been developed for the investigation of ultra-soft X-ray emission spectra which are excited in fluorescence using the synchrotron radiation of the storage ring DORIS at Hamburg. The 2 m grazing incidence concave grating spectrometer and the experimental layout are described.
Article
The theory of the concave grating has been examined in considerable detail by methods of geometrical optics. The results derived by means of geometrical optics were checked by comparing them with those based on physical optics in special cases. The conditions for image formation and aberrations in the image may be expressed by equations somewhat different from those given by Beutler which are shown to be in error. The astigmatism and other aberrations are treated with respect to finite length of slit illumination, finite grating size and deviations of the optical components from the Rowland plane but still lying on the Rowland cylinder. Within certain limitations, it is shown that the aberrations present in an off-plane Eagle mounting may be corrected by a very small rotation of the slit in a plane perpendicular to the optic axis. The optimum width of the grating, instrumental line half-width, and resolving power of the grating are also discussed. Finally, the Beutler treatment of the concave grating is examined in detail and the important errors are pointed out.
Article
After almost ten years of successful operation at a bending magnet beamline of HASYLAB, the Flipper monochromator was transferred to a 32-pole wiggler/undulator. With new grazing incidence optics the original energy range of 15–450 eV could be extended to more than 1500 eV. The gain in intensity is approximately two orders of magnitude. Undulator effects are observed between 36 and 900 eV and contribute considerably to this intensity gain. Monochromators of the Flipper type are ideally matching the emission characteristics of undulators. The main problem with wigglers at high energy storage rings is the heat load on the optical elements.
Article
The lead myristate multilayer analyzer has provided a basis for a relatively simple and efficient spectroscopy for the low‐energy x‐ray emissions in the 20–80‐Å region (where conventional crystal spectroscopy and grazing incidence grating spectroscopy are generally inefficient). The percent reflectivity, the integrated coefficient of reflection, and the Bragg diffraction width of the lead myristate analyzer have been measured and found to be consistent with the predictions of a simple theoretical model for multilayer diffraction. This multilayer spectroscopy at large Bragg angles has a high efficiency (high instrument transmission) as compared to grazing incidence grating spectroscopy in this 20–80‐Å region. However, the resolution is limited to that set by the diffraction width of the lead myristate analyzer of about 1 eV. Because the collimator‐crystal broadening function can be precisely defined, a simple and effective deconvolution procedure can be applied with this multilayer spectroscopy to bring the resolution into the sub‐electron‐volt region. To demonstrate the efficiency of lead myristate spectroscopy in the 20–80‐Å region, spectra were measured and analyzed from x‐ray excited fluorescent sources which are characteristically of low intensity. (X‐ray excitation yields a minimum of background spectra and of radiation damage.) These include the L 2,3 atomic spectrum of argon and the C‐K molecular spectrum of CO 2 , both in the gas phase, and the Cl‐L 2,3 and O‐K spectra from solid lithium perchlorate. Many samples undergo appreciable radiation‐induced chemical change during the exposure time that is required for measurement—even with an optimally fast spectrograph and with fluorescent excitation. A method has been developed to evaluate and to correct for radiation damage - by distributing the exposure over an effectively large sample volume either by gas flow or by rotating through multiple samples during measurement. Several spectral scans were made on the LiClO 4 using six samples. The total exposure time for each data point in each scan was recorded which permitted an extrapolation into a ’’zero’’ exposure spectrum. Finally, Fe‐L 2,3 /O‐K spectrum (from Fe 2 O 3 ) in the 17–25‐Å region is presented to illustrate the effectiveness of the lead myristate analyzer in third‐order diffraction. For this multilayer, the third‐order diffraction efficiency is one‐third that of the first order and is nearly twice that of the second order for this wavelength region.
Article
A new soft x‐ray spectrometer designed for use with photon excitation from synchrotron light sources is described and characterized. Special design features, including a close‐spaced input slit, large toroidal gratings, and a two‐dimensional charge‐coupled‐device array based detector system, provide exceptional measuring efficiency in a 5‐m Rowland circle design. Descriptions are given of the spectrometer’s mechanical and detector design, and of calibration and alignment procedures. The beam line providing photon excitation from a synchrotron light source is described. Typical electron beam and/or photon excited emission spectra of Al, Si, and LiF are presented and compared with those produced by other instruments.
Article
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
Although the microchannel plate (MCP) electron multipliers used in X-ray astronomy facilitate X-ray imaging with high spatial resolution, their intrinsic soft X-ray detection efficiencies of 1-10 percent are much lower than the near-unity values available with competing gas proportional counters. A high photoelectric yield material may be deposited on the MCP front surface and channel walls in order to enhance X-ray sensitivity at energies below a few keV. High 0.18-1.5 keV X-ray detection efficiencies are reported for MCPs bearing CsI deposition photocathodes, by which efficiency enhancement factors of up to 15 have been obtained. These results are especially pertinent to the sensitivity of such future X-ray astronomy experiments as the Roentgensatellit (Rosat) Wide Field Camera.
Article
Soft x-ray emission spectra of TiN, excited by monochromatized synchrotron radiation, have been recorded in a grazing-incidence spectrometer. The tunability of the excitation energy has allowed the pure N K and Ti LIII emission spectra to be isolated for the first time. A new type of resonance is observed in the Ti L emission spectrum, and interpreted as due to the decay of a quasiatomic intermediate state. The results call for a reinterpretation of Ti L spectra and a revision of the interpretation of 3d-metal L-emission spectra in general.
Clark (to be submitted for publication) Special topiCS 1696 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at
  • G Rubensson
  • R Bray
  • S Nyholm
  • N Cramm
  • J Mar
  • K.-L Nordgren
  • C H Tsang
  • T A Zhang
  • D L Callcott
  • C W Ederer
Rubensson, G. Bray, R. Nyholm, S. Cramm, N. Mar-tensson, and J. Nordgren, K.-L. Tsang, C. H. Zhang, T. A. Callcott, D. L. Ederer, and C. W. Clark (to be submitted for publication). Special topiCS 1696 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.233.210.97 On: Tue, 30 Sep 2014 15:36:03
uThe coordinate tables are from Micro Controle, France. 14The detector is a custom design version of a detector developed by Surface Science Instruments, Mountain View
  • r C Taylor
  • M C Hettrick
  • R F I1n
  • J.-E Wassdahl
12R. C. Taylor, M. C. Hettrick, and R. F. Malina, Rev. Sci. lnstrum. 54,171 (1983). uThe coordinate tables are from Micro Controle, France. 14The detector is a custom design version of a detector developed by Surface Science Instruments, Mountain View, CA. Rev. Sci. Instrum., Vol. 60, No.7, July 1989 16J._E. Rubensson, N. Wassdahl, G. Bray, J. Rindstedt, R. Nyholm, S. Cramm, N. Martensson, and J. Nordgren, Phys. Rev. Lett. 60, 1759 ( 1988). I1N. Wassdahl, J.-E.