Theofanis N Kitsopoulos

University of Crete | UOC · Department of Chemistry

Velocity-resolved kinetics is used to measure the thermal rate of formic acid desorption from Pd(111) between 228 and 273 K for four isotopologues: HCOOH, HCOOD, DCOOH, DCOOD. Upon molecular adsorption, formic acid undergoes decomposition to CO2 and H2 and thermal desorption. To disentangle the contributions of individual processes, we implement a...

A detailed velocity-resolved kinetics study of NH3 thermal desorption rates from p(2 × 2) O/Pt(111) is presented. We find a large reduction in the NH3 desorption rate due to adsorption of O-atoms on Pt(111). A physical model describing the interactions between adsorbed NH3 and O-atoms explains these observations. By fitting the model to the derived...

Thermal recombinative desorption rates of HD on Pd(111) and Pd(332) are reported from transient kinetic experiments performed between 523 and 1023 K. A detailed kinetic model accurately describes the competition between recombination of surface-adsorbed hydrogen and deuterium atoms and their diffusion into the bulk. By fitting the model to observed...

There is wide interest in developing accurate theories for predicting rates of chemical reactions that occur at metal surfaces, especially for applications in industrial catalysis. Conventional methods contain many approximations that lack experimental validation. In practice, there are few reactions where sufficiently accurate experimental data ex...

Invited for this month's cover are the groups of Alec M. Wodtke at the Georg‐August University in Göttigen (Germany) and at the University of Crete (Greece). The cover picture shows ammonia (NH3) molecules adsorbed on an atomically flat platinum surface. An ultra‐short laser pulse hits the surface and heats up the metal electron rapidly to several...

The Front Cover shows ammonia molecules adsorbed on an atomically flat platinum surface. An ultra‐short laser pulse hits the surface and heats up the metal electron rapidly to several thousand Kelvin within a few 100fs. This high electronic temperature causes a fraction of the ammonia molecules to desorb with hyperthermal velocity from the surface....

Most experimental methods for studying the kinetics of surface reactions – for example, temperature programmed desorption (TPD), molecular beam relaxation spectrometry (MBRS) and velocity‐resolved kinetics (VRK) – employ detection schemes that require thermal desorption. However, many adsorbates – for example reaction intermediates – never leave th...

We report accurate time-resolved measurements of NH3 desorption from Pt(111) and Pt(332) and use these results to determine elementary rate constants for desorption from steps, from (111) terrace sites and for diffusion on (111) terraces. Modeling the extracted rate constants with transition state theory, we find that conventional models for partit...

We have known for over 200 years that hydrogen undergoes rapid oxidation to water on Pt catalysts; yet the reaction mechanism remains unclear. Here, we report high temporal resolution measurements of the production rate of H2O from hydrogen oxidation catalysed by a Pt (111) single crystal surface with a known concentration of adsorbed oxygen atoms...

We report inelastic differential scattering experiments for energetic H and D atoms colliding at a Pt(111) surface with and without adsorbed O atoms. Dramatically, more energy loss is seen for scattering from the Pt(111) surface compared to p(2 × 2) O on Pt(111), indicating that O adsorption reduces the probability of electron–hole pair (EHP) excit...

We report nitric oxide (NO) desorption rates from Pd(111) and Pd(332) surfaces measured with velocity-resolved kinetics. The desorption rates at the surface temperatures from 620 to 800 K span more than 3 orders of magnitude, and competing processes, like dissociation, are absent. Applying transition state theory (TST) to model experimental data le...

Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment—a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement...

Nature of the molecule-surface encounter Adsorption is an important initial step in all heterogeneous chemical processes. However, detailed adsorption dynamics are complex and challenging to follow experimentally. Using the fact that vibrationally excited carbon monoxide molecules can be trapped on the Au(111) surface with all degrees of freedom be...

Up to now, the methods available for measuring the rate constants of reactions taking place on heterogeneous catalysts require that the catalyst be stable over long measurement times. But catalyst are often non-stationary, they may become activated under reaction conditions or become poisoned through use. It is therefore desirable to develop method...

The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as microkinetic models of atmospheric chemistry provided the predictive power that led to the Montreal Protocol reversing loss of stratospheric ozone, pursuing a microkinetic approach to heterogeneous catalysis has tremendous potential for societal impact. H...

The dream of theoretical surface chemistry is to predict the outcome of reactions in order to find the ideal catalyst for a certain application. Having a working ab initio theory in hand would not only enable these predictions but also provide insights into the mechanisms of surface reactions.The development of theoretical models can be assisted by...

We report measurements to investigate the effects of mechanical strain on the binding energy of carbon monoxide (CO) on the (111) surface of a 16 nm thin film of palladium (Pd) grown on rutile titanium dioxide (r-TiO2). The lattice mismatch between Pd and the r-TiO2 leads to a tensile mechanical in-plane stress in the Pd layer of approximately 0.38...

Catalysts are widely used to increase reaction rates. They function by stabilizing the transition state of the reaction at their active site, where the atomic arrangement ensures favourable interactions 1 . However, mechanistic understanding is often limited when catalysts possess multiple active sites-such as sites associated with either the step...

We describe a new instrument that uses ion imaging to study molecular beam-surface scattering and surfacedesorption kinetics, allowing independent determination of both residence times on the surface and scattering velocities of desorbing molecules. This instrument thus provides the capability to derive true kinetic traces, i.e., product flux versu...

We present a 1 + 1' resonance-enhanced multiphoton ionization (REMPI) scheme for acetylene via the linear G 4sσ (1)Πu Rydberg state, offering partial rotational resolution and the possibility to detect excitation in both the cis- and trans-bending modes. The resonant transition to the G state is driven by a vacuum ultraviolet (VUV) photon, generate...

The stereodynamics of methyl iodide photodissociation after excitation at 193 nm has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization (REMPI) detection of the methyl and iodine products. A weak anisotropic ring appearing in the image corresponding to vibrationally excited CH3(nu1=1) confirms the produc...

We present a new implementation of ion imaging for the study of surface scattering processes. The technique uses a combination of spatial ion imaging with laser slicing and delayed pulsed extraction. The scattering velocities of interest are parallel to the imaging plane, allowing speed and angular distributions to be extracted from a single image....

In the design of new chromophores, with high efficiency, chemical stability and low cost materials for dye sensitized solar cells, porphyrin macrocycles could play a very important role. Their successful use in nature during photosynthesis must be the inspiration for new artificial antenna systems. Porphyrins offer an excellent platform for buildin...

We present a new photo-fragment imaging spectrometer, which employs a movable repeller in a single field imaging geometry. This innovation offers two principal advantages. First, the optimal fields for velocity mapping can easily be achieved even using a large molecular beam diameter (5 mm); the velocity resolution (better than 1%) is sufficient to...

We identify a pair of near-degenerate states of opposite parity in atomic Xe, the $5p^5 10s \,\, ^2[3/2]_2^o$ at $\rm{E}=94759.927$ cm$^{-1}$ and $5p^5 6f \,\, ^2[5/2]_2$ at $\rm{E}= 94759.935$ cm$^{-1}$, for which parity- and time-odd effects are expected to be enhanced by the small energy separation. We present theoretical calculations which indi...

In this report we describe the use of a novel porphyrin triad (PPT) consisting of two zinc-metalated porphyrin units and one free-base porphyrin unit covalently linked through their peripheral amino-phenyl groups to a central s-triazine unit, in combination with PC70BM ([6,6]-phenyl C70 butyric acid methyl ester), as electron donor and electron acc...

A propeller-shaped, triazine-linked, unsymmetrical porphyrin triad – consisting of two zinc-metallated porphyrin units and one free-base porphyrin unit that is functionalized by a carboxylic acid – has been synthesized by stepwise amination reactions of cyanuric chloride. Photophysical, electrochemical, and DFT studies of the triad revealed no sign...

The photodissociation of methyl bromide at 193 nm is studied using slice imaging. From the measured photofragment translational energy and angular distributions we were able to extract methyl-state-specific dissociation channel yields and crossing probabilities between (3)Q0 and (1)Q1 surfaces. The angular distributions for the Bromine photofragmen...

A combination of a meso substituted zinc-porphyrin with two pyridyl groups and two benzoate groups at cis positions to each other (POR) with a mononuclear ruthenium(II) complex containing a 2,6-bis(1-methylbenzimidazol-2-yl)pyridine ligand and a carboxylic acid functionalized terpyridine ligand (SPS-G3) has been used as co-sensitizing system for th...

This study introduces the state resolved dynamics of CH3I when clustered in a supersonic expansion using He and Xe as seeding gases. Following excitation at 193 nm (6.4 eV), state specific alterations in photodissociation dynamics are observed. Four vibrational levels (v(2) = 0, 1, 2, 3) of the CH3 umbrella vibrational mode were probed, as were 1(P...

The photodissociation of nitromethane at 193 nm is reviewed in terms of new stereody- namical information provided by the measurement of the first four Dixon's bipolar moments, β02 (20), β0 (22), β02 (02) and β02 (22), using slice imaging. The measured speed-dependent β02 (20) (directly related with the spatial anisotropy parameter β ) indicates th...

We report vibrational excitation of CO from its ground (v = 0) to first excited (v = 1) vibrational state in collision with Au(111) at an incidence energy of translation of E(I) = 0.45 eV. Unlike past work, we can exclude an excitation mechanism involving temporary adsorption on the surface followed by thermalization and desorption. The angular dis...

We present slice imaging data demonstrating the influence of clustering on the photodissociation dynamics of a diatomic molecule: iodine monochloride (ICl) was dissociated at 235 nm in He and Xe seed gasses, probing both Cl and I photofragment energy and angular distributions. We observe that the kinetic energy releases of both Cl and I fragments c...

ICl photolysis in the ultraviolet region of the spectrum (235-265 nm) is studied using the Slice Imaging technique. The Cl∗((2)P(1/2))/Cl((2)P(3/2)) and the I∗((2)P(1/2))/I((2)P(3/2)) branching ratio between the I((2)P(3/2)) + Cl((2)P(3/2))∕Cl∗((2)P(1/2)) and I∗((2)P(1/2)) + Cl((2)P(3∕/2))∕Cl∗((2)P(1/2)) channels is extracted from the respective io...

We have implemented the velocity map imaging technique to study clustering in the pulsed supersonic expansions of hydrogen bromide in helium, argon, and xenon. The expansions are characterized by direct imaging of the beam velocity distributions. We have investigated the cluster generation by means of UV photodissociation and photoionization of HBr...

ICl photolysis in the visible region of the spectrum (400-570 nm) is studied using the slice imaging technique. The Cl((2)P(1/2))/Cl((2)P(3/2)) branching ratio between the I((2)P(3/2))+Cl((2)P(3/2)) and I((2)P(3/2))+Cl((2)P(1/2)) channels is extracted from the iodine photofragment images and it is found to range from 0 to 2.5, rising from 570 to 49...

We investigated the reaction dynamics of N-methylpyrrole (NMP) along the N-CH3 coordinate, upon excitation energies below 6.4 eV. Ours and previous experiments show clearly the existence of different reaction channels leading to slow and fast fragment production whose relative efficiency fluctuates with the changes in the excitation energy. Thanks...

The state-resolved photodissociation dynamics of CH3Br+ cations are studied using a combination of slice imaging and velocity mapping. Parent ions are prepared by (2 + 1) two-photon resonant three-photon ionization of CH3Br through the 5s Rydberg state in the energy region 29,069.8-29,976.0 cm(-1). Analysis of the photoelectron spectra reveals that...

In this work we present recent results on non-stoichiometric In<sub align="right">2</sub>O<sub align="right">3−x</sub> thin films grown by dc magnetron sputtering technique focusing on structural and surface characterisation using cross-section transmission electron microscopy (XTEM), Atomic force Microscopy (AFM) and X-ray diffraction (XRD). The f...

In this report we describe a process where chemical control can be used to create a molecular switch. Specifically we demonstrate that the fission of the N-H bond in pyrrole can be “controlled”, i.e., turned ON or OFF, by clustering the system to a single noble gas atom such as Xe. Using slice imaging experiments on the H-atom photofragments in com...

Targeting bio-sensing applications of ZnO in BioMEMS, thin films ZnO were grown by dc magnetron sputtering under different deposition parameters and their morphological and mechanical properties were analyzed using AFM, XRD, XTEM, and nanoindentation techniques. The films have nano structured granular surface suitable for bio material binding while...

The photodissociation of acetaldehyde in the molecular channel yielding CO and CH(4) at 248 nm has been studied, probing different rotational states of the CO(nu = 0) fragment by slice ion imaging using a 2+1 REMPI scheme at around 230 nm. From the slice images, clear evidence of the co-existence of two different mechanisms has been obtained. One o...

We investigate the photochemistry and photodissociation dynamics of uracil by two-colour photofragment Doppler spectroscopy and by two-colour slice imaging at excitation wavelengths between 268 and 235 nm. We observe the loss of a hydrogen atom upon excitation into the pipi* state. The angular distribution indicates a statistical process, while the...

Photo-induced hydrophilicity of titanium dioxide makes this material one of the most suitable for various coating applications in antifogging mirrors and self-cleaning glasses. The field of functional titanium dioxide coatings is expanding rapidly not only in applications for glass but also in applications for polymer, metal and ceramic materials....

In this work we present recent results on ZnO thin films grown by dc magnetron sputtering technique at room temperature (RT), focusing on structural and surface characterization using conventional cross-section transmission electron microscopy (XTEM) and high resolution cross section transmission electron microscopy (HRXTEM) in an attempt to unders...

Technological demands for the fabrication of nanostructured active coatings provide renewed motivation for understanding the properties that control the morphology of the thin film surface. With decreasing structure size, the issues of finite size and shape effects become non-negligible including issues of stability against decay and structural rea...

We study a heavy piston of mass M that separates finitely many ideal, unit mass gas particles moving in two or three dimensions. Neishtadt and Sinai previously determined a method for finding this system’s averaged equation and showed that its solutions oscillate periodically. Using averaging techniques, we prove that the actual motions of the pist...

The photolysis of pyrrole has been studied in a molecular beam at wavelengths of 250, 240, and 193.3 nm, using two different carrier gases, He and Xe. A broad bimodal distribution of H-atom fragment velocities has been observed at all wavelengths. Near threshold at both 240 and 250 nm, sharp features have been observed in the fast part of the H-ato...

Technological demands for the fabrication of nanostructured active coatings provide renewed motivation for understanding the properties that control the morphology of the thin film surface. With decreasing structure size, the issues of finite size and shape effects become non-negligible including issues of stability against decay and structural rea...

Pure and aluminum (Al) doped zinc oxide (ZnO and ZAO) thin films have been grown using direct current (dc) magnetron sputtering from pure metallic Zn and ceramic ZnO targets, as well as from Al-doped metallic ZnAl2at.% and ceramic ZnAl2at.%O targets at room temperature (RT). The effects of target composition on the film's surface topology, crystall...

In this paper, we present a new approach for studying the electronic properties of self-assembled monolayers and their interaction with a conductive substrate, the low-energy photoelectron imaging spectroscopy (LEPIS). LEPIS relies on imaging of photoelectrons ejected from a conductive substrate and subsequently transmitted through organic monolaye...

In this study we demonstrate that it is possible to perform velocity mapping and slice imaging of charged particle products using a single field (two electrodes) improving on the multielectrode geometries of the past. The performance of this new geometry is competitive to the multielectrode ones, presently limited by detector "spot" resolution. For...

In2O3-x thin films with a thickness of 100-990 nm were grown by dc magnetron sputtering. Their structural, electrical and ozone sensing properties were analyzed. Structural investigations carried out by electron probe micro analysis, secondary ion mass spectrometry and atomic force microscopy showed a strong correlation between stoichiometry, surfa...

We discuss the photochemistry and photodissociation dynamics of thymine as revealed by two-colour photofragment Doppler spectroscopy and by one-colour slice imaging. Thymine is optically excited into the pipi* state, known to deactivate quickly. The H atom photofragment spectra are dominated by two-photon excitation processes with subsequent statis...

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many appl...

Photoelectron imaging and time of flight mass spectrometry are used to study the multiphoton ionization and dissociation of pyrrole and its cation following excitation at 243 nm and at 364.7 nm. Our results confirm the 8.2 eV ionization potential of pyrrole and the 9.2 eV ionization threshold for forming the 2B1 first excited state of the cation. P...

The orientation and alignment of the (2)P(3/2) and (2)P(1/2) Br photofragments from the photodissociation of HBr is measured at 193 nm in terms of a(q) ((k))(p) parameters, using slice imaging. The A (1)Pi state is excited almost exclusively, and the measured a(q) ((k))(p) parameters and the spin-orbit branching ratio show that the dissociation pro...

Rotational state resolved center-of-mass angular scattering and kinetic energy release distributions have been determined for the HCl (v′ = 0, j′ = 0–6) products of the reaction of chlorine with n-butane using the photon-initiated reaction technique, coupled with velocity-map ion imaging. The angular and kinetic energy release distributions derived...

The combination of a standard velocity map imaging setup with three different slicing methods is demonstrated for the dissociation of O-2(X(3)Sigma(g)(-)) (v = 0) at 157 nm leading to the O(D-1) + O(P-3) channel, and I-2(X(1)Sigma(g)) (v = 0-4) at 490 nm leading to the I(P-2(3/2)) + I(P-2(1/2)) channel. The kinetic energy and angular distributions...

Slice imaging is used to measure the velocity distribution of the H-atom photofragments from the photodissociation of HBr at 243 nm. The achievable velocity resolution of our measurements is improved by a factor of 1.5 when (3 + 1) resonance enhanced multiphoton ionization (REMPI) via the 2p-excited state is used rather than the popular (2 + 1) REM...

The dynamics of H-atom abstraction reactions of Cl atoms with CH 3 Cl and CH 3 Br have been studied using velocity map imaging to obtain center-of-mass frame differential cross sections for HCl formed in its vibrational ground state and rotational levels with J) 2-5. The HCl products are preferentially backward scattered in both reactions, with bro...

Center-of-mass frame scattering angle distributions obtained directly from crossed molecular beam velocity map images are reported for HCl formed in different rotational levels of its vibrational ground state by reaction of Cl atoms with CH3OH and CH3OCH3. Products are observed to scatter over all angles, with peaks in the distribution in the forwa...

The hydrogen atom abstraction reaction between Cl(2P3/2) and ethane has been studied at a mean collision energy of 0.24 eV. The experiments were performed in a coexpansion of molecular chlorine and ethane, with the atomic Cl reactants generated by laser photodissociation of Cl2 at 355 nm. HCl(v',j') products were detected quantum state selectively...

The production of spin-polarized hydrogen atoms from the photodissociation of hydrogen chloride with circularly polarized 193-nanometer light is inferred from the measurement of the complete angular momentum distributions of ground state Cl(2P3/2)and excited state Cl(2P1/2)cofragments by slice imaging. The experimentally measured and ab initio pred...

The hydrogen abstraction reactions of atomic chlorine with ethane and n-butane are studied in a skimmerless crossed molecular beam experiment by imaging of state-selected products. The differential cross section for HCl(v=0, J=1–5) product is directly determined from the product velocity map image. The state selection of products is achieved using...

Charged particle imaging has revolutionized experimental studies of photodissociation and bimolecular collisions. Written in a tutorial style by some of the key practitioners in the field, this book gives a comprehensive account of the technique and describes many of its applications. The book is split into two parts. Part I is intended as a series...

Charged particle imaging has revolutionized experimental studies of photodissociation and bimolecular collisions. Written in a tutorial style by some of the key practitioners in the field, this book gives a comprehensive account of the technique and describes many of its applications. The book is split into two parts. Part I is intended as a series...

Slice imaging is used to measure the alignment of and photofragments from the photodissociation of HBr and HCl, respectively, at 193 nm. In both cases the state is predominantly optically excited, which correlates to mJ=±3/2 for the halogen atom photofragments. However, both the Br and Cl photofragments are measured to populate mostly mJ=±1/2, show...

State-to-state photodissociation experiments of OCS at 230 nm are reported using hexapole state selection of the parent molecule and velocity map ion imaging of the angular recoil of the CO photofragment. The role of the initial rovibrational state (ν2 = 0,1∣JlM) of OCS on the angular recoil distribution is investigated. The CO (X 1Σ+;v = 0∣J) rota...

Slice imaging is used to measure directly the alignment of Br(2P3/2) and Cl(2P3/2) photofragments from the photodissociation of Br2 and Cl2, respectively, at 355 nm, using an intensity-normalization method that does not require the wavelength scanning of the probe laser. In both cases the C 1Π(1u) state is optically excited. The Br photofragments a...

The photodissociation of CS2 at 193 nm (51 700) cm−1) is studied using the slice imaging technique, and velocity distributions for state selected S(3P2,1,0) and S(1D2) photofragments are measured. From the analysis of the speed distributions we determine the S(3P2,1,0)/S(1D2) branching ratio to be 1.5±0.4. The partially resolved S-atom speed distri...

A novel method to measure directly the photofragment alignment from Abel-invertible two-dimensional ion images, as a function of photofragment recoil velocity, is demonstrated for S(1D2) atoms from the photodissociation of carbonyl sulfide at 223 nm. The results are analyzed in terms of coherent and incoherent contributions from two dissociative st...