Dynamics of Surface-aligned Photochemistry. III. A quantum mechanical study of the photodissociation of HBr(ad)/LiF(001) (original) (raw)
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The Journal of Physical Chemistry B, 1997
We report a quasi-classical trajectory study of a chemical reaction between H and CO 2 at the LiF(001) surface. The reaction is initiated by photodissociation of well-aligned HBr(ad) at 193 nm, which produces a "hot" H atom directed toward a nearby CO 2 (ad). Single molecules of each reactant are placed on a static surface, and a full-dimensional HCO 2 potential derived from ab initio calculations is used. The adsorbate-substrate and the adsorbate-adsorbate potentials consist of both nonelectrostatic and electrostatic contributions. Several energetically favorable adsorption configurations are determined by a Monte Carlo method. Quasi-classical trajectories are calculated at 80 K for four different adsorption configurations. We find that the reactivity at some configurations is significantly enhanced compared with the corresponding gas-phase simulation. The calculated impact parameters and incident angles of the surface-aligned collisions indicate that the enhanced reactivity can be largely attributed to the closeness and alignment of the coadsorbates on the surface. Owing to the long-lived complex, product distributions, with the exception of a departure angle, show little memory with regard to the initial configuration and are similar to those obtained in the gas phase. A significant number of the unreacted hydrogen atoms retain sufficient energy to make subsequent reaction with other coadsorbates a possibility. We find evidence of several dynamic features pertinent to the use of the surface as a template for reactivity enhancement, including scattering at the surface, the squeezed atom effect, chattering, and caging.
A theoretical study of hydrogen diffraction following photodissociation of adsorbed molecules
The Journal of Chemical Physics, 1992
A new probe of surface structure is presented which is based on the photodissociation of hydrogen from an adsorbate molecule. The event creates an atomic hydrogen fragment, positioned between the adsorbate layer and the solid surface. Due to its light mass, the hydrogen dynamics is quantum mechanical in nature. A useful image is of the hydrogenic wave function behaving like a liquid able to fill all cracks. The coherent character of the hydrogenic wave function is crucial in the ability of the photodissociation experiment to act as a probe. A series of case studies has been carried out whose aim is to reveal the relation between the structure of the surface and the asymptotic energy resolved angular distribution of the hydrogen fragment. The dynamics of the hydrogen atom motion was modeled by the time dependent Schrijdinger equation. The cases studied include the dissociation of a single HBr adsorbate on flat and corrugated surfaces. A broad specular peak was observed, in addition to diffraction peaks which can be correlated with the corrugation. Moreover, selective adsorption peaks, which can be correlated with the attractive part of the surface potential, have been identified. Systems in which the hydrogenic wave function scatters from several adsorbates were also investigated. It was found that the scattering is dominated by the trapping of the wave function by unstable periodic orbits. The quantization rules of these periodic orbits have been identified, creating a link between the structure of the adsorbates and the asymptotic angular distributions.
J Phys Chem a, 1997
The photodissociation of CH 3 I on LiF(001) and NaCl(001) at 248 nm has been studied by probing the CH 3 fragments, using angular-resolved resonantly enhanced multiphoton ionization and time-of-flight mass spectrometry. At submonolayer and multilayer coverages, the translational energy, vibrational state, and angular distributions of the CH 3 photofragments were determined for both CH 3 I/LiF and CH 3 I/NaCl. The translational-energy distributions for the fast component of the CH 3 fragments resembled those for the gas phase, indicative of collision-free recoil from the substrate. The I* quantum yields obtained from adsorbed CH 3 I were, however, substantially lower than for the gas. At multilayer coverage the I* quantum yield from adsorbed CH 3 I was found to vary as a function of vibronic band of the CH 3 photofragments, from 0.76 for the 0 0 0 band to 0.34 for the 2 2 2 band. These results were rationalized on the basis of the Landau-Zener model for potential-energy surface hopping. The measured vibrational-state distributions of the umbrella mode also exhibited a strong dependence on reaction channel, on coverage, and on substrate. The angular distributions depended on the substrate; a sharp peak at 20°was observed in the angular distribution for the CH 3 I/LiF system, whereas a broader peak characteristic of collisional scattering was obtained for CH 3 I/NaCl. These angular distributions indicated that CH 3 I(ad) was close to upright on LiF but tilted further away from the normal on NaCl. The energy distributions, which showed evidence of greater collisional deexcitation on NaCl than on LiF, appeared consistent with these differing adlayer geometries.
Theoretical studies of surface diffusion and of photo-induced surface processes
Computer Physics Communications, 1994
Theoretical study of three types of processes associated with adsorbate-substrate interactions will be discussed. The first process considered is the surface diffusion mechanism of large adsorbates. It will be demonstrated that the conventional random walk mechanism, associated with the surface diffusion of small adsorbates, has to be largely modified when description of the motion of large adsorbates along the surface is desired. The results of these calculations shows good qualitative agreement when compared with experimental data. In the second study reported here, the branching ratio between desorption and dissociation for large model adsorbates on a laser heated substrate is examined. It is found that the branching ratio is very sensitive to a number of quantities including the adsorbate size, adsorption geometry and the relative strength between the adsorbate-surface bond and the intramolecular bond which is broken during the dissociation process. The last group of calculations investigate the dynamics of photodissociation products and their relation to the orientation and alignment of the adsorbates on the solid surface. The calculations are carried out for the HBr/LiF(001) system. Quantities such as the photoreaction probability, the angular and energy distributions of the scattered photofragment are shown to depend strongly on the initial alignment of the adsorbate. In addition, the quantum nature of the H photoproduct is examined using a 2-D quantum mechanical model. It is shown that for some systems the angular distribution of the scattered H-atom may exhibit a complicated diffraction pattern which can be related to the structure of the interface between the adsorbate layer and the substrate.
Photochemistry of adsorbed molecules
Surface Science, 1999
Methyl photofragments from the 193 nm photodissociation of CH 3 Br adsorbed on CaF 2 (111) were studied by angularly resolved time-of-flight mass spectrometry. The translational energy distributions, P(E T ∞), of the CH 3 photofragments showed evidence of four pathways, depending on the coverage. The pathways were termed 'direct' (DIR), 'indirect(1∞)' [IND(1∞)], 'indirect(1)' [IND(1)] and 'indirect(2)' [IND(2)] in order of decreasing peak E T ∞. The DIR methyl P(E T ∞) peaked at 2.4 eV, with a full-width at half-maximum (FWHM) of 0.6 eV, corresponding closely to that reported for gas-phase CH 3 (peak=2.5 eV, FWHM=0.5 eV). The P(E T ∞) of this DIR pathway was consistent with CH 3 escaping directly without collisions. For these DIR methyls the angular distribution, P(h∞), reflected the prior BrYC bond direction at the various coverages. The DIR P(h∞) indicated a change in the alignment of BrYCH 3 with increasing coverage, from one in which the Br-C axis was initially roughly parallel to the surface plane (0.25 ML), to one with the Br-C axis along the surface normal (0.5 and 2 ML), and finally to one with the Br-C axis 23°from the surface normal (7 ML). The IND(1∞) and IND(1) methyls, despite an energy loss of 0.7 and 1.3 eV, respectively, in a strong inelastic encounter, both exhibited similar energy and angular distributions to the DIR component. We propose that the IND(1) pathway is due to the occurrence of an 'intralayer' exchange reaction (CH 3 +BrCH 3 ∞ CH 3 Br+CH 3 ∞) in one layer, and in the case of the IND(1∞) pathway the same process occurring in 'interlayer' collisions, since IND(1∞) methyls were only observed at~2 ML coverage. This methyl-exchange reaction has been invoked previously to explain a similar inelastic pathway observed for photorecoiling CH 3 coming from CH 3 Br on LiF(001), NaCl(001) and MgO(001). The IND(2) pathway was characterized by a broader P(E T ∞) than the other three pathways, and a broad P(h∞) (cosn h∞, n=1.7-3.8), peaking at the surface normal. These attributes of P(h∞) for IND(2) are characteristic of strongly inelastic encounters in which memory of the initial BrYC bond direction is lost.
Local surface structure probed by photodissociation in the system: quantal and classical simulations
Surface Science, 1995
The photodissociation of HC1 on MgO(001 ) is studied by classical and quantum methods. The quantum aspect resulting from the hydrogen zero-point motion is also modeled in the classical simulation and has an important influence on the dynamics through the initial distribution of tilt angles from the surface normal. The angular distribution of the scattered photofragments and the kinetic energy release of hydrogen show characteristic structures due to rainbows, scattering shadow and resonances. Information about surface potential and adsorbate geometry can be obtained from them.
The Journal of chemical …, 2003
A sub-monolayer of atomic sodium was deposited on LiF͑001͒ at 40-90 K. The adsorbed sodium atoms and clusters were dosed with HBr, to form HBr¯Na n /LiF(001) (nϭ1,2,...) complexes which were then irradiated by 610 nm laser-light to induce charge-transfer reactions. The reaction-product atomic H(g) was observed leaving the surface, by two-color Rydberg-atom TOF spectroscopy. The H-atom translational energy in its ''fast'' ͑0.9 eV͒ component exhibited structure ͑40Ϯ10 meV spacing͒ attributed to vibration of the NaBr residue at the surface following photoinduced reaction in Na...HBr. The cross-section of the harpooning event was obtained as 7.5ϫ10 Ϫ19 cm 2 for the ''fast'' H-atom reaction-product. Investigation of the coverage and temperature dependencies of the H-atom signal and of temperature programmed desorption ͑TPD͒ and x-ray photoelectron spectra gave an activation barrier for surface diffusion of Na-atoms E di f f Ͻ170 meV. High-level ab initio calculations were employed to interpret the TOF and TPD spectra.
Photoreactions of small molecules at the surface of alkali metal halides
Catalysis Today, 2000
The photostimulated reactions of small molecules, i.e., O 2 photoadsorption and photodesorption, H 2 and CO photooxidation, and CO 2 photodecomposition on alkali halides (alkali halides) surface are discussed in this work. Most of 19 alkali halides show the activity in the reactions mentioned above. Three spectral regions of alkali halides stimulation by light are distinguished in relation to kinetics of oxygen photoadsorption and reactivity of adsorbed oxygen species. They are in correspondence with: (i) bulk exciton absorption bands; (ii) extrinsic and intrinsic bulk and surface absorption bands and (iii) color centers absorption bands. In the latter case the activity of alkali halides arises after pre-excitation in exciton bands, which leads to sensitization of alkali halides to visible light. The H 2 and CO photooxidation on alkali halides is two-step photoprocess involving some excited states of adsorbed oxygen. The latter is quenching by gaseous oxygen as it is shown for hydrogen oxidation over KBr. The photodecomposition of carbon dioxide is treated as CO 2 dissociative adsorption with CO evolution into the gas phase.
Molecular dynamics simulation of the photodissociation of adsorbed HCl on a MgO (001) surface
The Journal of Chemical Physics, 1995
The photodissociation of HCl/MgO ͑001͒ is studied by classical molecular dynamics of a single adsorbate system including the substrate phonon modes. An important quantum effect is accounted for by taking the hydrogen coordinates and momenta in the initial state from a vibrational ground state wave function. In the angular distribution of the scattered photofragments characteristic structures due to rainbows, scattering shadow and resonances are found, that are already well described within the rigid surface approximation. The hydrogen kinetic energy release also shows a pronounced peak structure corresponding to different energy transfer mechanisms and is significantly affected by inclusion of energy transfer to the phonon modes. Due to multiple collisions with the surface and the chlorine, the hydrogen can lose more than 3.5 eV of its 4.7 eV excess energy. The angular resolved energy spectrum is explained by several types of trajectories connected with the above mechanisms. The results suggest further that the different mechanisms can be separated in an experiment.
Photodissociation of HBr adsorbed on the surface and embedded in large Arn clusters
The Journal of Chemical Physics, 2000
Ultraviolet (UV) photodissociation experiments are carried out for Arn(HBr) clusters in which the HBr is adsorbed on the surface of the Arn, and also on isomers of these systems in which HBr is embedded within the rare-gas cluster. The mean size of the cluster distribution in the experiments is around n̄=130. The kinetic energy distribution (KED) of the hydrogen atoms that left the clusters is measured. Molecular dynamics (MD) simulations of the photodissociation of the chemically similar clusters Arn(HCl) are used to provide a qualitative interpretation of the experimental results. The clusters with embedded HBr give a very cold H-atom KED. The clusters with the surface-adsorbed HBr give a KED with two peaks, one corresponding to very low energy H atoms and the other pertaining to high energies, of the order of 1.35 eV. The theoretical simulations show that already for n=54, there is a strong cage effect for the “embedded” molecule case, resulting in slow H atoms. The surface-adsor...