Mixed quantum/classical simulation of the photolysis of HCl on MgO(001)

Abstract

Quantum and mixed quantum/classical calculations of the photolysis of a HCl adsorbate on a MgO surface are reported. In the quantum calculation of the hydrogen dynamics ͑with rigid surface and chlorine atoms͒ a strong oscillatory structure is found in the angular distribution of the photofragmented hydrogen as well as in the absorption spectrum. These resonances are caused by temporary trapping of the hydrogen atom between the chlorine atom and the surface and reflect the initial perpendicular adsorption geometry. Corrugation of the surface potential leads to a significant modification of these interference patterns, which exist even for a flat surface. Within a mixed quantum/classical time-dependent self-consistent field ͑Q/C TDSCF͒ propagation the influence of surface degrees of freedom on the interference patterns is investigated. The thermal motion of the surface and inelastic collisions of the hydrogen atom with the surface and the chlorine atom washes out most of the oscillatory structure. In the fully angular and energy resolved spectra nevertheless clearly distinguishable peaks are seen. They can be used in practice to extract information about adsorption geometry and surface potential parameters.

Classical and quantum studies of the photodissociation of a HX (X=Cl,F) molecule adsorbed on ice

“…16,43 For HF, the wave-packet dynamics appears more complicated mainly because of the initial orientation of the admolecule that is nearly perpendicular to the surface and allows the photofragment to explore more directions than in the case of HCl. Only the H motion has been solved, the halogen atom and the ice surface degrees of freedom being frozen.…”

“…The interpretation of these trajectories is related to the classical angular distribution ͑Fig. 16,43 A part of the wave packet is temporary trapped in a well along the Cl-O direction at the beginning of the propagation leading to an energy quantization. The classical angular distribution exhibits three intense peaks ͑34°, 65°, and 75°͒ ͑and a very small one: 76°͒ which can all be attributed to rainbow effects.…”

Photodissociation of a HCl molecule adsorbed on ice at T=210K

“…This would yield information on the energy transfer, kinetic and angular distributions of the desorbed H atoms. For ionic surfaces, classical simulations have shown that the energy loss of the hydrogen due to the collisions with the surface and with the chlorine are about 15% and 5%, respectively, [23]. Furthermore, energy dissipation through intramolecular vibrational modes and reactive interactions of H with H 2 O molecules should also be taken into account for a better modeling of the process by coupling time dependent quantum methods with quantum chemistry approaches.…”

“…There is a variety of approximate theories which include semiclassical calculations, reduced dimensional quantum methods, single or multiple configuration time dependent self consistent field (TDSCF) or mixed quantum/classical methods. Such methods have been applied to the case of a single molecule adsorbed on an ionic surface [21][22][23][24][25][26][27][28]. Exact or approximative quantum dynamics calculations have been performed in reduced dimensionality within the approximation of a perfect rigid surface [24][25][26]28].…”

“…Reported mixed quantum/classical time dependent studies do not account for the quantum properties of the substrate vibrations and are mainly related with low temperature processes, i.e. with limited adsorbate-substrate energy transfer [23].…”