Molecular dynamics simulation of the photodissociation of adsorbed HCl on a MgO (001) surface

Abstract

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 a HCl molecule adsorbed on ice at T=210K

“…The first one, which corresponds to the ice and the chlorine atom, can be described classically, and the second one, the hydrogen fragment, has to be treated quantum mechanically. As already mentioned in the introduction, such mixed quantum/classical approaches have been used for example to describe the photodissociation of HCl adsorbed on MgO(0 0 1) at 20 K [22] and the sticking of HCl on ice at 190 K [20]. Applying this method to the case of the ice surface appears more difficult, because the translational and rotational motions of the water molecules concern a larger number of degrees of freedom.…”

“…For example, Hintendener and coworkers have studied the photodissociation of HCl adsorbed on MgO(0 0 1) at a temperature of 20 K [22]. They used different levels of approximations: (i) a classical description of the photofragments and of the surface, (ii) a quantum mechanical treatment for a restricted number of degrees of freedom describing the photofragments motion, (iii) a mixed quantum/classical TDSCF approach.…”

“…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].…”

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

“…For the geometry of the system and the potentials we refer to our earlier work on the classical description of the system. 4 According to the results of a periodic Hartree-Fock calculation with Colle-Salveti correlation corrected energies, 13 HCl adsorbs perpendicular to the surface with the hydrogen atom pointing down to one of the oxygen ions of the surface. The equilibrium distance of the adsorbate is somewhat closer to the surface than it was assumed in Ref.…”

“…The large number of degrees of freedom made the studies focus predominantly on two situations. One is to use classical molecular dynamics, [1][2][3][4] which can be done with relative ease also in systems with a large number of coordinates. On the other hand, quantum calculations for the photofragments must rely on more stringent dynamical approximations like the one of a rigid surface.…”