DYNAMICS OF SURFACE-ALIGNED PHOTOCHEMISTRY .3. A QUANTUM-MECHANICAL STUDY OF THE PHOTODISSOCIATION OF HBR(AD) LIF(001)/

A quantum mechanical probe of surface and adsorbed layer structure is presented based on the photodissociation of ordered hydrogen halide ad sorbates. The photolysis of the adsorbate molecule released atomic hyd rogen which scattered from the surface as well as from neighboring ads orbed molecules. The coherent character of the hydrogenic wave functio n formed in the photodissociation process proved to be a revealing pro be of the surface and of nearby adsorbates by way of localized atomic scattering (LAS). The dynamics of the H-atom motion were modeled by th e time-dependent Schrodinger equation. This study includes two-dimensi onal simulations of the photodissociation of a single HBr molecule ads orbed on LiF(001) as well as photodissociation in full mono-layer and bilayer systems. It was found that for photolysis of an isolated adsor bate molecule, depending on the separation of the adsorbate from the s urface, the angular distribution showed either a single specular peak (small z) or diffraction (larger z), while for higher coverage a serie s of diffraction peaks as well as peaks which corresponded to trapped unstable periodic orbits were observed. The quantization rules of thes e periodic orbits were identified and served as a link between the obs erved angular distribution and the structure of the adsorbed layer(s).