PHOTODISSOCIATION OF MOLECULES PHYSISORBED ON INERT CRYSTALLINE SURFACES

This paper presents a theoretical treatment within the Franck-Condon and impulse approximations of direct photodissociation of polyatomic molecules sparsely physisorbed on static and UV transparent crystals. A separable local mode model is adopted for the ground state adsorbate and for the initially excited molecule in order to evaluate the multidimensional Franck-Condon excitation and impulse fragmentation amplitudes. Since physisorption forces are weak, the adsorbate excitation and decomposition processes remain intramolecular in nature. The photodissociation of surface physisorbed molecules thus differs from the corresponding gas phase process mainly because of substrate confinement on adsorbate rotation and translation degrees of freedom, which are all described by our theory. One result is that the photodissociation cross section depends on the laboratory frame observation angles. Another is the availability of fragment-substrate rearrangement channels which involve combined dissociation, fragment adsorption, and surface migration excitations. In particular, the excitation may lead to adsorbate desorption and/or migration in addition to adsorbate photofragmentation. Accordingly, the cross section contains six contributions depending on the excitation process and the fragment-substrate rearrangements.