A COHERENT TRANSITION MODEL FOR PHOTODESORPTION FROM METALS - NH3 CU/

A new time-dependent quantum mechanical scheme is proposed to study th e uv photodesorption dynamics of ammonia from Cu(111). The desorption is considered here as the result of substrate-mediated electronic exci tation and subsequent de-excitation. The transition from a short-lived excited electronic state to the ground state is described in a cohere nt manner. The dynamics of the system is represented by nuclear wave p ackets on two quasicoupled potential energy surfaces with two degrees of freedom (the desorption and N-H-3 inversion modes). The desorbed mo lecules are found to have significant vibrational excitation and their translational energy distributions are highly structured due to the d ominance of a predesorption mechanism. The desorption yield and the is otope effect are found to depend sensitively on the excited state life time. The results are compared with previous wave packet models and wi th experiments. (C) 1995 American Institute of Physics.