Thermal and photon-induced desorption of NH[sub 3] and ND[sub 3] from Ag{111}

Thermal desorption of NH3 and ND3 from Ag{111} reveals that adsorption is reversible with three desorption peaks at 145, 120, and 104 K. Irradiation of ammonia with 193 nm photons results in the desorption of molecular ammonia from the surface and in subsequent temperature programmed desorption there is no evidence for photodissociation. Below 3 monolayers (MLs) the cross section for photodesorption is less than 1X10(-20) cm(+2) for both isotopes, and above 3 ML the desorption yield increased linearly with coverage. Time-of-flight distributions were recorded as a function of coverage (3-50 ML) and characterized by two channels with average translational temperatures of 3150 and 600 K. The average flux-weighted kinetic energy of both channels is independent of laser pulse energy and the yield increases linearly with laser power, suggesting a nonthermal excitation mechanism. For multilayers, we propose a photodesorption model involving the direct excitation of ammonia. (C) 1997 American Vacuum Society.