Abstract

Recent attempts to clearly demarcate selective and nonselective effects in the IR-laser-stimulated vibrational excitation of an atom or molecule adsorbed on a solid surface¹ have included a number of model studies² and a few that have examined the detailed nature of the individual quantum states of the adsorption potential.^3,4 The presence of at least four time scales in such dynamical studies (experiment, laser, phonon, adbond) leads to difficulties in applying approximations such as the Markovian approximation (MA) which neglects memory effects in the generalized master equation (GME) describing the excitation. Use of the MA leads to the Pauli master equation (PME) which is easier to solve than the exact GME, and is therefore almost universally employed, but to our knowledge no realistic comparison has been made of the GME and the PME. We present below our results of the exact GME and the PME and find that for the few cases considered the two are drastically different in an (arbitrary) time range related to a time parameter τ necessary to force the MA in the first place. In the asymptotic (long time) limit the solutions can be made to converge. We also present the results of an approximation which gives results (for any time range) which are almost as good as those of the exact GME, but which are as easy to obtain as those of the PME.

© 1985 Optical Society of America