Generalized master equation for photodesorption by resonant IR laser-adsorbate vibrational coupling

Abstract

The validity of the master equation approach to kinetics of photodesorption of physisorbed molecules by resonant laser-molecular vibrational coupling is examined. Whereas use of the master equation in its standard form is usually justified for thermal desorption, in case of photodesorption, a more general approach taking into account slow dephasing, is necessary. The appropriate set of linear differential rate equations with constant coefficients is derived from first principles and conditions of its applicability are formulated. In case of fast dephasing of internal molecular vibrations, the standard master equation is recovered. In a more likely and opposite case, another set, called generalized master equation, is proposed. It is pointed out that the generalized master equation is a practical tool for numerical studies of the photodesorption process free of ambiguities associated with a proper choice of the laser line-width inherent in recent theoretical studies.