Isotope effect in vibration-mediated ultraviolet photodesorption: A short-time-limit harmonic approximation

Excitation and de-excitation of the electronic states of an adsorbed molecule on a solid surface result in intramolecular vibrational excitation. The internal vibrational energy can couple to the adsorbate–substrate bond and lead to molecular desorption. In this mechanism, the isotope effect in molecular desorption is determined not by the mass ratio of the leaving particles, but by that in the internal coordinate. This process is modeled within the context of semiclassical wave packet dynamics, which, in the short-time-limit harmonic approximation, allows analytical solution. Results are obtained for the ammonia/GaAs(100) system and are compared with experimental observations.