VIBRATIONAL-EXCITATION DYNAMICS IN PHOTODESORPTION

Quantum state specific detection of photodesorbed molecules enables me asuring their final state distributions in the translational and inter nal degrees of freedom, which contain a wealth of information about th e desorption mechanism and dynamics. Vibrational state populations are of particular interest because of the information they contain about the lifetime and nature of the electronic excited states responsible f or desorption. The measured vibrational distributions for nondissociat ive photodesorption of diatomic molecules tend to resemble Boltzmann d istributions with temperatures of 600-1200 K for desorption from metal surfaces, and 1700-2000 K for semiconductors and oxides. Two-dimensio nal quantum dynamics calculations of the desorption process show that these vibrational distributions can be reproduced only if the intramol ecular equilibrium bondlength in the electronic excited state is remar kably similar to that of the ground state. In particular, the results are inconsistent with a desorption mechanism in which the intramolecul ar bondlength change upon excitation is similar to that of electron ca pture in the gas phase. (C) 1997 Elsevier Science B.V.