CUBIC ANHARMONICITY AND MULTIPHONON VIBRATIONAL-RELAXATION OF ADSORBED MOLECULES

For a localized vibration of a linearly bonded adsorbate, with a relative high-frequency displacement coordinate q(a), coupled to a substrate mode with displacement coordinate q(s), we propose a new contribution to the linewidth arising from the cubic terms q(a)2q(s) and q(a)q(s)2 by going to third and higher order in perturbation theory. For third- or higher-order multiphonon processes, this contribution is shown through a specific example to be much more important than the leading-order damping contribution from the term q(a)q(s)n (n > 2), which has been considered by others.