VIBRATIONAL-RELAXATION ON METAL-SURFACES - MOLECULAR-ORBITAL THEORY AND APPLICATION TO CO/CU(100)

A nonempirical theory of vibrational relaxation at metal surfaces via nonadiabatic coupling to conduction electrons is presented. Using a single determinant Hartree-Fock (HF) description of the electronic states of the system, an expression for the lifetime of an excited vibration is obtained. Under certain additional assumptions, all the quantities necessary to calculate the lifetime can be obtained from the results of ab initio HF calculations on cluster models of the adsorbate-metal system. As a practical test of this procedure, the lifetime of the excited upsilon = 1 vibrational state of CO on Cu(100) is calculated using clusters of 6, 10, and 14 copper atoms. Results ranging between 1.1 and 3.5 ps are obtained, with our preferred procedure yielding 1.7 ps for the largest cluster, in good agreement with experiment. Extensions of this approach may also be valuable for treating other nonadiabatic phenomena at metal surfaces.