Infrared laser excitation of the internal vibrational mode of a diatomic molecule adsorbed on a metal surface

The infrared-laser excitation of the internal vibrational mode of a diatomic molecule adsorbed on a metal surface is analyzed theoretically. This vibrational energy is damped into the metal by electron-hole excitations. Simple expressions for the populations of the vibrational levels, the mean number of vibrational quanta, and the rate of energy transfer between the infrared laser and the metal surface in the steady state are derived. An equation of evolution can readily be solved numerically to determine the time necessary to reach this steady state. The criteria of applicability of the Markov approximation (which leads to the golden rule) is clearly established, where it is seen that this approximation may not be used to compute the evolution of the populations of the vibrational levels. The random-phase approximation is shown to give the correct kinetic equation for the populations of the vibrational levels. The excitation of carbon monoxide adsorbed on a copper surface is analyzed quantitatively.