ELECTRONIC DAMPING OF MOLECULAR VIBRATIONS AT METAL-SURFACES - SUBPICOSECOND TRANSIENT PUMP-PROBE SPECTROSCOPY

A theoretical study is made of the vibrational dynamics of molecules adsorbed on metal surfaces in connection with the sub-picosecond transient pump-probe spectroscopy. The rate equation formalism is used in order to describe the vibrational dynamics in the time domain. The expression for the induced polarization is derived within the rotating-wave approximation. It is shown that in the negative time delay the induced polarization relaxes through the phase and in the positive time delay the induced polarization relaxes through the population within the rate equation formalism. Furthermore, on the assumption that the relaxation mainly proceeds through the coupling of vibrational degrees of freedom to the surface electronic system, the relaxation time involved in the expression for the induced polarization is estimated from a microscopic standpoint. It is shown that the expression for the phase relaxation time is the same as that for the population relaxation time within the Markov approximation.