APPLICATION OF A BOSON HUBBARD-MODEL TO VIBRATIONAL DYNAMICS IN ADSORBATE LAYERS

I study the nature of the fundamental (n = 0 --> 1) and "hot-band" (1 --> 2) optical-absorption spectra of vibrational modes in adsorbed layers, as probed by weak, time-delayed, ir pulses after the system has been pumped by a strong ir pulse. The discussion is based on a boson Hubbard model and the theoretical results compare favorably with experimental studies (transient ir spectroscopy) of vibrational dynamics in adsorbed layers. In particular, the observation by Guyot-Sionnest of a large broadening of the hot-band (1 --> 2) Si-H stretch vibration in the H/Si(111) 1 X 1 adsorbate system is reproduced by the theory. The theory gives a qualitative insight into how this broadening occurs and how other characteristic features of the absorption spectra result.