On vibrational lineshapes of adsorbed molecules

The possible information content in lineshapes observed in vibrational spectroscopy of molecules adsorbed on surfaces is considered by drawing analogies with similar situations in other spectroscopic areas where the systematics are more completely understood. When possible, simple physical models are introduced which illustrate the points to be stressed. Particular emphasis is placed on the relative roles of T1 (dissipative decay) versus T'2 (pure dephasing) processes in determining linewidths, on the roles of substrate electron-hole pairs, phonons, and photons in T1 processes, and possible ways to establish which broadening mechanism is operative in a given situation. A particular kind of dephasing that is important for vibrational lineshapes in molecular crystals, so called exchange-coupling, is suggested as playing a significant role for molecular adsorbates as well. Some recent experimental studies are analyzed in the light of concepts introduced in this paper. © 1984.