Semiclassical vibration–rotation spectra of gaseous and physisorbed molecules

A semiclassical spectral intensity method is applied to the calculation of vibration–rotation spectra both of isolated molecules and of molecules physically adsorbed on a solid surface. For the case of an isolated HCl molecule, we are able to generate discrete vibration–rotation spectral lines, the frequencies and integrated intensities of which agree well with the available literature values. Line shapes obtained for this case exhibit no evidence of broadening beyond the theoretical resolution of the calculation. Physisorption of the HCl on an Ar(111) surface leads, however, to a collapse of the R, P band structure at low rotational energies, with the free rotor dynamical limit being reached only at higher energies. The transition between dynamical regimes is associated with substantial line broadening and shifts in the line centers as well as with an increase in the desorption rate. A loss of rotational phase coherence appears to represent the principle line broadening mechanism in the present calculations.