ROTATIONAL SPECTRUM OF A QUANTUM ROTOR ADSORBED ON A ROUGH AND DISORDERED SURFACE - PARA-H-2 AND ORTHO-H-2 ON AMORPHOUS ICE

Experimental Raman spectra are presented for the J = 0 --> J = 2 transition of para-H-2 and for the J = 1 --> J = 3 transition of ortho-H-2 adsorbed on the amorphous ice surface. Analysis of the spectra is based on a path integral Monte Carlo simulation of the H-2 adsorbate-surface system. Rotational line broadening is due to the removal of the (2J + 1)-fold degeneracy of the J manifolds of the \M] states. The line broadening pattern is traced to the characteristics of the anisotropic gas-surface interaction. The theoretical model, which does not contain any adjustable parameters, provides a simple quantitative interpretation of the spectra.