Librational motion of CO in solid Ar: Raman and IR spectra and quantum simulations

Rovibrational Raman spectra of CO molecules isolated in solid Ar are measured at temperatures of 9–30 K and compared to past and present IR spectra. The fundamental band appears as a triplet-split structure, while the center peak has completely different IR and Raman responses to temperature. The Raman peak is sharp and stable but broadens reversibly beyond recognition in the IR upon annealing. The red-shifted, intense line of the triplet is thermally inert in both spectroscopies. The third line is the weakest, and since it is concentration dependent, it is ascribed to a dimer, as before. The CO-H2O impurity complex is identified as a side band. We employ crystal field and quantum chemical modeling to interpret the disparity between the spectroscopies. The stable and broadened lines are assigned to double- and single-substitution sites, respectively. Thermal excitation is not effective in the former case of an angularly tight-confined, deep potential well. In the single-substitution case, the librational ...