LIBRATIONAL BROADENING OF MOLECULAR-TRANSITIONS IN LIQUIDS - THE 5TH C-H STRETCHING LOCAL MODE OVERTONE IN BENZENE

Franck-Condon progressions in the librational subspace are explored as a possible source of broadening of molecular transitions in the liquid phase. The band shape of the fifth overtone of the benzene CH stretching vibration, as seen in liquid phase thermal lensing spectroscopy, is rationalized from this point of view. The librational subspace is modeled by a one-dimensional symmetric potential which is taken to be perturbed by the molecular transition. Franck-Condon factors are computed for this subspace in which thermalization plays a significant role. Band shapes are computed for cases in which the individual librational transitions carry homogeneous line widths of 4 cm-1 and 90 cm-1, the latter corresponding to the reported homogeneous broadening of the fifth overtone transition in the gas phase. The librational model predicts a strong dependence on temperature of the band shape as shown by simulations at 1 K and 300 K. © 1979.