Influence of hot bands on vibrational spectra of shock compressed materials

For polyatomic molecules, extracting vibrational frequency shifts from spec tra recorded under shock loading conditions is complicated by anharmonic co upling of modes and thermal population of low lying vibrational levels, whi ch produce hot bands in the vibrational spectra. Most vibrational frequency shifts with shock pressure reported in the literature have been extracted from inadequately resolved spectral bands fitted with single Gaussians. Thi s paper shows that, in general, for polyatomic molecules, the effect of the hot bands is to push the centroid of the fitted Gaussian away from the fre quency of the vibrational fundamental. The direction and magnitude of this effect requires knowledge of the anharmonic coupling coefficients (which ar e in all likelihood also pressure dependent). To completely characterize th e intra- and intermolecular potentials, it is therefore necessary to use th e highest spectral resolution possible, and to independently measure or cal culate the pressure and temperature dependence of the anharmonic coupling c oefficients for the molecules of interest.