Vibrational relaxation in crystal SO2

The vibrational relaxation of some internal and external phonons in crystal SO2 as a function of temperature is reported and discussed in terms of depopulation and/or dephasing decay mechanisms. By picosecond CARS spectroscopy the lifetime of the polar omega-1 (A1) mode (symmetric stretching) was studied in the temperature range 35-170 K. High-resolution Raman spectroscopy was used to measure the linewidth of the second Davydov component, omega-1 (A2), and of lattice modes at 76 and 102 cm-1 in the same temperature range. The experimental data are analyzed on the basis of current theories on vibrational decay in molecular crystals. Using a recently proposed intermolecular potential for crystal SO2 including atom-atom and electrostatic interactions, the contributions to linewidth from third-order depopulation processes were calculated. In the case of lattice phonons, the calculated bandwidths are in good agreement with experimental results at low temperature. At higher temperature fourth-order decay mechanisms become efficient. As to the internal phonons, calculations show that depopulation has negligible importance for decay. Higher-order contributions to linewidth must be considered in this case. We have estimated the four-phonon intraband scattering and we have found that this mechanism is mostly responsible for the observed behaviour with temperature. It is suggested that fourth-order intraband processes may be important also for the decay of other well isolated phonons in molecular crystals.