Knowledge Bank

With pulsed or modulated infrared lasers selective excitation of molecular-vibrational energy levels is possible. From spectroscopic study of the time dependence of energy-level populations the kinetics of vibrational energy transfer processes may be measured. Experiments of this type have been carried out on many molecules including $CH4$, $CO2$, HCI, DCI, HBr and $SF6$. One example is the laser-excited vibrational fluorescence experiments on HCl and mixtures of HCl with other gases. These experiments give both the rates of vibrational relaxation to translation and rotation and of energy transfer among the different vibrational degrees of freedom in mixtures. An area to which laser experiments have made a considerable contribution is that of nearresonant $V\to V$ energy transfer. Cross sections for $V\to V$ processes and their dependence on temperature, and on net change in vibrational energy have been measured. For processes involving small changes in total vibrational energy the observed cross sections often decrease with increasing temperature. This behavior is consistent with an interaction which occurs at distances for which intermolecular potential energy does not greatly exceed kT. For few cross sections the calculated interaction of transition dipole moments reproduces the experimental observations.