Vibrational energy relaxation and exchange in liquid N2–CO–OCS mixtures

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Abstract
The laser induced fluorescence technique has been applied to the vibrational dynamics of liquid N2 doped with small (∼1–100 ppm) quantities of CO and OCS. For N2–CO mixtures, the vibrational energy is shown to equilibrate among the CO and N2 molecules on a time scale rapid compared to vibrational decay processes. These decay processes are dominated by radiative effects for both N2 (radiative lifetime 60 s and CO (radiative lifetime 0.023 s). OCS(00°1) vibrational decay is much more rapid (∼2±0.5 ns) and has been measured by infrared saturation techniques. In N2–CO–OCS mixtures, the OCS acts as a rapid deactivation center for vibrational energy and the rate limiting processes are the vibrational exchange rates between the initially excited N2 and the CO and OCS dopants. The measured rate constants are KN2*,CO*=2.7±0.6×10−15 cm3 s−1, KN2*,OCS*=1.0±0.2×10−15 cm3 s−1, and KCO*,OCS*=4.6±0.8×10−12 cm3 s−1, where KX*,Y* refers to vibrational transfer from X to Y. The N2*→CO* rate constant is identical with previous gas phase measurements and is shown to be inconsistent with a simple isolated binary collision model of a liquid.