Vibrational energy transfer map for OCS

Abstract

Fluorescence has been detected from the OCS ν1, ν2, ν3, 4ν2, and 2ν2+ν1 levels following initial excitation of the OCS bending overtone, 2ν2, with a pulsed CO2 laser. Analysis of each time-dependent fluorescence yields multiple exponentials which are consistent with a rather simple energy transfer path. The excess population placed in 2ν2 by the laser rapidly equilibrates through ladder climbing processes with the rest of the ν2 bending manifold at a near gas kinetic rate. Subsequently, the 4ν2 level couples to the asymmetric stretch, ν3, with a rate constant k4ν2→ν3=12±2 msec−1 Torr−1 (680 gas kinetic collisions); 2ν2 then fills the symmetric stretch ν1 with a rate constant k2ν2→ν1=3.3±0.5 msec−1 Torr−1 (2400 gas kinetic collisions). This intermode vibrational energy transfer process is followed closely by overall relaxation of the vibrational energy into rotations and translations with a rate constant kVT=2.3±0.5 msec−1 Torr−1 (3400 gas kinetic collisions). Vibrational relaxation from ν2 to ν1 may also occur but the maximum rate constant for this process is kν2→ν1?0.3 msec−1 Torr−1(?24 000 gas kinetic collisions). Theoretical probabilities derived from a short range repulsive force model are in reasonable agreement with experimentally derived probabilities when proper anharmonically mixed states are used in the transition matrix elements. A model calculation of the probability for the 4ν2→ν2 coupling based on long range attractive forces is about 3 times the measured value. The vibrational energy transfer mechanism and rates for OCS are compared with those for three other triatomics, CO2, N2O, and SO2, which have been extensively investigated. In all of these molecules, the vibrational relaxation of the lower states is consistent with a few, simple ’’propensity rules.’’