Monte Carlo analysis of T-1 pyrazine collisional vibrational relaxation: Evidence for supercollisions

The collisional loss of vibrational energy from polyatomic molecules in tri plet electronic states has been studied in new detail through a variant of the competitive radiationless decay (CRD) method. Experimental transient ab sorption kinetics for T-1 pyrazine vapor in the presence of helium relaxer reveals the competition between unimolecular radiationless decay and collis ional vibrational relaxation. These data have been simulated with Monte Car lo stochastic calculations equivalent to full master equation solutions tha t model the distribution of donor vibrational energies during relaxation. T he simulations included energy-dependent processes of T-1 --> S-0 radiation less decay, T-n <-- T-1 optical absorption, and collisional energy loss. Th e simulation results confirm earlier findings of energy loss tendencies tha t increase strongly for pyrazine vibrational energies above similar to 2000 cm(-1). It is also found that the experimental data are not accurately sim ulated over a range of relaxer pressures if a simple exponential step-size distribution function is used to model collisional energy changes. Improved simulations are obtained by including an additional, low-probability chann el representing large energy changes. This second channel would represent " supercollisions," which have not previously been recognized in the vibratio nal relaxation of triplet state polyatomics. (C) 2000 American Institute of Physics. [S0021-9606(00)00523-7].