VIBRATIONAL-ENERGY DISTRIBUTIONS THROUGH KINETIC-ANALYSIS - EARLY COLLISIONAL RELAXATION OF T-1 PYRAZINE

A new kinetics-based method is described for determining the distribut ion of vibrational energy contents in a highly excited, precollisional polyatomic sample. The method can be applied to excited species whose population decay constant depends on vibrational energy. In such case s a sample's decay kinetics will show a distribution of rate constants corresponding to the underlying initial distribution of vibrational e nergies. This method is illustrated using low-pressure pyrazine sample s whose total T-1 population is measured by triplet-triplet transient absorption and kinetically analyzed to obtain a decay constant distrib ution modeled as a sum of Gaussian peaks. The result is then transform ed into a vibrational energy distribution using an independently deter mined calibration curve. The sample's relatively narrow nascent energy distribution appears to evolve into a bimodal form through collisions with helium atoms. It is estimated that approximately 0.7% (on averag e) of the gas kinetic encounters between an excited triplet pyrazine m olecule and a helium atom lead to vibrational deactivation of ca. 2000 cm(-1).