Collisional vibrational relaxation of a triplet state: Energy-dependent energy loss from T1 pyrazine

The loss of vibrational energy from gas phase T1 pyrazine molecules has been measured for thermal collisions with helium, argon, H2, SF6, and ground state pyrazine. Triplet pyrazine was prepared with a well defined vibrational energy of 5433 cm-1 through S1→T1 intersystem crossing following optical excitation to the 8a1 level of S1. The time-dependent vibrational energy content of the excited pyrazine molecules was then deduced using a recently developed ''direct'' method involving the kinetics of subsequent T1→S0 intersystem crossing. For each of the collision partners studied, it was possible to find the average energy lost per gas kinetic collision for donor energies ranging from ca. 2000 to 5433 cm-1. The magnitudes of these energy losses generally increased with the mass and vibrational complexity of the relaxing collision partner. For vibrational energy contents near 5000 cm-1, relaxation of the triplet pyrazine was enhanced by factors of as much as 24 relative to S0 benzene at a similar vibrational energy. In addition, with all collision partners studied the average energy lost per collision showed appparent threshold behavior near 3000 cm-1, increasing by approximately an order of magnitude as the donor's energy increased from 2500 to 5000 cm-1. The findings of this first quantitative study of triplet relaxation suggest that collisional vibrational energy transfer from organic triplet states may proceed by mechanisms different from those that dominate ground state relaxation