Vibrational relaxation of T1 pyrazine:: Results from the refined competitive radiationless decay method

Gas phase collisional vibrational relaxation of pyrazine in its lowest triplet electronic state has been further investigated using a refined version of the competitive radiationless decay (CRD) method. Improvements to the experimental apparatus now provide primary kinetic data of much greater accuracy and precision, allowing the use of sample pressures low enough to ensure negligible self-relaxation. A major refinement of the data handling procedure permits the sample's energy-dependent triplet-triplet molar absorptivity to be determined through an iterative analysis. To validate the refined CRD method, average energy loss per collision has been deduced as a function of donor energy under conditions of varying sample pressure, relaxer pressure, excitation beam energy, and excitation wavelength. Consistent results have been obtained for all of these variations, suggesting the absence of major systematic errors. The two main findings of earlier pyrazine triplet relaxation studies are confirmed by the new measurements: the presence of a threshold donor energy above which relaxation becomes much more efficient, and relatively large values of average energy loss per collision. Comparing to results from a recent ground state study [L. A. Miller and J. R. Barker, J. Chem. Phys. 105, 1383 (1996)], it is found that pyrazine containing 5000 cm(-1) of vibrational energy is relaxed by a variety of monatomic, diatomic, and polyatomic gases approximately seven times more efficiently when the donor is in its triplet rather than its ground electronic state. The order of relaxer efficiencies toward triplet pyrazine is found to be He<H-2<Ne<D-2<N-2< Kr<Ar<Xe<CH4<CO<CO2<CH3F<H2O. Energy loss efficiencies correlate rather well with relaxer boiling points. (C) 1998 American Institute of Physics.