Efficient collisional vibrational relaxation of triplet state molecules: Pyrazine deuteration and methylation effects

To further explore the collisional vibrational relaxation of triplet state polyatomic molecules, studies of pyrazine have been extended to its fully deuterated isotopomer (pyrazine-d(4)) and methyl derivative (2-methylpyrazine). The lowest triplet states of these compounds were prepared with 5596 and 5149 cm(-1), respectively, of vibrational excitation through pulsed optical excitation followed by intersystem crossing. Collisional vibrational energy loss was then monitored using the refined competitive radiationless decay (CRD) method. The average energy losses per gas kinetic collision were measured as a function of donor vibrational energy for relaxation by He, Ne, Ar, Kr, Xe, H-2,D-2,CO2,H2O, CH4, and CH3F. Deuteration is found to cause very little change in the efficient energy loss behavior measured earlier in T-1 pyrazine-h(4). By contrast, methylation significantly increases the average energy lost per collision and also seems to lower the energy threshold for rapid relaxation. It is deduced that vibronic coupling between T-1 and nearby triplet states is largely responsible for the high susceptibility to collisional energy loss. Many other molecules with closely spaced excited states may be expected to show collisional vibrational relaxation that is significantly enhanced by electronic excitation. (C) 1999 American Institute of Physics. [S0021-9606(99)00111-7].