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 d euterated isotopomer (pyrazine-d(4)) and methyl derivative (2-methylpyrazin e). The lowest triplet states of these compounds were prepared with 5596 an d 5149 cm(-1), respectively, of vibrational excitation through pulsed optic al excitation followed by intersystem crossing. Collisional vibrational ene rgy loss was then monitored using the refined competitive radiationless dec ay (CRD) method. The average energy losses per gas kinetic collision were m easured 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 i n T-1 pyrazine-h(4). By contrast, methylation significantly increases the a verage energy lost per collision and also seems to lower the energy thresho ld for rapid relaxation. It is deduced that vibronic coupling between T-1 a nd nearby triplet states is largely responsible for the high susceptibility to collisional energy loss. Many other molecules with closely spaced excit ed states may be expected to show collisional vibrational relaxation that i s significantly enhanced by electronic excitation. (C) 1999 American Instit ute of Physics. [S0021-9606(99)00111-7].