ELECTRONIC AND CARS SPECTROSCOPY OF GAS PHASE TRANSIENTS:PHOTODYNAMICS OF AZOMETHANE AND PYRIDAZINE

[摘要] An apparatus has been constructed to perform gas phase excited state spectroscopy and dynamics with nanosecond time resolution. Two different transient spectroscopic techniques were employed to study azomethane and pyridazine. The photodissociation of azomethane into N(,2) and two methyl radicals was studied using the method of transient coherent anti-Stokes Raman scattering (CARS). The previously undetected v = 0 (--->) 1 transitions for the (nu)(,1) (a(,1)) symmetric C-H stretch of methyl radicals in their ('2)A(,2)'' (D(,3h)) electronic ground state was observed and the following spectroscopic constants were obtained for this mode: the vibrational origin frequency (nu)(,0) = 3004.8 cm('-1), B(,1) = 9.493 cm('-1), and C(,1) = 4.694 cm('-1). A spectral examination of N(,2) photofragments indicated that their nascent vibrational distribution following dissociation is 84 (+OR-) 3% in v = 0, 16 (+OR-) 2% in v = 1, and1. Temporal observations were found to show the prompt appearance (<2 ns after excitation) of both methyl radicals and N(,2). On the basis of these observations, a sequential mechanism for azomethane dissociation is suggested. The excited state behavior of pyridazine was studied using transient electronic spectroscopy. A sharp, promptly appearing feature that decays rapidly into a broad, featureless transient was observed at 26048 cm('-1). The prompt feature is assigned to S(,1) (('1)B(,1)) and the broad feature is assigned to vibrationally hot S(,0). Stimulated emission, originating from the S(,1) state, was observed at the 6a(,1)('0) frequency. Based on temporal observations of this feature, the S(,1) lifetime was determined to be 3.1 ns. It was found that the S(,1) state is efficiently quenched by various collision partners. It is suggested that the product of S(,1) quenching is a higher-energy ('1)B(,1) state that undergoes rapid internal conversion (in less than 2 ns) to form hot S(,0). Finally, the estimated photochemical yield for gas phase pyridazine decomposition following 0('0) excitation is 4 (+OR-) 2%. It is thought that pyridazine vapor phase photochemistry occurs through vibrationally hot S(,0).

[发布机构] Rice University

[效力级别] chemistry