Dynamics of the two-step photodissociation of azomethane

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Abstract
Time-resolved coherent anti-Stokes Raman spectroscopy (CARS) measurements have revealed aspects of energy disposal in the 355 nm photodissociation of gas phase azomethane. Interpretation of these results is aided by earlier experiments that kinetically resolved the two steps of dissociation leading to two methyl radicals and a nitrogen molecule. Methyl radicals were observed with zero to four quanta of vibrational excitation in the ν2 out-of-plane deformation mode. Kinetic analysis showed the first-step methyl radicals to carry far more ν2 excitation than the second-step methyl radicals. Through simulation of band contours, a rotational temperature was estimated for the vibrationally unexcited second-step methyls. In addition, nascent vibrational and rotational populations were determined for the nitrogen photoproduct, which is formed in the second dissociative step. These experimental findings are compared to the results of impulsive and statistical models of energy partitioning in this system. It appears that the first step may have significant impulsive character. In the second step, some experimental findings are predicted accurately by the separate statistical ensemble (SSE) model, while other findings fall outside the range spanned by the impulsive and SSE predictions. It is suggested that the second-step energy distributions may reflect the effects of specific exit channel interactions superimposed on statistical partitioning of available energy. Quantum chemical computations on the second step’s reaction path (leading from the methyldiazenyl radical intermediate to the methyl radical and nitrogen photoproducts) should permit deeper understanding of the dissociation dynamics.