Photodissociation of CH2. III. Two-dimensional dynamics of the dissociation of CH2, CD2, and CHD through the first excited triplet state.

We present quantitative results on photodissociation of CH2 (X 3B1) and its isotopomers CHD and CD2 through the first excited triplet state (1 3A1). A two-dimensional wave packet method employing the light–heavy–light approximation was used to perform the dynamics. The potential energy surfaces and the transition dipole moment function used were all taken from ab initio calculations. The peak positions in the calculated CH2 and CD2 spectra nearly coincide with the positions of unassigned peaks in experimental CH2 and CD2 3+1 resonance enhanced multiphoton ionization spectra, provided that the experimental peaks are interpreted as two-photon transitions. Comparing the photodissociation of CH2 and its isotopomers to photodissociation of water in the first absorption band, we find these processes to be very similar in all aspects discussed in this work. These aspects include the origin of the diffuse structure and the overall shape of the total absorption spectra of vibrationless and vibrationally excited CH2 , trends seen in the fragment vibrational level distribution of the different isotopomers, and selectivity of photodissociation of both vibrationless and vibrationally excited CHD. In particular, we find that the CD/CH branching ratio exceeds two for all wavelengths in photodissociation of vibrationless CHD. [ABSTRACT FROM AUTHOR]