The photodissociation of CH3SCH3 and CD3SCD3 at 220-231 nm investigated by velocity map ion imaging

The photodissociation of the two isotopomers of dimethyl sulfide, CH3SCH3 and CD3SCD3, through the first electronic absorption band at wavelengths 220-231 nm has been studied employing velocity map ion imaging to detect the methyl products. Translational energy and recoil angle distributions have been determined for the CH3 and CD3 fragments either in the vibrational ground state or without product state selection. The measurements indicate that the S-C bond photolysis yields strongly anisotropic product scattering distributions characterized by beta parameters within the range (-0.4, -1.0). The beta values closest to the limiting value of -1.0 (corresponding to a dipole perpendicular transition) are observed for the methyl fragments produced in the vibrational ground state. The product recoil energy distributions are centered at approximate to 1.5-1.7 eV (i.e., 65%-75% of the available energy) and are quite narrow (full width at half maximum, FWHM approximate to 0.3-0.5 eV), which indicates that methyl and methylthio fragments are born internally cold but with substantial translational excitation. The recoil energy distribution is practically independent of the photolysis wavelength within the interval studied, indicating that changes in the photon energy correlate primarily with the rovibrational excitation of the CH3S (or CD3S) fragment whose internal dynamics appear to be more active in the dissociation process than that of the CH3 (or CD3) partner. (C) 2001 American Institute of Physics.