Photodissociation study of CH3Br in the first continuum

The photolysis of CH3Br is studied in the energy region of the A band between 4.94 and 5.76 eV using ion imaging. Velocity distributions for both the bromine-atom and methyl-radical photofragments are determined. Our results indicate that transitions to the 3Q0 and 3Q1 states dominate the absorption cross section and the partial cross section to each state is determined. The [Br*]/[Br] branching ratio is found to be strongly dependent on the excitation energy varying between 0.6 and 1.5. Both the bromine-atom and the methyl-radical translational energy distributions suggest that the vibrational distribution in the nascent CH3 is nonstatistical with appreciable excitation in the v2 umbrella mode. The lifetime of the A band is estimated at τ=120±40 fs.