Time-dependent dynamics of methyl iodide photodissociation in the first continuum

The photodissociation dynamics of methyl iodide (CH₃I) and its deuterated counterpart (CD₃I) in the A band is studied using a time-dependent quantum mechanical method, based on the fast Fourier transform (FFT) method of Kosloff and Kosloff. The calculation uses a pseudotriatomic approximation in which the umbrella mode of CH₃ (CD₃) is approximated by a C-X (X = H₃, D₃) stretch. In addition, the I-C-X dissociation is assumed collinear and angular momentum is ignored. The wave packet is propagated on coupled electronic potential surfaces which we have developed by revising Shapiro's potentials to fit recent experimental data. Good agreement between our calculations and experiment has been obtained for the absorption spectrum, the vibrational distributions for the methyl umbrella mode, and I*/I branching ratios for both CH₃I and CD ₃I at 248 nm. According to our model, the CH₃ fragment in the I* channel at 266 nm is predominantly at its ground vibrational state, while the vibrational distribution in the lower I channel extends to ν = 6 and has a peak at ν = 1.