Quantum nonadiabatic effects in the photodissociation of vibrationally excited CH₃I

Photodissociation of vibrationally excited CH₃I is studied using a time-dependent quantum mechanical formalism based on the fast Fourier transform (FFT) method. The dissociation dynamics is modeled with two active degrees of freedom, i.e., the dissociation coordinate and the C-H₃ umbrella coordinate. The ground state vibrational wave functions are calculated using a time-dependent relaxation method proposed by Kosloff and Tal-Ezer. Two coupled excited states are explicitly considered in this model and the potential energy functions are taken from a previous study that was able to reproduce experiments for photodissociation of the CH₃I ground state. We investigate the dissociation dynamics of the system after initial vibrational excitation, with particular attention paid to nonadiabatic transitions during the dissociation process. Our calculations show that vibrational excitation can significantly change the product I*/I branching ratio. In particular, it is found that there are significant dips in the I* yield at energies associated with minima in the absorption spectrum. These dips can be attributed to differences in Franck-Condon factors associated with the two excited state potential surfaces. Other observables of the dissociation process, such as the absorption spectrum and fragment vibrational state distributions, have also been investigated.