The photodissociation of CH3I in the red edge of the A-band: Comparison between slice imaging experiments and multisurface wave packet calculations

The photodissociation of methyl iodide at different wavelengths in the red edge of the A-band (286-333 nm) has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the methyl fragment in the vibrational ground state (nu=0). The kinetic energy distributions (KED) of the produced CH3(nu=0) fragments show a vibrational structure, both in the I(P-2(3/2)) and I-*(P-2(1/2)) channels, due to the contribution to the overall process of initial vibrational excitation in the nu(3)(C-I) mode of the parent CH3I. The structures observed in the KEDs shift toward upper vibrational excited levels of CH3I when the photolysis wavelength is increased. The I(P-2(3/2))/I-*(P-2(1/2)) branching ratios, photofragment anisotropies, and the contribution of vibrational excitation of the parent CH3I are explained in terms of the contribution of the three excited surfaces involved in the photodissociation process, (3)Q(0), (1)Q(1), and (3)Q(1), as well as the probability of nonadiabatic curve crossing (1)Q(1)<-(3)Q(0). The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and nonadiabatic couplings, employing a reduced dimensionality (pseudotriatomic) model. A general qualitative good agreement has been found between theory and experiment, the most important discrepancies being in the I(P-2(3/2))/[I(P-2(3/2))+I-*(P-2(1/2))] branching ratios. Inaccuracies of the available potential energy surfaces are the main reason for the discrepancies.