Ab initio configuration interaction study of the B- and C-band photodissociation of methyl iodide

Multireference spin-orbit configuration interaction calculations have been carried out for the valence and low-lying Rydberg states of CH3I. Potential energy surfaces along the C-I dissociation coordinate (minimal energy paths with respect to the umbrella angle) have been obtained as well as transition moments for excitation of the Rydberg states. It is shown that the B and C absorption bands of CH3I are dominated by the perpendicular R-3(1), R-1 (E) <- (X) over tilde A(1) transitions, while the R-3(2)(E), R-3(0+) (A(1)) <- (X) over tilde A(1) transitions are very weak. It is demonstrated that the bound Rydberg states of the B and C bands are predissociated due to the interaction with the repulsive E and A(2) components of the (3)A(1) state, with the (3)A(1)(E) state being the main decay channel. It is predicted that the only possibility to obtain the I(P-2(3/2)) ground state atoms from the CH3I photodissociation in the B band is by interaction of the R-3(1)(E) state with the repulsive (1)Q(E) valence state at excitation energies above 55 000 cm(-1). The calculated ab initio data are used to analyze the influence of the Rydberg state vibrational excitation on the decay process. It is shown that, in contrast to intuition, excitation of the nu(3) C-I stretching mode supresses the predissociation, whereas the nu(6) rocking vibration enhances the predissociation rate. (C) 2011 American Institute of Physics. [doi:10.1063/1.3532926]