Photodissociation processes in the CH molecule

Abstract

The photodissociation processes in the CH molecule have been investigated using ab initio self-consistent-field with configuration-interaction (CI) methods. Potential energy curves of all states which dissociate into the lowest four atomic limits have been calculated, as well as those of several higher-lying states with Rydberg character. Transition dipole moments connecting the excited states with the lowest states have been obtained. Direct photodissociation of CH may occur by absorption into the continua of the B 2∑− and 2 2∑+ states, and cross sections for absorption from the X 2Π v″=0 state into these states are reported. Indirect photodissociation of CH may take place by absorption into bound states, which are subsequently dissociated through interactions with continuum states. The A 2Δ vibrational levels couple with the continuum of the X 2Π state through the nuclear rotational operator. The nuclear rotational coupling elements have been obtained from the CI wave functions, and predissociation rates are presented. The C 2∑+ v′=0 state is shown to be predissociated efficiently by spin-orbit coupling with the B 2∑− state. The 2, 3, and 4 2Π states of CH interact strongly through the radial nuclear kinetic energy operator. The nuclear radial coupling elements between these states have been computed by numerical differentiation of the CI wave functions. The coupled equations for the nuclear wave functions have been solved in a diabatic formulation. The calculated cross section for absorption into the coupled states is similar to that found recently for absorption into the 2 2Π and 3 2Π states of the OH molecule: it consists of a series of resonances with asymmetric Beutler–Fano profiles superimposed on a strong continuous background. The 3(F)2∑+ state, as well as the higher-lying (Rydberg) states, may interact with the 2Π states through rotational coupling or with the 2 4∑− state through spin-orbit coupling. Due to uncertainties in the coupling elements, the efficiencies with which these states are dissociated are not well determined, but they are probably small for most states. The photodissociation rate of CH by the unattenuated interstellar radiation field has been computed. The total rate is 9.5×10−10 s−1. The 2 2∑+ and coupled 2Π channels are most effective in the photodissociation of interstellar CH.