Photodissociation of CH2. V. Three-dimensional adiabatic potential energy surfaces and transition dipole moments

Abstract

Full three-dimensional adiabatic potential energy surfaces are presented for the lowest five 3A″ and five 3A′ states of CH2. Both the 1 3A′ and 2 3A″ states are dissociative with respect to the C–H coordinates, consistent with our earlier two-dimensional results. All higher lying states are found to be bound for this coordinate, although the barrier toward dissociation is small for some states. In terms of angle dependence, the 1 3A′ state shows a flat behavior, but tends towards larger angles as dissociation proceeds. Most excited 3A′ states are somewhat bent with only a small barrier to linearity. Transition dipole moments connecting the ground state with the excited triplet states are presented as well. The 1 3A′ state is the only state of that symmetry with a large transition dipole moment in the Franck–Condon region. Other 3A′ states exhibit large values only if one bond is stretched compared with the ground state equilibrium geometry. The 1 3A″, 3 3A″, and 4 3A″ states are also slightly bent with a small barrier to linearity. However, the 2 3A″ state has an absolute minimum at very small angles (less than 60°), and shows a considerable local minimum (∼1.5 eV) for the linear configuration. The 5 3A″ state prefers the linear shape. The 3 3A″ state has the largest transition dipole moment function in the Franck–Condon region, but the transition moments to other 3A″ states can exhibit large values outside this region. The 2 3A″ and 3 3A″ states undergo an avoided crossing in the Franck–Condon region, so that a coupled states treatment is necessary for a correct description of the photodissociation dynamics. In order to provide the corresponding transition dipole moments in an appropriate form, a transformation to the principal axes of inertia was performed. The adopted transformations are discussed in detail.