Photodissociation of methane: Exploring potential energy surfaces

The potential energy surface for the first excited singlet state (S-1) of methane is explored using multireference singles and doubles configuration interaction calculations, employing a valence triple zeta basis set. A larger valence quadruple zeta basis is used to calculate the vertical excitation energy and dissociation energies. All stationary points found on the S-1 surface are saddle points and have imaginary frequencies for symmetry-breaking vibrations. By studying several two-dimensional cuts through the potential energy surfaces, it is argued that CH4 in the S-1 state will distort to planar structures. Several conical intersection seams between the ground state surface S-0 and the S-1 surface have been identified at planar geometries. The conical intersections provide electronically nonadiabatic pathways towards products CH3(X (2)A(2)')+H, CH2(a (1)A(1))+H-2, or CH2(X B-3(1))+H+H. The present results thereby make it plausible that the CH3(X (2)A(2)')+H and CH2(a (1)A(1))+H-2 channels are major dissociation channels, as has been observed experimentally. (c) 2006 American Institute of Physics.