AB-INITIO POTENTIAL-ENERGY SURFACES FOR C2H-]C-2+H PHOTODISSOCIATION

The C2H photodissociation processes have been studied using ab initio multireference perturbative configuration interaction methods. The calculated structural parameters of the two linear lowest lying states X (2) Sigma(+) and A (2) Pi were found to be in good agreement with previous theoretical and experimental works. Construction of the correlation diagram indicates that C2H cannot photodissociate into C + CH, Therefore, two-dimensional potential energy surfaces of the six lowest lying A' and A'' doublet states have been calculated as functions of the bond angle and the C-H bond distance, keeping the C-C distance at the equilibrium C2H value. It appears that the 1 (2)A', 2 (2)A', and 1 (2)A'' states correlating to the X (2) Sigma(+) and A (2) Pi states in linear geometry remain bound at all angles, whereas the 3 (2)A', 4 (2)A', and 4 (2)A'' are dissociative. However, the structure of the energy surfaces is complicated by the presence of numerous avoided crossings between these states. Transition dipole moments connecting the excited states with the X (2) Sigma(+) and A (2) Pi states have been also obtained in linear geometry. They were calculated to be very weak, leading to the conclusion that the photodissociation rate of C2H will be negligible below 7 eV. Higher lying states, including Rydberg states, were not considered here, since they cannot dissociate directly into C-2 + H.