Photodissociation of CO−3: Product kinetic energy measurements as a probe of excited state potential surfaces and dissociation dynamics

Abstract

The photodissociation process CO−3 +hν→O−+CO2 has been investigated at photon energies of 2.41, 2.50, 2.54, 2.60, and 2.71 eV. Experiments were conducted by crossing a mass-selected, 8 keV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and laser polarization, angular distributions were obtained at photon energies of 2.41 and 2.54 eV. The photon energy dependence of the average photofragment kinetic energies shows conclusively that photodissociation at these photon energies does not proceed by a direct dissociation process on a repulsive potential surface, or by a statistical vibrational predissociation process on a bound surface. The photofragment angular distributions are isotropic, providing further evidence that precludes direct photodissociation on a repulsive potential surface. Ab initio calculations were performed using the gaussian86 programs. These calculations indicate that ground state CO−3 has a planar D3h geometry, and 2A′2 electronic symmetry. This ground state correlates adiabatically to the CO−2 +O dissociation asymptote, not the lower energy O−+CO2 asymptote. Taken together, these new experimental and theoretical results suggest that the photodissociation of CO−3 at these energies occurs via the interaction of bound and repulsive excited state potential surfaces. A new model of the potential surfaces of CO−3 is proposed.