Photodissociation of the vinoxy radical through conical, and avoided, intersections

The mechanism for predissociation of the vinoxy radical after the excitation (X) over tilde (2)A"-->(B) over tilde (2)A" is studied theoretically. Critical points on the (A) over tilde (2)A(') and (B) over tilde (2)A" potential energy surfaces are determined at the multireference configuration interaction level using expansions of five million configuration state functions. A mechanism consisting of a sequence of internal conversions to the ground state is proposed. (B) over tilde (2)A'"-(A) over tilde (2)A(') conical intersections are shown to be too high in energy to participate in this process. The (B) over tilde (2)A"-(A) over tilde (2)A(') radiationless transition is enabled by an avoided crossing which is accessible after a barrier of ca. 2000 cm(-1), in agreement with experimental observations. Subsequently the (A) over tilde (2)A'-->(X) over tilde (2)A" radiationless transition can occur very efficiently through easily accessible conical intersections. A one-dimensional coupled adiabatic state model of the process is developed. The radiationless decay rates of the vibrational levels obtained from this model serve to validate the proposed mechanism. (C) 2002 American Institute of Physics.