The photodissociation channels of nitrosobenzene (PhNO) induced by a 255 nm
photolytic wavelength have been studied with the complete active space self-consistent
(CASSCF) method and the multistate second-order multiconfigurational perturbation
theory (MS-CASPT2). It is found that there exists a triplet route for photodissociation of
the molecule. The reaction mechanism consists on a complex cascade of nonadiabatic
electronic transitions involving triple and double conical intersections as well as
intersystem crossing. Several of the relevant states (S2, S4, and S5 states) correspond to
double excitations. It is worthy to note that the last step of the photodissociation implies
an internal conversion process. The experimentally observed velocity pattern of the NO
fragment is a signature of such a conical intersection.
