Multiple surface intersections and strong nonadiabatic coupling effects between the (D)over-tilde2E1u and (E)over-tilde2B2u states of C6H6+

The multimode vibronic interactions between the (D) over tilde(2)E(1u) and (E) over tilde(2)B(2u) excited states of the benzene radical cation are investigated theoretically. In first order of the nuclear displacements the four Jahn-Teller active (e(2g)) vibrational modes can couple the two electronic states. Ab initio Green's function calculations including electron correlation effects have been carried out to determine the pertinent inter-state coupling constants as well as the Jahn-Teller coupling constants for the (D) over tilde(2)E(1u) state. After a slight readjustment of some of the parameters the diffuse shape of the corresponding photoelectron bands is well reproduced by theory. Ensuing wave-packet dynamical calculations reveal a (E) over tilde --> (D) over tilde electronic population decay (internal conversion process) on a timescale of several fs. The importance of this ultrafast population decay for the fragmentation dynamics of electronically excited C6H6+ is pointed out. (C) 1997 Elsevier Science B.V.