MULTIPLE SURFACE INTERSECTIONS AND STRONG NONADIABATIC COUPLING EFFECTS BETWEEN THE (D)OVER-TILDE(2)E(1U) AND (E)OVER-TILDE(2)B(2U) 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 cati on are investigated theoretically. In first order of the nuclear displ acements the four Jahn-Teller active (e(2g)) vibrational modes can cou ple the two electronic states. Ab initio Green's function calculations including electron correlation effects have been carried out to deter mine 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 t he corresponding photoelectron bands is well reproduced by theory. Ens uing 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 popula tion decay for the fragmentation dynamics of electronically excited C6 H6+ is pointed out. (C) 1997 Elsevier Science B.V.