Multistate vibronic interactions in the benzene radical cation. II. Quantum dynamical simulations.

The multistate vibronic dynamics in the &Xtilde;²E[sub 1g]-&Etilde;²B[sub 2u] electronic states of the benzene radical cation is investigated theoretically by an ab initio quantum-dynamical approach. The vibronic coupling scheme and the ab initio values of the system parameters are adopted from the previous Paper I. Vibronic line spectra are obtained with the Lanczos procedure. Extensive calculations on wave-packet propagation have been performed with the aid of the multiconfiguration time-dependent Hartree method. Up to five coupled electronic potential energy surfaces and 13 vibrational degrees of freedom have been included in these calculations. As a result, the impact of a third electronic state (&Xtilde; or &Btilde;) on a strongly coupled manifold (&Btidel;-&Ctilde; or &Dtilde;-&Etilde; states) is quantitatively assessed. It leads to a restructuring of the spectral envelope which is stronger for the &Btilde;-&Dtilde;-&Etilde; than for the &Xtilde;-&Btilde;-&Ctilde; system. The internal conversion dynamics is characterized by a stepwise transfer of electronic population to the lowest electronic state on a time scale of ∼100 fs, if the system is prepared initially on the highest potential energy surface. Companion calculations have also been performed for the case when the system is prepared in the intermediate state at t = 0; they show a branching of the electronic populations. These are all novel findings which are discussed in terms of a series of conical intersections between the various potential energy surfaces. The importance of such multistate vibronic interactions for the photophysics and photochemistry of medium-sized systems is pointed out. [ABSTRACT FROM AUTHOR]