A model study of the wavepacket dynamics around a Jahn-Teller conical intersection in a symmetric charge-transfer system

A model system with a Jahn-Teller conical intersection is investigated. Two vibrational modes are considered: a tuning mode corresponding to a vibrati onal motion that reduces the symmetry, thus removing the degeneration betwe en the electronic states and a coupling mode, which is responsible of the e lectronic coupling. While the above scheme may be quite general, we conside r in particular a charge-transfer system, in which the charge (an electron) is exchanged between identical moieties and the hopping term is null for t he reference geometry. A typical realization may be that in biphenyl-like c ompounds in which the two rings are held at 90 degrees by bulky substituent s. We perform calculations of the time-dependent populations of the two dia batic (charge separated) states, assuming that the system is initially dist orted, being trapped in one of the two minima, while a short pulse excites the wave-packet vertically. We not only discuss the ordinary situation in w hich we deal with the response of an ensemble of molecules, but also the on e in which an individual molecule is interrogated. In the latter case, mimi cking what may happen in a molecular electronic device grounded on a single molecule, quantum jumps come out. The role of adding energy quanta in the coupling mode is also investigated, showing that this gives rise to an acce lerated charge-transfer dynamics. From the observation that no electronic c oherence is produced between diabatic states during the time evolution, we are led to try with a very simple model (in the spirit of surface hopping b ut working with diabatic surfaces) which is shown to reproduce quite well t he exact results. (C) 2000 Elsevier Science B.V. All rights reserved.