The effect of a model environment on the S-2 absorption spectrum of pyrazine: A wave packet study treating all 24 vibrational modes

The absorption spectrum of pyrazine to the S-2 electronic state can be usefully described by a 4-mode system interacting with a 20-mode bath. In this paper wave packet propagation techniques, using the multiconfiguration time-dependent Hartree approach, are used to study this problem. The investigation was made in stages so as to study the nature of the wave function needed to correctly describe various properties of this multimode problem: the absorption spectrum; the energy exchange between the system and the bath; and the rate of inter-state crossing. It was found that, despite the relatively weak system-bath coupling, a multiconfigurational wave function was necessary to describe the interaction between the two parts of the problem. While it was not possible to treat the full 24-mode problem with such a wave function, the spectrum for a 14-mode system, which includes all the important bath modes, has been calculated in this way. The results, in agreement with the path integral calculations of Krempl et al. [J. Chem. Phys. 100, 926 (1994)], show that the effect of a model bath linearly coupled to the system is to reduce the vibrational structure of the spectrum, so as to produce a broad envelope analogous to that observed experimentally. The details of the spectrum are however different for the two methods. The effect of introducing anharmonicity to the bath was also studied, with the result that this leads to a yet broader spectrum. (C) 1996 American Institute of Physics.