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 multicon figuration time-dependent Hartree approach, are used to study this pro blem. The investigation was made in stages so as to study the nature o f 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 int eraction between the two parts of the problem. While it was not possib le to treat the full 24-mode problem with such a wave function, the sp ectrum for a 14-mode system, which includes all the important bath mod es, has been calculated in this way. The results, in agreement with th e 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.