A microscopic description of dissipation in systems with strong vibronic coupling:: the S1 and S2 absorption spectra of pyrazine

The photoabsorption spectrum of the vibronically coupled electronic S-1/S-2 states of pyrazine has been calculated. The dynamics of the molecule is described within the reduced density matrix formalism. The four most important vibrational modes are included in the reduced density matrix and the 20 remaining modes are considered as bath. In the master equation, the system-bath interaction is treated within second-order perturbation theory and the corresponding operators are derived from a microscopic model. The resulting non-Markovian dissipation operator is evaluated numerically. The calculated spectra are in good agreement with experiment and benchmark MCTDH calculations. (C) 1998 Elsevier Science B.V. All rights reserved.