Molecular dynamics of pyrazine after excitation to the S-2 electronic state using a realistic 24-mode model Hamiltonian

The molecular dynamics of pyrazine after excitation to the S-2 electronic s tate is investigated using the S-2 absorption spectrum as a benchmark. We f irst present a realistic model Hamiltonian including all 24 vibrational mod es of the pyrazine molecule. Using this model, we determined the potential energy surfaces of the lowest two excited states, S-1 and S-2, which are st rongly coupled to each other. We then treated the nuclear motion of all 24 vibrational modes using the multiconfiguration time-dependent Hartree (MCTD H) wave packet propagation method. This method obtains results of good accu racy with acceptable computational effort for such a large system. The calc ulated spectrum is in good agreement with the experimental one. Furthermore , our results shed light on the role of the 20 modes which are only weakly coupled to the system, and demonstrate that essential physical features, su ch as symmetries, have to be considered when one wants to treat the molecul ar dynamics of pyrazine realistically. (C) 1999 American Institute of Physi cs. [S0021-9606(99)00802-8].