Anharmonic effects in photoinduced electron transfer

Electron transfer in condensed phase media is typically studied within the standard spin-boson model. The electron transfer is described between two coupled electronic states that are coupled linearly to a bath of displaced harmonic oscillators (bosonic degrees of freedom). The dynamics of this dissipative two level system is rich and well studied as a function of the extent of the coupling to the harmonic bath. Many recent experimental studies have focused on systems where the electron transfer is coupled to a set of high frequency intramolecular modes and lower frequency solvent modes. From semiclassical modeling and experimental studies, it is found that these modes can show anharmonic distortions. This work is a study of the effects of anharmonicities in the nuclear degrees of freedom on the electron transfer process. An approximation method, the Gaussian wave packet dynamics-path integral approach, developed by Coalson [J. Phys. Chem. 100, 7896 (1996)], is adapted to study condensed phase electron transfer in all coupling regimes. Along each path in the path integral sum, the anharmonic nuclear dynamics is calculated using the Gaussian wave packet approximation. The effects of anharmonic intramolecular modes and the effects of anharmonicities in a weakly coupled solvent bath on the donor electronic state population are found to vary greatly with system variables including exothermicity, the bath spectral density, and the nonadiabatic coupling strength. (C) 2000 American Institute of Physics. [S0021-9606(00)50732-6].