THE RELAXATION DYNAMICS AND SHORT-TIME OPTICAL-RESPONSE OF A MULTIMODE OPEN SYSTEM

When dividing a large system into a subsystem and a bath, one sometime s needs to include multiple nuclear coordinates in the subsystem in or der to treat the remaining bath modes by Markovian relaxation theory ( which assumes fast bath relaxation). This paper examines the effects o f Redfield relaxation on the time-resolved fluorescence signal and on the loss of energy from a three-mode system. Simulations are compared for system coordinates coupled to the same bath mode and to independen t bath modes, for coupling linear and quadratic in the system coordina tes, and for different temperatures. To make these comparisons meaning ful, a criterion is proposed for normalization of the coupling strengt hs in the different cases. The fluorescence Stokes shift and the syste m energy are shown to be sensitive to different relaxation processes. The coupling of multiple system coordinates to the same bath mode resu lts in a wide range of relaxation rates. Simple descriptions in terms of individual population relaxation and coherence dephasing rates are inadequate due to sequential processes and to coupling between populat ions and coherences. These results have implications not only for Redf ield treatments but also for other relaxation theories such as the Bro wnian oscillator model.