Validity of the discrete nonlinear Schrodinger equation in the context of the fluorescence depolarization of a spin-boson system

The validity of the discrete nonlinear Schrodinger equation and related semi-classical transport equations is studied with the help of an exact calculation of the spin-boson system in the experimentally relevant context of the fluorescence depolarization of stick dimers. The consequences of an initial excitation corresponding to low-energy dynamics are investigated. A number of features are found including the sensitive role of the polarization angle of incident light in determining the regimes of validity. Our earlier finding that the semi-classical approximation improves in validity in a well defined ''massive oscillator'' limit involving small phonon frequency is borne out. In the opposite Limit of large phonon frequency, the semiclassical approximation and the discrete nonlinear Schrodinger equation are found to coincide with each other but to differ significantly from the exact evolution.