Femtosecond secondary emission arising from the nonadiabatic photoisomerization in rhodopsin

Abstract

A microscopic quantum-mechanical model of the femtosecond photodynamics and the associated secondary emission of rhodopsin is presented. The formulation consists of a two-state two-mode model describing the nonadiabatic photoisomerization of retinal, a harmonic multi-mode ansatz accounting for the remaining Raman-active modes, and a low-frequency bath accounting for the coupling of retinal to the protein environment. The interaction between the various subsystems of the model is described in a mean-field approximation. Explicit simulations of absorption, resonance Raman and fluorescence spectra of rhodopsin are presented and compared to available experimental data. The model assumptions and the validity of the approximations involved are discussed in some detail. Furthermore, it is studied to what extent the secondary emission spectra reflect the photochemical reaction of the molecular system. It is shown that standard continuous-wave techniques such as absorption, resonance Raman and fluorescence spectra may yield only little direct information on the photoreaction. Considering the time- and frequency-resolved fluorescence spectrum, on the other hand, the time evolution of the excited-state wave function can be monitored, thereby providing a real-time measurement of the nonadiabatic photoreaction. Moreover, the proposed model of rhodopsin reveals recurrences of time-resolved emission which are shown to reflect coherent vibrational motion on coupled potential-energy surfaces.