Femtosecond secondary emission arising from the nonadiabatic photoisomerization in rhodopsin

A microscopic quantum-mechanical model of the femtosecond photodynamics and the associated secondary emission of rhodopsin is presented. The formulati on consists of a two-state two-mode model describing the nonadiabatic photo isomerization of retinal, a harmonic multi-mode ansatz accounting for the r emaining Raman-active modes, and a low-frequency bath accounting for the co upling of retinal to the protein environment. The interaction between the v arious subsystems of the model is described in a mean-field approximation. Explicit simulations of absorption, resonance Raman and fluorescence spectr a of rhodopsin are presented and compared to available experimental data. T he model assumptions and the validity of the approximations involved are di scussed in some detail. Furthermore, it is studied to what extent the secon dary emission spectra reflect the photochemical reaction of the molecular s ystem. It is shown that standard continuous-wave techniques such as absorpt ion, resonance Raman and fluorescence spectra may yield only little direct information on the photoreaction. Considering the time- and frequency-resol ved fluorescence spectrum, on the other hand, the time evolution of the exc ited-state wave function can be monitored, thereby providing a real-time me asurement of the nonadiabatic photoreaction. Moreover, the proposed model o f rhodopsin reveals recurrences of time-resolved emission which are shown t o reflect coherent vibrational motion on coupled potential-energy surfaces. (C) 2000 Elsevier Science B.V. All rights reserved.