Knowledge Bank

Time-resolved resonance Raman $(TR3)$ spectroscopy has been used to record the vibrational spectra and to monitor the dynamical properties of transient intermediates found in the reactions of visual chromophores. The experimental approach derives from pump-probe configurations using two independently tunable dye lasers with time resolution extending to ps. Results will be presented on the excited and ground-state mechanisms for isomerization and conformational changes in retinals and in the bacteriorhodopsin photocycle. The vibrational Raman spectra of the conformational and structural intermediates found during these reactions also will be described. These data lead to clear distinctions between the reaction pathways that proceed along either excited or ground state potential surfaces. The availability of $TR3$ spectra for these intermediates permits these mechanistic routes to be viewed in terms of the structural changes occurring in the retinal chromophore. These experiments also comment on the photolytic interruption of retinal isomerization occurring on ground-state potential surface by the high laser intensities used to initiate and probe the reactions.