Picosecond Time-Resolved Resonance Raman Anti-Stokes Spectroscopy: Vibrationally-Excited Populations in the Bacteriorhodopsin Photocycle
Picosecond transient absorption (PTA) and picosecond time-resolved fluorescence (PTRF) results suggest that vibrationally-excited 1Ag retinal is formed following 5 ps optical initiation of the bacteriorhodopsin (BR) photocycle . This interpretation has been confirmed experimentally by the observation of antiStokes resonance Raman (RR) scattering arising from vibrationally “hot” 1Ag populations in Br-570 (BR’) . These picosecond time-resolved RR (PTR3) antiStokes data demonstrate that the BR’ levels are populated by non-radiative decay from the excited electronic state of BR (BR*) produced by 5 ps optical excitation. PTR3 measurements using a pump-probe experimental arrangement also have shown that the BR’ levels relax with a rate of ≈ 7 ps. . Shifts of the positions of band maxima to lower ferquency displacements on both antiStokes and Stokes RR bands provide an estimate of the 1Ag vibrational anharmonicities. A detailed kinetic model of the excitation process in the BR phtocycle derived from PTA and PTRF data together with the intensity dependencies of the antiStokes RR bands can be used to establish a clearer understanding of the role of vibrational relaxation and energy redistribution in both the optical excitation process and the BR photocycle.
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