Femtosecond optical techniques were employed to investigate the ultrafast cis-trans isomerization reaction of retinal in the bovine rhodopsin photocycle. Probing at near-IR wavelengths allowed detection of stimulated emission, the rise and decay of which were found to have an upper limit of 40 fs. This implies motion of the excited state population out of the Franck-Condon region along a coordinate other than the torsion coordinate since the isomerization takes place on a longer (200 fs) timescale. A polarized pump-probe measurement of the formation of the first isomerization product, bathorhodopsin, shows that the initial electronic transition dipole moment direction could be as much as 30( away from the original direction as indicated by an anisotropy of 0.25. The anisotropy then rises exponentially, with a time constant of 6.0(0.6 x 1012 s-1, to 0.34(0.01. This final anisotropy value indicates that the S0.S1 transition dipole moment direction of all-trans retinal is shifte d by 16.5( relative to that of 11-cis retinal. The coherent vibrational oscillations in the product well, which manifest themselves as oscillations in the magic angle signal only, are shown to involve less than 4( reorientations of the transition dipole direction. A new transient absorption at ca. 700 nm was revealed and characterizes that part of the initially excited rhodopsin population that is unreactive. The lifetime of this species was found to be significantly longer than the formation time of isomerized product. These results support a multidimensional free energy surface involving C=C bond stretching preceding the torsional motion leading to product formation.
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