title: Femtosecond stimulated Raman study of excited-state evolution in bacteriorhodopsin.
abstract: Femtosecond time-resolved stimulated Raman spectroscopy (FSRS) is used to examine the photoisomerization dynamics in the excited state of bacteriorhodopsin. Near-IR stimulated emission is observed in the FSRS probe window that decays with a 400-600-fs time constant. Additionally, dispersive vibrational lines appear at the locations of the ground-state vibrational frequencies and decay with a 260-fs time constant. The dispersive line shapes are caused by a nonlinear effect we term Raman initiated by nonlinear emission (RINE) that generates vibrational coherence on the ground-state surface. Theoretical expressions for the RINE line shapes are developed and used to fit the spectral and temporal evolution of the spectra. The rapid 260-fs decay of the RINE peak intensity, compared to the slower evolution of the stimulated emission, indicates that the excited-state population moves in approximately 260 fs to a region on the potential energy surface where the RINE signal is attenuated. This loss of RINE signal is best explained by structural evolution of the excited-state population along multiple low-frequency modes that carry the molecule out of the harmonic photochemically inactive Franck-Condon region and into the photochemically active geometry.
authors.affiliation: Department of Chemistry, University of California, Berkeley, California 94720, USA.
authors.forename: David W
authors.initials: DW
authors.lastname: McCamant
authors.forename: Philipp
authors.initials: P
authors.lastname: Kukura
authors.forename: Richard A
authors.initials: RA
authors.lastname: Mathies
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry*; D019617:Halobacterium salinarum / Q000737:chemistry; D007536:Isomerism; D013059:Spectrum Analysis, Raman / Q000379:methods*
chemical_list: D001436:Bacteriorhodopsins