Femtosecond pump-probe spectroscopy of bacteriochlorophyll a monomers in solution.
One- and two-color absorption difference profiles were obtained for BChl a in 1-propanol with approximately 50-fs resolution, using a self-mode-locked Ti:sapphire laser system. Time evolution in the BChl a absorption difference spectrum produces nonexponential photobleaching/stimulated emission (PB/SE) decay kinetics in 800-nm one-color experiments. Nonexponential PB/SE rise behavior occurs for some combinations of pump and probe wavelengths in two-color experiments. Optimized parameters from triexponential fits to the absorption difference profiles depend markedly on the fitting time window; they typically include a minor component with lifetime in the hundreds of fs. Much of the latter component is due to vibrational relaxation and/or intramolecular vibrational redistribution, rather than solvent dielectric relaxation. Measurements of the pump-probe anisotropy indicate that the electronic transition moment for the broad Qy excited state absorption band that overlaps the Qy steady-state absorption spectrum makes an angle of at most 20 degrees from that of the ground-->Qy state transition. No coherent oscillations are observed at early times. Our results bear directly on the interpretation of fs pump-probe experiments on BChl a-containing pigment-protein complexes.
Ames Laboratory, U. S. Department of Energy, Ames, Iowa.
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Savikhin
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Struve
D001429:Bacteriochlorophylls / Q000737:chemistry* / Q000528:radiation effects; D055592:Biophysical Phenomena; D001703:Biophysics; D004735:Energy Transfer; D007834:Lasers; D008968:Molecular Conformation; D012996:Solutions; D013053:Spectrophotometry / Q000379:methods*
D001429:Bacteriochlorophylls; D012996:Solutions