Studies on Short-Lived Molecular Species by Raman Spectroscopy

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Two lines of research have been pursued, and the following results have been obtained.(1) Picosecond time-resolved resonance Raman scattering from excited electronic states of carotenoids in vivo and in vitro.The vibrational spectroscopy and population dynamics of excited singlet (2^1Ag), excited triplet (^3Bu), and the ground (^1Ag) electronic states of carotenoids in chromatophores of Chromatium vinosum (mainly spirilloxanthin and rhodopin) and of the same carotenoids in benzene solutions are examined by picosecond time-resolved resonance Raman scattering. Compariosons of the time-dependent intensities of 2^1Ag and ^1Ag resonance Raman bands from both in vitro and in vivo carotenoids suggest that vibrationally-excited livels in ^1Ag are populated directly by the decay of the 2^1Ag state and that these levels relax into a thermalized distribution in <50 ps. The appearance of asymmetrically broadened ground-state resonance Raman bands supports this conclusion. Formation of the ^3Bu sta … More te is observed for carotenoids in chromatophores, but not for in vitro spirilloxanthin indicating that the ^3Bu state is formed by fission processes originating from the spatial organization of pigments within chromatophores.(2) Vibrational Relaxation in Carotenoids in Vivo and in Vitro.The ground-state ^1Ag vibrational relaxation in the population dynamics of carotenoids in pigment-protein complexes of photosynthetic bacteria (in vivo) and of spirilloxanthin in benzene and toluene solutions (in vitro) is quantitatively characterized using picosecond time-resolved anti-Stokes resonance Raman spectroscopy. For in vitro samples, anti-stokes resonance Raman signals reach maxima <approximately equal>10 ps after optical excitation before decaying with a rate of <approximately equal>15 ps to intensities which reflect a thermalized ^1Ag vibrational population. For the in vivo case, the maximum band intensities are found within the cross-correlation time (<approximately equal>8 ps) defined by the 567 nm pump (6 ps FWHM) and 576 nm probe (6 ps FWHM) laser pulses while the decay rate is similar to that of the in vitro samp Less