Vibrational Relaxation in β-Carotene Probed by Picosecond Stokes and Anti-Stokes Resonance Raman Spectroscopy

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Abstract
Picosecond time-resolved Stokes and anti-Stokes resonance Raman spectra of all-trans-β-carotene are obtained and analyzed to reveal the dynamics of excited-state (S1) population and decay, as well as ground-state vibrational relaxation. Time-resolved Stokes spectra show that the ground state recovers with a 12.6 ps time constant, in agreement with the observed decay of the unique S1 Stokes bands. The anti-Stokes spectra exhibit no peaks attributable to the S1 (2Ag-) state, indicating that vibrational relaxation in S1 must be nearly complete within 2 ps. After photoexcitation there is a large increase in anti-Stokes scattering from ground-state modes that are vibrationally excited through internal conversion. The anti-Stokes data are fit to a kinetic scheme in which the CC mode relaxes in 0.7 ps, the C−C mode relaxes in 5.4 ps and the C−CH3 mode relaxes in 12.1 ps. These results are consistent with a model for S1−S0 internal conversion in which the CC mode is the primary acceptor, the C−C mode is a minor acceptor, and the C−CH3 mode is excited via intramolecular vibrational energy redistribution.

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