Vibrational relaxation in β-carotene probed by picosecond Stokes and anti-Stokes resonance Raman spectroscopy

Picosecond time-resolved Stokes and anti-Stokes resonance Raman spectra of all-trans-beta-carotene are obtained and analyzed to reveal the dynamics of excited-state (SI) 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 S-1 Stokes bands. The anti-Stokes spectra exhibit no peaks attributable to the S-1 (2A(g)(-)) state, indicating that vibrational relaxation in S-1 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 C=C 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 S-1-S-0 internal conversion in which the C=C 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.