Probe-wavelength dependence of picosecond time-resolved anti-Stokes Raman spectrum of canthaxanthin: Determination of energy states of vibrationally excited molecules generated via internal conversion from the lowest excited singlet state

The vibrational relaxation process in the ground electronic state (S₀) of canthaxanthin after internal conversion from the lowest excited electronic state (S₁) to the S₀ state is studied by picosecond time-resolved anti-Stokes Raman spectroscopy. The pump-induced intensities of two strong anti-Stokes Raman bands reach their maxima at delay time ∼12 ps from the pump pulse, and decay with a time constant of 15-20 ps. The peak position of the transient "in-phase" C=C stretching anti-Stokes Raman band shows a small shift to a lower frequency from that observed in the stationary (cw) spectrum. In order to determine the energy levels on which the observed vibrationally excited molecules are populated, the probe-wavelength dependence of the pump-induced anti-Stokes Raman intensities are analyzed. Most of the vibrationally excited transients giving rise to the transient 1520 cm^-1 band at delay time 12 ps are on the first excited vibrational level of the C=C stretching mode in the S0 state. This result suggests that the intramolecular vibrational redistribution (IVR) process is very fast and contributes only to the rise part of the time dependence of the anti-Stokes Raman intensity. The observed shift of the C=C stretching band is considered to arise from anharmonic coupling with various other vibrational modes which are excited through the IVR process, rather than from contributions of molecules on highly excited levels of the C=C stretching mode generated immediately after the internal conversion from the S₁ state.