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 (S-0 ) of canthaxanthin after internal conversion from the lowest excited e lectronic state (S-1) to the S-0 state is studied by picosecond time-r esolved anti-Stokes Raman spectroscopy. The pump-induced intensities o f two strong anti-Stokes Raman bands reach their maxima at delay time similar to 12 ps from the pump pulse, and decay with a time constant o f 15-20 ps. The peak position of the transient ''in-phase'' C=C stretc hing anti-Stokes Raman band shows a small shift to a lower frequency f rom that observed in the stationary (cw) spectrum. In order to determi ne the energy levels on which the observed vibrationally excited molec ules are populated, the probe-wavelength dependence of the pump-induce d anti-Stokes Raman intensities are analyzed. Most of the vibrationall y 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 So state. This result suggests that the intram olecular vibrational redistribution (IVR) process is very fast and con tributes only to the rise part of the time dependence of the anti-Stok es Raman intensity. The observed shift of the C=C stretching band is c onsidered to arise from anharmonic coupling with various other vibrati onal modes which are excited through the IVR process, rather than from contributions of molecules on highly excited levels of the C=C stretc hing mode generated immediately after the internal conversion from the S-1 state.