Vibrational relaxation of the2Ag−excited state in all-trans-β-carotene obtained by femtosecond time-resolved Raman spectroscopy

Relaxation kinetics in all-trans-β-carotene has been investigated by recently developed femtosecond Raman spectroscopy. We observed the stimulated Raman signal of the ${2A}_g^{-}$ excited state which is converted from the photogenerated ${1B}_u^{+}$ state. The vibrational relaxation in the ${2A}_g^{-}$ state takes place within 0.6 ps until the $l=1\mathrm{}$ vibrational level, but the relaxation from the $l=1\mathrm{}$ level to the $l=0\mathrm{}$ level is slower than 10 ps. The unusual kinetics is explained in terms of the ${1B}_u^{-}$ state which exists between the $l=1\mathrm{}$ and $l=2\mathrm{}$ levels of the ${2A}_g^{-}$ state.