Vibrational relaxation of the 2 A⁻_g 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 2 A⁻_g excited state which is converted from the photogenerated 1B⁺_u state. The vibrational relaxation in the 2 A⁻_g state takes place within 0.6 ps until the l = 1 vibrational level, but the relaxation from the l = 1 evel to the l = 0 level is slower than 10 ps. The unusual kinetics is explained in terms of the 1 B⁻_u state which exists between the l = 1 and l = 2 levels of the 2 A⁻_g state.