VIBRATIONAL-RELAXATION AND REDISTRIBUTION IN THE 2A(G)(-) STATE OF ALL-TRANS-LYCOPENE AS REVEALED BY PICOSECOND TIME-RESOLVED ABSORPTION-SPECTROSCOPY

Vibrational relaxation in the 2A(g)(-) (S-1) state of all-trans-lycope ne, which takes place in a time scale slower than that of internal con version, has been found by picosecond time-resolved absorption spectro scopy: The time-resolved S-n (2B(u)(+)) <--S-1 (2A(g)(-)) absorption s pectra of this carotenoid in quinoline and CS2 solutions can be explai ned in terms of vibrational structures originating from the C=C (v(1)) and C-C (v(2)) stretching modes, and the time scales of vibrational r elaxation (k(v1) and k(v2) defined for the lowest vibrational transiti on) relative to that of internal conversion (k) are estimated by spect ral simulation; the values of k(v1)/k and k(v2)/k are around 0.6 and 0 .3 in the quinoline solution and 0.3 and 1.6 in the CS2 solution. The slower vibrational relaxations are discussed in light of the light-har vesting function of carotenoids in photosynthesis.