S1 AND T1 SPECIES OF BETA-CAROTENE GENERATED BY DIRECT PHOTOEXCITATION FROM THE ALL-TRANS, 9-CIS, 13-CIS, AND 15-CIS ISOMERS AS REVEALED BY PICOSECOND TRANSIENT ABSORPTION AND TRANSIENT RAMAN SPECTROSCOPIES

Time-resolved absorption spectra, upon excitation of the all-trans, 9-cis, 13-cis, and 15-cis isomers of beta-carotene, were recorded by using 20-25-ps, 355-nm pump and white-continuum interrogating pulses. The isomers showed the S(n) <-- S1 absorption at 556 (all-trans), 565 (9-cis), 560 (13-cis), and 562 (15-cis) nm; no time-dependent changes of the absorptions were detected. (The S1 lifetimes for the all-trans and 15-cis isomers were determined precisely, by using 0.5-ps pump and interrogating pulses, to be 12.4 +/- 0.5 and 14.0 +/- 0.5 ps, respectively.) After the decay of the S(n) <-- S1 absorption, a weak absorption ascribable to the T(n) <-- T1 transition remained in the 510-520-nm region from which the quantum yield of intersystem crossing was estimated to be on the order of 10(-3). Transient Raman spectra were recorded by using 355-nm pump and 532-nm probe mode-locked (76 MHz), Q-switched (800 Hz) pulse trains (pulse duration approximately 100 ps). An S1 Raman spectrum was extracted by subtraction between a pair of pump-and-probe spectra with different delay times, i.e. 0 and 500 ps. A T1 Raman spectrum was extracted by subtracting a probe-only spectrum from a pump-and-probe spectrum with the delay time 500 ps. Each isomer showed a unique S1 Raman spectrum, which was interpreted by assuming the configuration of the particular isomer. On the other hand, the 13-cis and 15-cis isomers showed T1 Raman spectra that are essentially the same as that shown by the all-trans isomer; the 9-cis isomer showed a unique T1 Raman spectrum. The above results indicate (1) that no isomerization takes place in the S1 state and (2) that the T1 state is generated through intersystem crossing.