MOLECULAR-DYNAMICS OF BETA-CAROTENE IN SOLUTION MEASURED BY SUBPICOSECOND TRANSIENT OPTICAL KERR EFFECT

Optical anisotropy was created and detected in diluted solutions of all-trans beta-carotene in n-dodecane by femtosecond pump and probe pulses. At low pump intensity the anisotropy is due to photoselection produced by one-photon absorption, and it decays with the lifetime (8 ps) of the lowest electronic excited state of beta-carotene. At high pump intensity optical Kerr effect can be detected thanks to the amplification of the solute non-linear susceptibility due to the resonance of the probe beam with the first electronic transition of beta-carotene. Besides the instantaneous electronic response, two relaxation times (250 fs and 215 ps) are observed and attributed to the dynamics of beta-carotene molecules in the ground electronic state.