Ultrafast Nonlinear Optical Responses Induced by Multiphoton Excitation in All-trans-$\beta$-Carotene: Nonresonant Excitation to the Optically Allowed S2 State

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The ultrafast nonlinear optical responses of all-trans-$\beta$-carotene have been investigated by femtosecond pump–probe spectroscopy under a nonresonant excitation condition to the optically allowed S2 (11Bu+) state. Instantaneous signals were ascribed to coherent nonlinear optical effects in a three-level electronic system in $\beta$-carotene without any convolution of the population of excited states. The temporal responses of the coherent signals were identified with an instrumental response function of the system. At a longer delay (${>}1$ ps), transient signals due to the dark S1 (21Ag-) state induced by two-photon excitation and to the lowest triplet T1 state induced by four-photon excitation were well temporally and spectrally resolved. The S1 decay time and T1 formation time were determined to be 9.5 and 20 ps, respectively. We show the availability of multiphoton excitation spectroscopy for clarifying the ultrafast relaxation kinetics of a complex system.
2009-10-15