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

抄録

The ultrafast nonlinear optical responses of all-trans-β-carotene have been investigated by femtosecond pump–probe spectroscopy under a nonresonant excitation condition to the optically allowed S₂ (1¹B_u⁺) state. Instantaneous signals were ascribed to coherent nonlinear optical effects in a three-level electronic system in β-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 (2¹A_g⁻) 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.