Ultrafast Nonlinear Optical Responses Induced by Multiphoton Excitation in All-trans-β-Carotene: Nonresonant Excitation to the Optically Allowed S2State

Abstract

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 S 2 (1 1 B 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 S 1 (2 1 A g - ) state induced by two-photon excitation and to the lowest triplet T 1 state induced by four-photon excitation were well temporally and spectrally resolved. The S 1 decay time and T 1 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.