Two-photon resonances in femtosecond time-resolved four-wave mixing spectroscopy: beta-carotene.

Femtosecond time-resolved pump-degenerate four-wave mixing (pump-DFWM) spectroscopy has been used to study the ultrafast dynamics of beta-carotene involving several electronic and vibrational states. An initial pump pulse, resonant with the S(0)-to-S(2) transition, excites the molecular system and a DFWM process, resonant with the S(1)-to-S(n) transition, is used to probe the relaxation pathways. The transient shows a peculiar decay behavior, which is due to the contributions of resonant DFWM signal of the excited S(1) state, nonresonant DFWM signal of the ground S(0) state and vibrational hot S(0)* state, and the two-photon resonant DFWM signal of the ground S(0) state. We have used a kinetic model including all the signal contributions to successfully fit the transient. The time constants extracted are in very good agreement with the known values for beta-carotene. For comparison, a two-pulse pump-probe experiment was performed measuring the transient absorption at the wavelength of the DFWM experiment.