Ultrafast Dynamics of All-trans-β-Carotene Explored by Resonant and Nonresonant Photoexcitations

Excitation energy dependence of transmittance change has been investigated in $\beta$-carotene. The signal induced by nonresonant excitation is ascribed to the ac Stark effect and the two-photon absorption of the excitation and probe pulses in three-level systems. The ultrafast response following resonant excitation is assigned to the two-photon absorption and the transient absorption of the photogenerated $S_2(1{}^{1}B_u^{+})$ state with a lifetime of 150 fs. The long-debated $S_2\mathrm{\text{−}}{S}_1$ relaxation in $\beta$-carotene can be explained by a two-state model $(S_2,S_1)$ without involving any intermediate states.