Ultrafast optical responses of three-level systems in β-carotene: Resonance to a high-lying n 1 A g − excited state

Ultrafast optical phenomena in all-trans-β-carotene have been investigated by femtosecond absorption and fluorescence spectroscopy. Following a resonant pump pulse, both fluorescence and absorbance changes have a decay time of 150 fs. The signals are assigned to the lowest optically allowed singlet excited state, 11 B u + . Transmittance changes induced by nonresonant pump pulses depend on the pump photon energy. They are interpreted in terms of the ac Stark effect in three-level systems and two-photon absorption of the pump and probe pulses generating a high-lying n 1 A g − state.