Femtosecond time-resolved stimulated Raman spectroscopy of the S2 (1Bu+) excited state of β-carotene

The electronic and vibrational structure of beta-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived (similar to160 fs) second excited singlet state (S-2, 1B(u)(+)) of beta-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at similar to1100, 1300, and 1650 cm(-1) that decay with a time constant corresponding to the electronic lifetime Of S-2. The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1B(u)(+) and 2A(g)(-)) in the ultrafast relaxation pathway of beta\-carotene.