Femtosecond Time-Resolved Stimulated Raman Spectroscopy of the S2 (1Bu+) Excited State of β-Carotene

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Abstract
The electronic and vibrational structure of β-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived (∼160 fs) second excited singlet state (S2, 1Bu+) of β-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at ∼1100, 1300, and 1650 cm-1 that decay with a time constant corresponding to the electronic lifetime of S2. The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1Bu+ and 2Ag-) in the ultrafast relaxation pathway of β-carotene.