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

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, 1B_u⁺) 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 (1B_u⁺ and 2A_g^-) in the ultrafast relaxation pathway of β-carotene.