Femtosecond Stimulated Raman Spectroscopy of Linear Carotenoids.

Femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption data measured in a single experiment are used to determine the vibronic properties of the S state of linear carotenoids with different conjugation lengths. The Raman band corresponding to the C═C stretching mode in the S state peaks at 1799 cm (neurosporene), 1802 cm (spheroidene), and 1791 cm (lycopene). Contrary to the ground state C═C mode, variation of the C═C stretching mode in the S state is small and does not follow a linear dependence on . The lifetime of the Raman band matches the S decays obtained from transient absorption, confirming its S state origin. Direct comparison of transient absorption and FSRS signals allowed us to assign Raman signatures of nonrelaxed S and S states. For lycopene, FSRS data identified a component associated with a downshifted ground state C═C mode, which matches the dynamics of the S* signal observed in transient absorption data.