[beta]-Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy

[beta]-Carotene in n-hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals. Overlaid on the decay of S1 excited-state absorption, a picosecond process is found that is absent when the C9-methyl group is replaced by ethyl or isopropyl. It is attributed to reorganization on the S1 potential energy surface, involving dihedral angles between C6 and C9. In Raman studies, electronic states S2 or S1 were selected through resonance conditions. We observe a broad vibrational band at 1770cm-1 in S2 already. With 200fs it decays and transforms into the well-known S1 Raman line for an asymmetric C=C stretching mode. Low-frequency activity (