Resonance Raman spectra (5800–40 cm−1) of All‐trans and 15‐cis isomers of β‐carotene in the solid state and in solution. Measurements with various laser lines from ultraviolet to red

Abstract The Raman spectra (5800–40 cm −1 ) of the all‐ trans and 15‐ cis isomers of β‐carotene were observed in the solid state at 77 K and in cyclohexane solutions using laser lines of various wavelengths for excitation. The Raman spectra of the solid samples exhibited not only the well known strong bands but also a great number of sharp bands (mostly weak in intensity) which could not be observed or were observed broadly in solutions at room temperature. The molecules of both isomers in solution seem to have structures slightly different (‘relaxed’) from those in the solid state. All the bands in the region of 5800–1800 cm −1 were satisfactorily accounted for as overtones or combination tones of relatively strong fundamentals. Rough excitation profiles and depolarization degrees measured for cyclohexane solutions gave information on the mechanism of resonance enhancement. The polarized bands of the all‐ trans isomer at 1597 and 1525 cm −1 are both resonant with the 1 B u ← 1 A g absorption around 480 nm. In addition, the 1597 cm −1 band is resonant with the 2 1 B u ← 1 A g absorption around 275 nm. The polarized bands at 1597, 1533, 1194, 1156 and 1004 cm −1 of the 15‐ cis isomer are resonant with the ‘ cis ‐peak’ around 340 nm as well as with the 1 B 1 ← 1 A 1 absorption around 475 nm. The polarized bands at 1241, 1052 and 953 cm −1 of the 15‐ cis isomer are characteristic of this isomer, but these are only slightly resonant with the ‘ cis ‐peak’. The intensity of the depolarized band at 1564 cm −1 of the 15‐ cis isomer is enhanced by the vibronic interaction between the excited 1 A and 1 B 1 states.