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Two samples of solid all-trans-β-carotene which differed in their degrees of crystallinity were studied using visible absorption and resonance Raman spectroscopy. The absorption spectrum of a commercial crystalline β-carotene sample revealed two bands at 18 450 and 20 040 cm−1 which were not present in the absorption spectrum of a prepared amorphous sample. Instead, the spectrum of the amorphous material showed a band at 19 268 cm−1 which is in a position approximately half way between the 18 450 and 20 040 cm−1 bands. From these observations these two latter bands are assigned to the Davydov components of the crystalline β-carotene. The resonance Raman spectrum of crystalline all-trans-β-carotene revealed several frequencies for the backbone CC stretching mode, νcc, as the laser excitation wavelength was varied. This frequency was observed to vary between 1516 and 1525 cm−1. The amorphous carotene sample also exhibited a wavelength-dependent Raman spectrum where νcc ranged from 1516 to 1522 cm−1 initially. On further exposure to laser radiation (30–60 min) at 514.5 nm, the range increased to an upper limit of 1525 cm−1, which was attributed to a photochemical product formed in both the crystalline and amorphous samples. The wavelength-dependent νcc frequency of these carotene samples is interpreted as being due to resonance-enhanced Raman bands of β-carotene molecules with different effective double bond chain lengths originating from separate micro-domains in the solid samples. |
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