Mechanism Activating the 2¹A_g State in all-trans-β-Carotene Crystal to Resonance Raman Scattering

Abstract

The mechanism governing the additional excitation peak of the C=C stretching Raman line was studied by comparing randomly oriented crystals of all- trans -β-carotene at 11 K with microcrystals dispersed in a KBr disk at 11 K, a spin-coated film at 11 K and an isopentane solution at 160 K. Only the randomly oriented crystals gave rise to the additional peak. The peak at 14500 cm -1 is assigned to the electronic forbidden transition to the 2 1 A g state, and its oscillator strength is estimated as f a ∼1×10 -2 . An asymmetric distortion in the crystal is proposed to activate the forbidden transition at low temperature.