Excitation followed by stimulated-emission from diabatic levels in all-trans- and 15-cis-β-carotenes: Effects of molecular symmetry and solvent polarity

Upon-excitation, with subpicosecond pulses, to the pair of overlapped optically-allowed 1 B u + (1) and optically-forbidden 3 A g - (2) levels (‘diabatic levels’), we observed stimulated-emission from five diabatic levels, above and below the original pair, which included 1 B u - (1), 3 A g - (0), 3 A g - (1), 3 A g - (2) and 3 A g - (3). We ascribed this to diabatic excitation followed by radiative transitions between the 3 A g - and 1 B u + (or 1 B u - ) vibronic levels in aggregates. The number of observed stimulated-emission peaks increased from two to five in n-hexane on going from all-trans- to 15-cis-β-carotene, and in all-trans-β-carotene from non-polar to polar solvent, which we ascribed to configurational change and electronic polarization in the conjugated chain.