The state energy and the displacements of the potential minima of the 2Ag− state in all-trans-β-carotene as determined by fluorescence spectroscopy

The fluorescence spectrum of all-trans-β-carotene was recorded at 170 K. The 1Bu+ → 1Ag− fluorescence exhibited clear vibrational structures constituting a mirror image with those of the 1Bu+ ← 1Ag− absorption, and the deconvolution of the entire spectrum identified the 2Ag−(0) → 1Ag−(0) transition at 14 500 cm−1. The displacements of the 1Bu+ and 2Ag− potential minima along ν1 and ν2 (the CC stretching and C–C stretching normal coordinates, respectively) were determined to be 1.2 and 0.9, and 1.6 and 1.5, respectively. Thus, much larger potential displacements in the 2Ag− state than in the 1Bu+ state have been shown.