Site selective photoselection study of indole in argon matrix: location of the 1La origin

Polarized fluorescence excitation and dispersed fluorescence spectra of indole in solid Ar at 20 K under site selective conditions have revealed that ν28 is Herzberg-Teller (HT) active, both in the excitation and the fluorescence spectrum. The two lines 455 and 480 cm−1 above the 1Lb origin are a Fermi doublet involving 2801 and the out-of-plane combination 39014101 and derive 1La optical character via the HT coupling by ν28. The true 1La origin appears to be about 1400 cm−1 above the 1Lb origin in vapor, as discerned by the differential red shift in the Ar matrix.