The S1–S(1B2u–1A g) transition of p-difluorobenzene cooled in a supersonic free jet expansion. Excitation and dispersed fluorescence spectra, vibrational assignments, Fermi resonances, and forbidden transitions

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Abstract
The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p‐difluorobenzene (000 at 36 838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been identified. The prevalent Fermi resonance between ν’5 and 2ν6 has been analyzed with the assistance of both excitation and dispersed fluorescence spectroscopy, yielding a coupling matrix element [g566]=−1 cm−1. Thirty‐one matrix elements describing cubic anharmonicity and involving a variety of vibrational modes have been estimated. The majority of the coupling matrix elements lie within the range ±2 cm−1.

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