Determination of the geometry change of benzimidazole upon electronic excitation from a combined Franck–Condon/rotational constants fit

•Fluorescence excitation and single vibronic level fluorescence spectra of benzimidazole were recorded in a molecular beam.•We determined the complete changes of the heavy atom structures in ground and lowest excited singlet states of benzimidazole.•A combined Franck–Condon/rotational constants fit has been performed.•Perturbations due to Herzberg–Teller coupling have been detected.

Single vibronic level fluorescence spectra of the electronic origin and of seven vibronic bands between 0,0 and 0,0 + 1265 cm−1 have been measured and analyzed by means of a combined Franck–Condon/rotational constants fit. The rotational constants in ground and lowest electronically excited singlet state of four different isotopologues have been taken from previous rotationally resolved measurements of Schmitt et al. (2006). The intensities of 182 vibronic emission bands and of 8 rotational constants have been used for a fit of the complete heavy atom geometry changes upon electronic excitation. Vibronic modes, about 1000 cm−1 above the electronic origin, show strong deviations from Franck–Condon behavior in emission. Herzberg–Teller coupling contributes to this effect. 1300 cm−1 above the origin, we observe the onset of intramolecular vibrational redistribution in the emission spectra.