Fluorescence excitation and resonant two-photon ionization spectroscopy of fluorene cooled in pulsed molecular beams: Vibronic assignment based on a RCIS calculation

Fluorescence excitation and resonance two-photon ionization spectra in the range of 285-302 nm were measured for fluorene (FR)-h10 and -d10 molecules cooled in pulsed supersonic expansion of He. The vibrational bands above the electronic origin were assigned on the basis of the S₁ state vibrational frequencies evaluated with the RCIS/6-31G(d) method. We observed a sizable discrepancy between the 0-0 band observed in the present study for FR-h10, 33, 791 cm^-1, and the vertical excitation energy of 44,209 cm^-1 evaluated with the RCIS/6-31G(d) method. However, the experimental frequency ratios of v_H/v_D populated in the range from 1.05 to 1.09 were found to be consistent with the theoretical ratios spanning 1.03-1.17. The findings are quite informative as to the assignment of the vibronic peaks.