Spectroscopic Investigation of cis-2,4-Difluorophenol Cation by Mass-analyzed Threshold Ionization Spectroscopy

We applied the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of 2,4-difluorophenol. As supported by our theoretical calculations, only the cis form of 2,4-difluorophenol involves in the two-photon photoexcitation and pulsed field ionization processes. The band origin of the S1 ← S0 electronic transition of cis-2,4-difluorophenol appears at 35 647 ± 2 cm and the adiabatic ionization energy is determined to be 70 030 ± 5 cm, respectively. Most of the observed active vibrations in the electronically excited S1 and cationic ground D0 states mainly involve in-plane ring deformation vibrations. Comparing these data of cis-2,4-difluorophenol with those of phenol, cis-2fluorophenol, and 4-fluorophenol, we found that there is an additivity rule associated with the energy shift resulting from the additional fluorine substitution.