Mass analyzed threshold ionization spectroscopy of p-fluorophenol cation and the p-fluoro substitution effect

The origin of the S1 ← S0 electronic transition and the adiabatic ionization energy of p-fluorophenol are determined to be 35 117 ± 2 and 68 577 ± 5 cm−1, which are lower than those of phenol by 1232 and 48 cm−1, respectively. Most of the active vibrations in the electronic excited S1 and cationic ground D0 states are related to the in-plane ring modes. The p-fluoro substitution not only causes red shifts in the transition energies but also leads to a decrease in the frequencies of the observed vibrations. The present experimental findings are well supported by our ab initio and density functional calculations.