Dispersed fluorescence spectra of jet-cooled benzophenone ketyl radical: Assignment of the low-frequency vibrational modes

We measured the laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of jet-cooled benzophenone ketyl radical (BPK), which was generated by the 248 nm photolysis of α-phenyl benzoin or benzhydrol. The several single vibronic level (SVL) DF spectra of BPK were obtained to analyze the low-frequency vibrational modes. We identified three low-frequency vibrations of 47, 74, and 98 cm−1 in the ground state, which are assigned to the out-of-phase torsional motion, the in-phase torsional motion and Ph–C–Ph bending motion, respectively, with the aid of the density functional theory (DFT) calculation. The corresponding vibrational frequencies are 60, 65 and 84 cm−1, respectively, in the excited state. The geometries of BPK in the ground and excited states are discussed based on the experimental results, ab initio calculations, and one-dimensional Franck–Condon simulation.