Butterfly vibration of the tetrafluorobenzene cation studied by pulsed field ionization-zero kinetic energy photoelectron spectroscopy

Pulsed field ionization-zero kinetic energy photoelectron spectroscopy has been applied to 1,2,4,5-tetrafluorobenzene in a supersonic jet. The spectrum measured by selecting a specific vibronic level of the butterfly vibrational mode in S1 by the first laser shows a well-resolved vibrational structure of the cation. A long progression has been assigned to the out-of-plane butterfly vibrational mode 11 with an even quantum number in the cation. From the harmonisity and Franck-Condon factor, it has been concluded that the molecular structure of the tetrafluorobenzene cation is flat, though that in S1 is non-planer along the butterfly vibrational mode. The geometrical change upon ionization is discussed in terms of the electronic structure.