Electronic spectra of isolated cations in supersonic jets by mass-selected ion-dip spectroscopy. Cations of benzene, p-difluorobenzene, and 1,3,5-trifluorobenzene

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Abstract
The electronic transitions of the cations of benzene, p‐difluorobenzene (p‐DFB), and 1,3,5‐trifluorobenzene (1,3,5‐TFB) have been measured by mass‐selected ion‐dip spectroscopy which utilizes the dissociation of a parent cation in an excited state. This spectroscopy was successfully applied to the vibrational level selected cation in the ground electronic state which was prepared by 1+1 REMPI (resonant enhanced multiphoton ionization) of the neutral molecule in a supersonic jet. For all the cations, the spectra due to the transition from the ground state to the excited π,π state were observed. Ion‐dip spectra having sharp vibrational structures were found for p‐DFB and 1,3,5‐TFB cations, while a broad spectrum was observed for a benzene cation. It was also found that the vibrational structure of the ion‐dip spectrum of the 1,3,5‐TFB cation is quite different from that of the fluorescence excitation spectrum. The assignments of the ion‐dip spectra and dissociation mechanisms of the excited cations will be discussed.