Spectroscopy of the ionic ground state of monohalogenated benzenes

We present multiphoton ionization photoelectron (PE) spectra of monofluoro-, -chloro-, and -bromobenzene. The population of the vibrational levels in the cation after one-color, two-photon ionization via various vibronic intermediate states of the neutral molecule has been investigated. The observed structure has been assigned, and the frequencies of some vibrations in the ionic X-state were determined, providing new data for these molecular ions. Our results show that the PE spectra reflect Fermi resonances and Duschinsky rotations in the neutral intermediate states, allowing an interpretation of these states. The consequences of our results for neutral as well as ion spectroscopy will be discussed.