A simple threshold photoelectron energy selector was installed in the source of a conventional photoionization mass spectrometer to measure photoionization thresholds. In this work, HF, DF, and F2 were investigated. A number of vibronic levels of the HF+ and DF+ ions, not observed by HeI photoelectron spectroscopy, have been detected, from which spectroscopic constants of the molecular ions could be inferred for comparison with recent emission spectroscopy data. The threshold photoelectron spectra show some complicated structure associated with resonant autoionization of molecular Rydberg states belonging to series converging to the various vibronic levels of the ions.

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