: The resolution that can be achieved by photoelectron spectroscopy has been continually improved over the past 50 years and is now sufficiently high for the rovibronic energy level structure of polyatomic molecular cations to be measured accurately. Ionisation potentials, molecular
constants, and in some cases the potential energy surfaces of both the neutral and the ionic states connected by the photoionising transitions can be extracted from photoelectron spectra and used to test ab initio quantum chemical calculations. Ab initio quantum chemistry represents
an essential tool to assign photoelectron spectra and to rationalise the experimental observations. Because either the neutral or the ionised species, or even both, connected by a photoionising transition possesses at least one unpaired electron, photoelectron spectroscopy represents a very
convenient method to study openshell molecules. Unfortunately, much fewer highly accurate ab initio quantum chemical calculations have been reported on open-shell than on closed-shell molecules, and such calculations would be desired to assist in the interpretation of photoelectron
spectra. This contribution illustrates the fruitful interplay between ab initio quantum chemistry and photoelectron spectroscopy with examples chosen from our recent work on the photoelectron spectra of the rare gas dimers and of small polyatomic hydrides and hydrocarbons. It also briefly
reviews the experimental progress that makes it possible today to measure the fine and hyperfine structure in molecular ions by experimental techniques closely related to photoelectron spectroscopy.
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