| Summary: | New results for the threshold ionisation of the molecules, HF, DF, HCl, DCl, HBr, DBr, HI and F<SUB>2</SUB> have been recorded using threshold photoelectron spectroscopy (TPES) at resolutions down to 3 meV. The entire outer valence ionisation region has been studied for all of these molecules, and additionally, the inner valence region of the molecules not containing fluorine has been recorded. This represents a significant volume of consistent spectroscopic data for the halogen containing diatomic systems. The completeness of the study utilising both hydrogen isotopes has proven very useful for the in-depth analysis of the TPE spectra, allowing each system to be analysed by combining the data for both hydrogen isotopes. This has permitted the production of improved spectroscopic constants compared to previous TPE work. In all study molecules, the TPE spectrum of the lowest electronic (X) state shows extended vibrational progressions due to the autoionisation of super-excited neutral (Rydberg) states, producing the molecular ion and a threshold electron. The symmetry of the autoionising Rydberg states can be tentatively assigned from the position of enhancements in the TPE spectra. The assignments suggest that the Rydberg states that autoionise to produce threshold electrons do not have the same symmetry as the Rydberg states seen most strongly in photoabsorption and photoionisation spectra. The similarity between the recorded TPE spectra and fluorescence excitation spectra in the literature suggests the predominant autoionisation route leading to threshold electron production could be via repulsive intermediate states. The first excited (A) ion state for HX and DX species is well known to be predissociated for all systems except X=F. The TPE spectra of the predissociated vibrational levels in the HCl, DCl, HBr, and DBr systems show evidence of an additional ionisation mechanism that is not seen for non-resonant ionisation. These additional mechanisms have been modeled as being due to a propensity for a large loss of rotational angular momentum from the ion core during the ionisation process. It is suggested that these mechanisms involve the rotational autoionisation of dissociative Rydberg states into dissociative ion states accompanied by the ejection of a near threshold electron.
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