| Summary: | In the present work, a beam of monoenergetic positrons has been used to investigate the ionization of atoms and small molecules at energies below 1 keV. The beam was produced from the radioactive decay of a ²²Na source combined with W-mesh moderators and a magnetic guidance system. The first measurements of the cross-sections for excited-state positronium formation from Xe and simultaneous ionization–excitation cross-sections for positron impact on CO₂ and N₂ have been performed. Near-complete characterization of the detection system coupled with the ability to measure several processes simultaneously allowed the collection of data sets which were internally self-consistent. By normalizing the total ionization cross-section, an absolute scale could be applied to all measurements. A number of methods for achieving this were employed, as a check on external consistency. The cross-section for excited-state positronium formation from Xe completed a study (Murtagh et al., 2009) in Ps formation from the noble gases. The measurement has defined a trend of increasing maximal fraction of Ps formed into the 2P state with increasing atomic number. The measurements of ionization–excitation for molecular targets (Cooke et al., 2010a) reveal that this process is enhanced over the equivalent interaction involving electrons. This enhancement arises mainly (or exclusively, in the case of CO₂) from the effect of positronium formation, over and above the corresponding enhancement in the total ionization cross-section. Based on this observation, and the comparative lack of excited-state Ps detected in these targets, a mechanism for the enhancement involving an accidental resonance between a neutral excited molecular state and an ionic state with Ps formation has been proposed. The cross-sections for ionization–excitation were measured contemporaneously with a full suite of ionization cross-sections.
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