A new experimental method is described which allows the range of scattering angles to be extended to cover the whole of the backward hemisphere, i.e. up to , in measurements of elastic electron scattering. This is achieved by adding a localized magnetic field to the interaction region of a conventional electrostatic electron spectrometer. Measurements are presented of the differential cross section from 30 - for elastic scattering of electrons by argon at an energy of 10 eV, and of the resonance structures due to the states of at the fixed scattering angles of , and .

The Ar 2+ and Ne 2+ spectra have been studied from their respective thresholds up to their respective triple ionization potentials by threshold photoelectrons coincidence (TPEsCO) spectroscopy. The np 4 , nsnp 5 and ns 0 np 6 (n = 2, 3 in Ne and Ar, respectively) states of the dications as well as the np 3 n l satellite states, where two electrons are ejected and one is promoted into an unfilled orbital, have been observed. This study is the first extensive observation of the satellite states of dications by electron emission spectroscopy and extends the previous observations by optical emission spectroscopy to the region of the triple ionization threshold. New features have been discerned in the spectra and an assignment is proposed based on previous results from optical spectroscopy and on calculations with an atomic multiconfigurational Dirac-Fock code.

Differential cross sections for electron impact excitation of the , , , and , states of mercury have been measured at the incident energies of 12.2 and 15 eV and over the range of scattering angles from . The cross sections for the 6s6p states at 15 eV are compared with the results of recent relativistic distorted-wave calculations. The integral cross sections for the 6s6p states have also been obtained, and are compared with those used in electron swarm studies of mercury.

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The production of C + and O + ion pairs in carbon monoxide at photon energies below the adiabatic double-ionization threshold of 41.25 eV has been probed in a thresholdphotoelectron-photoelectron coincidence (TPEPECO) experiment using tunable VUV radiation and a sensitive electron spectrometer. The TPEPECO spectra provide evidence of C + + O + production that does not involve creation and dissociation of a molecular dication, but instead results from complete dissociation of a molecular cation followed by autoionization of the atomic oxygen fragment. Furthermore, an electron-electron coincidence signal has been detected at photon energies as low as 36.5 eV, well below the previously measured onset for C + + O + production.Adiabatic photo-double ionization of a molecule corresponds to absorption of an energetic photon with the immediate ejection of two electrons. For a small molecule, the resulting doubly charged positive ion (dication) is always unstable against dissociation into two singly charged positive ion (cation) fragments. However, a binding force produced by exchange interactions between the remaining electrons may create a local potential minimum capable of supporting vibrational motion for a short time. Cation pair production can also occur at photon energies below a molecule's double-ionization threshold (here, double-ionization threshold refers to the adiabatic formation energy of the lowest dication state); this was first observed in photoion-photoion coincidence (PIPICO) studies of water by Winkoun et al (1988). In this case, the two cation fragments must result from indirect double-ionization mechanisms.Cation pair production below the double-ionization threshold has also been observed in CO. The double-ionization threshold of CO is 41.25 eV (Hochlaf et al 1996), and the lowest lying C + + O + dissociation limit is 35.99 eV (Moore 1958, Huber andHerzberg 1979). In PIPICO studies of CO, Lablanquie et al (1989) observed an onset for C + +O + pair production at a photon energy of 38.4 eV. To explain this observation, the authors suggested the existence of a two-step double-ionization mechanism. This two-step mechanism, also