Absolute cross sections for single and multiple ionization of Ne by fast (0.75–3.50 MeV) protons have been measured. The comparison with available theoretical calculations shows that the inclusion of the contribution from time-delayed post-collisional mechanisms is needed for a good description of the experimental results for double and triple ionization at high velocities.

Absolute cross sections, σq+, for multiple ionization of He (q = 1), Ne (q = 1, 2, and 3), Ar (q = 1, 2, 3, and 4), Kr (q = 1, 2, 3, and 4), and Xe (q = 1, 2, 3, 4, and 5) atoms by protons in the 0.75–3.5 MeV impact energy range have been measured. The present results are in good agreement with the previously published data in the case of the single-and double-ionization cross sections, but some non-systematic discrepancies appear for higher charge states. A detailed comparison with electron impact cross sections in these gases is performed, and it shows that the cross sections for single ionization by high-velocity protons are substantially smaller than those corresponding to equi-velocity electrons for heavier targets.

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Using high-resolution recoil-ion momentum spectroscopy we have measured the differential cross sections of single-electron capture and target single-ionization processes for 8.7 MeV 07'-He collisions as functions of scattering angle. A transverse momentum molution of hO.2 a", which componds to an angular resolution of about I I .5 x rad for the projectile scattering angle, was obtained by intersecfing a well collimated 07+ beam with a target of 3. supersonic He jet from a pre-cooled gas and by measuring the recoil-ion transverse momentum. For the single capture reaction, information on the n-value of the eleclron final state in O"(lsn1) is oblained from the longiludinal momentum of the recoil ions. In pure singleelectron capture. the dominant contributions to capture were found to be thwe from the n = 4 and higher states. whereas single capture accompanied by lhe ionization of the second target electron mainly populates n = 2 lo n = 4 states. Furthermore, the measured VansverSe momentum distribution differs significantly between pure single capture and capture with simultaneous ionization, The measured data for the pure capture process compare favourably with thenretical results based on a molecular-state expansion method. Other experimental data are discussed in terms of the classical overbarrier model.

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