Positron and electron impact double ionisation of helium

Abstract

The ratio of the double to single ionisation cross section, R'", has been measured over a wide range of energies for positron and electron impact on He. At high velocities (>1-2 MeV amu-') the electron results, in accord with previous studies, merge with those for antiproton impact. Using a beam of low-energy positrons, a similar effect is found with proton data, further supporting the hypothesis that the observed difference in the electron and proton double ionisation cross sections is a result of their opposite charge.

High-velocity limit for the ratio of helium double-to-single ionization for highly charged, bare-ion impact

“…As a further consequence, the high-velocity limit of R is predicted to be independent of the sign of the projectile charge q and velocity vp. Systematic experiments with singly charged projectiles for electron [4,5], proton [6], antiproton [7,8], and positron [9] impact indeed yielded identical ratios within the experimental errors between 0.24% and 0.28%. This value is in good agreement with theoretical results of ab initio calculations by Ford and Reading [10] for charged particle impact of R =0.259%.…”

“…This gives us confidence that the asymptotic high-velocity limit has been reached and a value of R =0.257% ±0.010% has been established. In Table I our result for highly charged ion impact is compared to the high-p/> limit observed for other projectiles and good agreement within the experimental uncertain- Ratio R of helium double-to-single ionization for different projectiles as a function of the projectile velocity (lower scale: in atomic units) and energy (upper scale: in MeV/u) Ne ,0+ (full circles, present work); Ni 28+ (full squares, present work); Kr 36+ (full triangles [16]); N 7+ (open circles, for projectile energies between 10 MeV/u and 30 MeV/u [11] and 40 MeV/u [15]); positrons (e + , small full squares [9]); the lines with different symbols for electrons {e~ [4]), protons (p + [7]), and antiprotons (p ~ [7,8]) are fits through the experimental data. Open square close to the line vp ^c (velocity of light) is for electron impact [5].…”

The lives and deaths of positrons in the interstellar medium

“…These distributions can be approximated by an exponential law with an energy scale of ≈18.5 eV. We neglected the He double ionization by a positron since its cross section is ≈3 × 10 −3 lower than that for single ionization (Charlton et al 1988;CH2001).…”

High-charge-state limit for the double-to-single ionization ratio of helium in the strong-coupling regime

“…Such deviations are also observed in systematic measurement where fast (3.6 MeV/u ∼ 1 GeV/u) and highly charged (q = 24 ∼ 92) ions are used [14]. As stated by previous authors, helium double ionization for very highly charged ion impact (q/v > 1) is still unclear in strong coupling regimes [3][4][5][6][7][8][9][10][11][12][13][14][15].…”

“…The SO mechanism states that the first electron is ejected through a binary interaction with the projectile, while the second is ejected due to a sudden change in the effective potential after the removal of the first electron. The limit is expected to be 0.26% and has been established experimentally for fast proton [5], antiproton [6], electron [7], and positron [8] impact.…”