Ionization of helium and molecular hydrogen by slow antiprotons

Abstract

Measurements of the cross sections for single and double ionization of helium as well as for the creation of H2+ and H+ ions from H2 for impact of antiprotons in the energy range 13-500 keV are presented. The results are compared with our earlier, less accurate data and with data for equivelocity proton impact. The single ionization cross section of helium agrees remarkably well with a continuum distorted wave (CDW-EIS) calculation. The ratio between the double and the single ionization cross sections of helium increases dramatically with decreasing projectile energy. New theoretical calculations are called for.

Collisions of low-energy antiprotons with molecular hydrogen: ionization, excitation and stopping power

“…1 the present H 2 results for ionization and electron loss byp and p impact, respectively, are compared with cross sections given in the literature and results for atomic hydrogen multiplied by two. It can be seen that the naive picture of the H 2 molecule as being practically the same as two independent H atoms only holds at high impact energies E. For high and intermediate impact energies the present p results agree with the experiments by Anderson et al [1] and Hvelplund et al [5] although they are slightly lower than the measured data. At low energies E < 20 keV, however, the experimental results are significantly smaller than the present calculations which partly can be explained with experimental problems not accounted for at that time [6].…”

“…At low energies E < 20 keV, however, the experimental results are significantly smaller than the present calculations which partly can be explained with experimental problems not accounted for at that time [6]. These problems also occurred forp + He cross sections measured with the same apparatus [5] and were identified by very recent He measurement [7]. The other theoretical curve [2] satisfyingly.…”

“…data. The ionization cross sections [1] used as input for the analysis in [4] were, however, found to be erroneous for E < 200 keV [5,6]. On the other hand, the slope of the present curve for E > 100 keV is remarkably similar to that of the experimentalp curve but for E > 160 keV.…”

“…In the case of hydrogen also a number of contributions exist for atomic targets whereas the knowledge on low-energyp collisions with the simplest two-electron molecule H 2 is rather poor [1,5], especially, concerning theory [2,11]. Precise data onp + H 2 are, however, of great interest in many fields.…”

Stopping power of antiprotons in H,H2, and He targets

“…The shown S µr esults depend therefore also on additional data which were taken from literature. The experimentalp ionization cross sections σ ion [34] used in order to determine S µ¯, however, were later on found to be erroneous for E < 200 keV [2,35]. Due to these substantial uncertainties it is one aim of this work to discriminate with the help of the present findings between the different experimental results.…”