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

Abstract

A time-dependent coupled-channel approach was used to calculate ionization, excitation, and energyloss cross sections as well as energy spectra for antiproton and proton collisions with molecular hydrogen for impact energies 8 < E < 4000 keV.

Collisions of antiprotons with hydrogen molecular ions

“…This is similar to the findings for hydrogen atoms but larger than for alkali-metal atom targets [31]. Calculations for p + H 2 [38,51] in which the target was described by an atomic effective one-electron model [40] yielded maxima for ionization and excitation which lie at lower and higher energies, respectively, than the present maxima for H + 2 .…”

“…It also means that for E ≥ 100 keV an appropriate onecenter model potential might be sufficient in particular for the description of the ionization process. For these high energies satisfactory results were obtained in calculations forp + H 2 collisions [38,51] using a one-center one-electron model for the description of the H 2 molecule [40]. The calculations reproduced the experimental data [1,33] for E ≥ 90 keV.…”

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

“…Also given in Fig. 7 are the ionization potentials being the upper limits for bound state transitions as well as the minimum energy transfer into the first excited states for the three targets which are all independent of E. Theǭ exc curves for all three targets stay close to the minimum lines for all E. This is in accordance to the fact that the first excited dipole-allowed state is the dominant excitation channel as was observed in [4,27,29] and is in agreement with measurements for e¯+ H 2 collisions [45]. For decreasing E the contribution of the higher excited states increases for H and H 2 while it decreases slightly in the case of He.…”

Interaction of antiprotons with Rb atoms and a comparison of antiproton stopping powers of the atoms H, Li, Na, K, and Rb

“…All curves are smooth, fall off with increasing ǫ and no resonance structures can be seen forp impact which is also the case forp + H 2 collisions [5]. This is in contrast to the spectra for electron loss in the case of p collisions where a pronounced resonance can be observed for ǫ = E/Mp = v 2 /2 [7]. The resonance originates from the electron capture process by the p. Thereby, the ionized and then captured electron moves with approximately the velocity v of the p.…”