Differential positronium-formation cross sections for Ne, Ar, Kr, and Xe

Abstract

Experimental determinations of the absolute differential positronium-formation cross sections near 0 • for Ne, Ar, Kr, and Xe are presented and compared with theory. The degree of forward collimation, expressed by the ratios of the differential-to-integral positronium-formation cross sections, is also computed and compared with theories and other targets. Trends among targets and structures at low energies emerge when considered as a function of the reduced total energy.

Deep Minimum and a Vortex for Positronium Formation in Low-Energy Positron-Helium Collisions

“…The Ps-formation DCS near the forward direction has been recently measured for positron collisions for a number of targets including helium [4][5][6]. While a dip was found in the measured near-forward direction Ps-formation DCS for positron collisions from Ne, Ar, Kr and Xe [5], it was not observed in the cross section for positron-helium collisions for the energy range of the experiment [4,6].…”

“…The Ps-formation DCS near the forward direction has been recently measured for positron collisions for a number of targets including helium [4][5][6]. While a dip was found in the measured near-forward direction Ps-formation DCS for positron collisions from Ne, Ar, Kr and Xe [5], it was not observed in the cross section for positron-helium collisions for the energy range of the experiment [4,6]. However, we theoretically obtained a minimum in the Ps-formation DCS (θ = 0 • ) for positron-helium collisions at 17.9 eV, which is at a lower energy than the energies considered in the experiment for helium [4,6].…”

“…Positronium (Ps) formation, a rearrangement process for positron collisions, is of interest in both theoretical and experimental studies [1][2][3][4][5][6]. Recently, we obtained two zeros in the Ps-formation scattering amplitude, f Ps , for positron-hydrogen collisions in the Ore gap [7].…”

“…We determine the Ps-formation DCS as a function of k and θ, where θ is the angle of the outgoing Ps with respect to the z-axis, which is taken to be parallel to the momentum of the incident positron k. Due to the experimental interest of the near-forward direction Ps-formation DCS for positron-helium collisions [4][5][6], we also compute this cross section using the complex Kohn K-matrices and using different fits for the K-matrices. In the near-forward direction, the Ps-formation DCS for positron collisions from Ne, Ar, Kr and Xe has recently been measured and a dip at a low energy is present in this cross section for each of these target atoms [5]. Fayer et al [5] raised the question of whether the low-energy features that they obtained for the Ps-formation DCS near 0 • are related to quantum vortices.…”

“…In the near-forward direction, the Ps-formation DCS for positron collisions from Ne, Ar, Kr and Xe has recently been measured and a dip at a low energy is present in this cross section for each of these target atoms [5]. Fayer et al [5] raised the question of whether the low-energy features that they obtained for the Ps-formation DCS near 0 • are related to quantum vortices. Interestingly, we find that there is a valley in the logarithm of the Ps-formation DCS for positron-helium collisions that contains a minimum in the forward direction and a deep minimum that is connected to a vortex in v ext .…”