Dissociative recombination of antiprotonic helium ions:e+p¯He2+→p¯+

Abstract

Rearrangement reaction in electron scattering from antiprotonic helium ions ͑pHe 2+ ͒ in highly excited states, i.e., e + pHe 2+ → p +He + , is theoretically investigated by using the R-matrix method. This type of reaction can be identified as dissociative recombination ͑DR͒ which is well known in molecular physics. If the concept of the adiabatic Born-Oppenheimer separation is applied, the present exotic reaction has a similarity to the DR of HeH + molecules in the sense that the system has no crossings of the adiabatic potential curves. The present exotic DR reaction shows a variety of resonances, which can be described as a Rydberg electron attached to a pHe 2+ ion excited to an energetically forbidden state. The cross sections and rate coefficients for the present DR are calculated. As the bound motion of p-He 2+ is closer to a circular orbit, the DR reaction, which leads to the destruction of pHe 2+ , occurs less frequently. For the antiprotonic ion pHe 2+ , it turns out that the circular states are the most stable against electron impacts.

Cross sections for antiproton capture by helium ions

“…The detailed description of the theoretical formulation and the numerical method for the collision system of a heavy negative particle and a He + ion can be found elsewhere [10,25,29] and is not repeated here. Only a brief summary is given.…”

“…We needed to further propagate the R matrix from r = a to a large distance r = 50 a.u. to eliminate long-range coupling effects [10,29].…”

“…The resonance states relevant to Eq. (1) must be composed of excited He + and rather can be characterized as a collision complex closely correlated with the product channel (i.e., a Rydberg electron attached to an ion corepHe 2+ [10]). If the total energy is less than the separation limit D ofp + He + (i.e., the collision channelp + He + is closed), the resonance states primarily composed of He + (1s) are named antiprotonic helium.…”

“…Recently, careful and rigorous QM studies of the finalstate selected capture (or its inverse) process were made for the systemsp + He + [10], µ − + He + [25], and µ − + H [26,27] by means of a time-independent R-matrix method [28] combined with the direct numerical solution [29]. In these studies, the type II coordinates were adopted, and hence the final-state distribution could be calculated accurately.…”

“…Owing to the Coulomb attraction betweenp and He + , this remains true even at E → 0. In the previous R-matrix study for thē p + He + system [10], only a limited range of J was taken into account. To obtain the capture cross sections, here we perform the R-matrix calculations for J = 0-40.…”