Allison Harris

Four-body charge transfer processes in proton–helium collisions

Recent advancements in experimental techniques now allow for the study of fully differential cross sections (FDCS) for four-body collisions. The simplest four-body problem is a charged particle collision with a helium atom, in which both atomic electrons change state. This type of collision can result in many different outcomes, such as double excitation, excitation ionization, double ionization, transfer excitation, transfer ionization and double charge transfer. In this paper, we compare absolute experimental proton–helium FDCS for transfer excitation with the fully quantum mechanical 4BTTE (four-body transfer with target excitation) model. This model was previously used to study TTE for proton energies between 25 and 75 keV and reasonable agreement was found with the experimental data for large scattering angles, but not small angles. Since this is a first-order model, which contains contributions from all higher order terms, one would expect improved agreement with increasing energy and the purpose of this work was to look at higher energies. We found that the agreement with the magnitude of the experimental data became worse with increasing energy while the agreement with the shape of the data was reasonably good. Consequently, we conclude that the model contains the physical effects that determine the shape but not the magnitude of the cross section.

Post collision interactions in fully differential cross sections for four-body charge transfer processes

Recently, experimental fully differential cross sections have been reported for proton?helium collisions. In this work we will examine single capture, transfer with target excitation and double capture for proton?helium collisions. For single capture, the proton captures one electron from helium and leaves the other electron in the ground state. For the transfer-excitation case, the target is excited to arbitrary excited states. In the case of double capture, the proton captures both of the electrons from helium and leaves the collision as an H??ion. We introduce a fully quantum mechanical four-body model that includes the final state post collision interactions between all two-particle pairs exactly. In the initial state, the two-particle interactions are included asymptotically by using an Eikonal wavefunction. The results of this model are in better agreement with experimental data than previous quantum mechanical calculations.

Comprehensive study of 3-body and 4-body models of single ionization of helium

The frozen core approximation has been successfully used for many years to model 4-body collisions as 3-body collisions. We present a comprehensive comparison of 3-body and 4-body models for the process of single ionization of helium by electron impact using our 4-body distorted wave model. Differences between the two models are observed in both magnitude and peak locations. We identify four possible sources for the discrepancies between the models, and isolate the specific physical causes of the discrepancies.

Theoretical fully differential cross sections for double-charge-transfer collisions

We present a four-body model for double charge transfer, called the four-body double-capture model. This model explicitly treats all four particles in the collision, and we apply it here to fully differential cross sections (FDCSs) for proton+helium collisions. The effects of initial- and final-state electron correlations are studied, as well as the role of the projectile-nucleus interaction. We also present results for proton+helium single capture, as well as single-capture:double-capture ratios of FDCSs.

Reply to 'comment on "Four-Body Charge Transfer Processes in Proton Helium Collisions"'

Houamer and Popov have performed a first Born approximation calculation (FBA) for TTE (charge transfer with target excitation) for 300 keV proton–helium collisions. Their results are in reasonable agreement with the absolute measurements of Schoffler (2006 PhD thesis, University of Frankfurt am Main) whereas our FBA results yielded the shape of the experimental data reasonably well (except for small scattering angles) but with a magnitude that was a factor of 144 larger than experiment. Consequently, Houamer and Popov conclude that our results must have huge numerical errors. We have extensively tested our codes and we do not find any evidence to support this claim.

Fully differential cross section for four body charge transfer process

Recently experimental fully differential cross sections (FDCS) have been reported for double capture in proton helium collisions which disagree with existing theoretical calculations by two orders of magnitude. We introduce here a theoretical model for charge transfer processes which is fully quantum mechanical and takes all post collision interactions (PCI) between the particles into account exactly. The results of this model are in much better agreement with experimental data.

Four body Charge Transfer Process in Proton Helium Collision

Recent advancements in experimental techniques now allow for the study of fully differential cross sections for 4-body collisions. Theoretical fully differential cross sections will be presented and compared with absolute experimental data for transfer-excitation in proton-helium collisions. The role of different scattering mechanism will be discussed.