Positron-impact Ionization Of Hydrogen Research Articles

Differential Analysis of the Positron Impact Ionization of Hydrogen in Debye Plasmas

In this work, a theoretical differential analysis of the positron impact ionization of hydrogen embedded in weakly coupled plasmas at an impact energy of 80 eV is developed. While the total and singly differential cross sections are analyzed within the classical trajectory Monte Carlo method, a Born-3DW model for screened environments, recently introduced by the authors, is used to provide a fully differential view of the process. The present results suggest that the electron emission spectra are strongly affected by the level of screening of the surrounding medium, mainly due to the loss of the postcollisional interaction mechanism.

Deep Minima in the Triply Differential Cross Section for Ionization of Atomic Hydrogen by Electron and Positron Impact

We investigate ionization of atomic hydrogen by electron- and positron-impact. We apply the Coulomb–Born (CB1) approximation, various modified CB1 approximations, the three body distorted wave (3DW) approximation, and the time-dependent close-coupling (TDCC) method to electron-impact ionization of hydrogen. For electron-impact ionization of hydrogen for an incident energy of approximately 76.45 eV, we obtain a deep minimum in the CB1 triply differential cross section (TDCS). However, the TDCC for 74.45 eV and the 3DW for 74.46 eV gave a dip in the TDCS. For positron-hydrogen ionization (breakup) we apply the CB1 approximation and a modified CB1 approximation. We obtain a deep minimum in the TDCS and a zero in the CB1 transition matrix element for an incident energy of 100 eV with a gun angle of 56.13 ° . Corresponding to a zero in the CB1 transition matrix element, there is a vortex in the velocity field associated with this element. For both electron- and positron-impact ionization of hydrogen the velocity field rotates in the same direction, which is anticlockwise. All calculations are performed for a doubly symmetric geometry; the electron-impact ionization is in-plane and the positron-impact ionization is out-of-plane.

Vortices in the three-body electron–positron–proton continuum system induced by the positron-impact ionization of hydrogen

By means of a systematic theoretical study of the transition matrix element T for the ionization collisions in a collinear geometry, we uncover the presence of three isolated zeros at intermediate impact energies. We demonstrate that these zeros actually represent vortices in the generalized velocity field associated to T. One of these vortices is shown to be related to a deep minimum observed more than two decades ago by Brauner and Briggs in 1991 at extremely large impact energies, where the corresponding fully differential cross section was too small to be experimentally accessible. Here we elucidate that it might still be present at much smaller impact energies, thus being amenable to experimental investigation. Furthermore, we discovered that this vortex is paired with a second one of opposite circulation, in accordance with one of the scenarios for their emergence. We study the location of these vortices and find that they seem to be located at, emerge from, or move towards specific points which could be related to particular collision mechanisms, as the direct ionization and capture to the continuum cusps, or the saddle-point and Thomas processes.

Near-threshold positron impact ionization of hydrogen

The hyperspherical hidden crossing method (HHCM) is used to investigate positron impact ionization of hydrogen near threshold. An important feature of this method is that it can provide valuable insight into scattering processes. In the calculation of positron–hydrogen ionization, the adiabatic Hamiltonian is expanded about the Wannier saddle point; anharmonic corrections are treated perturbatively. The S-wave results are consistent with the Wannier threshold law and with the extended threshold law that was previously derived using the HHCM. We have extended the previous HHCM calculation to higher angular momenta L and have calculated the absolute ionization cross-section for L=0, 1 and 2. The HHCM calculation confirms that the S-wave ionization cross-section is small and provides the reason why it is small. The HHCM ionization cross-section (summed over the lowest partial waves) is compared with a convergent close-coupling calculation, a 33-state close-coupling calculation and experimental data.

Positron-impact ionisation of hydrogen near the threshold

The near-threshold behavior for positron-impact ionization (breakup) of hydrogen is considered. The two-centre convergent close-coupling method is used. We find that convergence in the cross section is only practically obtained if two near-complete expansions are used, one centred around H and the other around Ps. The contribution to the breakup cross section becomes the same for both centres as the threshold is approached. The calculations are found to be in good agreement with the threshold law predicted by Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)].

Coulomb correlations in electron and positron impact ionization of hydrogen at intermediate and higher energies

Investigating the relation between the asymptotic condition and the dynamic Coulomb correlation for single and multiple ionization the authors discuss a complete set of spatially separable N-electron final-state wavefunctions, satisfying multiple ionization boundary conditions. They apply these results to electron and positron impact ionization of atomic hydrogen in the energy range 54.4 and 250 eV on the basis of a parameter-free model formulated within the scope of the multiple scattering approach. A comparison between their results and available experimental data and alternative theoretical calculations are made and discussed. The role of exchange is also investigated. The contribution of exchange in the triple differential cross section is found to be virtually negligible whereas the asymmetry parameter A is significantly affected.

Structures in differential cross sections for positron impact ionization of hydrogen

A singular structure in the differential cross section for electron ejection by positron impact occurs when the positron and the ionized electron have low relative momentum in the final channel. This is the analogue of the process of 'electron capture to the continuum', well known in positive-ion impact ionization. Near the singularity a dip, due to interference between two double-binary scattering processes, is predicted if the scattering angle of positron and electron equals the critical angle of 45'. Dip and singularity coincide in the high energy limit.

Triple differential cross-sections for the fast electron/positron impact ionization of hydrogen and helium in the Glauber approximation

Triple differential cross-section for electron and positron impact ionization of hydrogen and helium are calculated by using the Glauber approximation along with post-collision interaction effects which are estimated classically. The present results are compared with the recent absolute data of Ehrhardt, Jung and coworkers for the electron impact case. The positron impact case is found to lead to a larger binary to recoil peak maxima ratio (compared to the electron impact case) which further increases when post-collision interaction effects are included.

Triple differential cross sections for the positron-impact ionization of hydrogen and helium

Triple differential cross sections for the positron-impact ionization of hydrogen and helium in a coplanar, asymmetric geometry are calculated by using the second Born and the modified Glauber approximations. The present results are compared with those for the corresponding electron-impact case. The positron gives a larger binary- to recoil-peak maxima ratio than the electron.