Role of final state correlations in electron and positron impact ionization of H and He

Single ionization of H and double ionization of He are investigated using a multiple scattering approach and a correlated final state wavefunction, which satisfies the correct asymptotic condition for the electron and positron impact ionization of atoms. Analyses presented here show the influence of the Coulomb correlations in the final state on the three- and fivefold differential cross sections. Situations are discussed in which consideration of every pair of electron-electron and/or electron-positron correlation in the final state becomes important for physically satisfactory results.     

Excitation of molecular vibrations by positron impact

Absolute cross sections for the vibrational excitation of CO, CO2, and H2 by positron impact are presented for incident positron energies from 0.5 eV to several electron volts. The measurements use a novel technique that exploits the adiabatic motion of a positron beam in a strong magnetic field. This work is the first systematic experimental study of vibrational excitation by positron impact, and extends to energies where positron measurements have traditionally been difficult. The measured cross sections are compared with available theoretical calculations.     

Laser-assisted positron-impact ionization of atomic hydrogen

We study the ionization of atomic hydrogen by a fast positron in the presence of an external linearly polarized laser field. We concentrate on the limit of a small momentum transfer and describe the fast positron's continuum states by Volkov wave functions. The ejected electron is described by a Coulomb-Volkov wave function. We are limited to small laser intensities such that the dressed state of the target is treatable within the time-dependent perturbation theory, even though the laser intensity is still quite high by laboratory standards. Numerical results for the triply differential cross sections and their dependencies on laser-field parameters are discussed and compared with the results of laser-assisted ionization by electron impact.     

Perturbation of the excited states of the hydrogen atom and hydrogen-like ions by a neutral atom

Owing to the degeneracy of the energy levels, the wavefunction of the electron in the excited states of the hydrogen atom and hydrogen-like ions perturbed by a neutral atom B is significantly different from the wavefunction of the unperturbed state. The perturbed function has a wide high maximum in the region of atom B, which is explained by multiple collisions of the electron with atom B, because the classical trajectories in the Coulomb field are closed and the size of atom B is much smaller than the size of the excited-state orbit. The radiative lifetimes of the excited states are much larger than those of unperturbed states. The orbital angular momentum L of the excited electron is strongly changed in collisions with atom B owing to the quantum interference or mixing of the temporal phases of adiabatic wavefunctions. The cross sections for such a change in the orbital angular momentum are several orders of magnitude larger than the cross sections found in early investigations in the approximation of the single collision of the electron with atom B.     

K-shell ionisation of atoms by positron and electron impacts

Recent developments in the calculation of the K-shell ionisation cross sections of atoms by positron and electron impacts are reviewed. The application of the first Born and other approximations derived by the modification of the first Born are discussed. It is pointed out that the first Born approximation modified to include exchange and the effect of the atomic field on the projectile yields satisfactory results for the light atoms.     

Absolute cross-section measurements of inner-shell ionization

Cross section ratios for K- and L-shell ionization of thin silver and gold targets by positron and electron impact have been determined at projectile energies of 30–70 keV. The experimental results are confirmed by calculations in plane wave Born approximation (PWBA) which include an electron exchange term and account for the deceleration or acceleration of the incident projectile in the nuclear field of the target atom. We report first absolute cross sections for K- and L-shell ionization of silver and gold targets by lepton impact in the threshold region. We have measured the corresponding cross sections for electron (e^–) impact with an electron gun and the same experimental set-up.     

Positronium formation for positron scattering from metastable hydrogen

Positronium(Ps) formation for positron impact on metastable hydrogen in 2s state has been studied by using the twochannel, two-center eikonal final state-continuum initial distorted wave(EFS-CDW) method. The differential, integrated,and total cross sections for Ps formation in different states have been calculated from each channel opening thresholds to high energy region. The results are compared with other theoretical calculations available in the literature. For Ps formation in s-state at intermediate and high energies, our results are in good agreement with the prediction of distorted wave theory.Those formed in p-states and the total Ps formation cross sections are reported for the first time. It is shown that the total Ps formation cross sections for positron scattering from H(2s) state are significantly larger at relatively low energies, while smaller at high energies, compared with those obtained from hydrogen in ground state.     

Inner-shell ionization by positron and electron impact

Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30–70 keV are reported. The experiments were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum. This source and the experimental equipment will be described briefly and the experimental results will be presented in detail. In addition, plane wave Born (PWBA) calculations were performed. They include an electron exchange term and the deceleration or acceleration of the incident projectile in the nuclear field of the target atom. The present experimental results agree well with these calculations; for L-shell ionization, they are at variance with a previous experiment where a different energy dependence was observed.     

Integrated Cross Sections of High-Energy <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>–</Superscript> Pair Electroproduction by an Electron in an Atomic Field

The integrated cross sections of e⁺e^– pair electroproduction by an ultrarelativistic electron in an atomic field are studied. The importance of various contributions to the integrated cross sections is discussed. The Coulomb corrections have turned out to make a large contribution to both differential and integrated cross sections even at moderate values of the nuclear charge number. The interaction of the electron emitting a virtual photon with the atomic field is shown to contribute significantly to the cross sections differential with respect to the electron transverse momentum. However, this interaction affects only slightly the cross section differential only with respect to the positron transverse momentum.     

Single differential and total ionization cross section of the helium atom by positron impact

Single differential (SDCS) and total (TCS) cross sections are calculated for the single ionization process of a helium atom by positron impact in the intermediate and high energy range (50–300 eV). To study the charge asymmetry, cross sections for electron impact ionization are also presented for comparison with the positron data. The TCS results for positron impact have been compared with existing measurements. A good agreement is noted in the high energy regime ( 100–300 eV).     

About the disparity between differential cross sections of electron and positron bremsstrahlung

The electron and positron bremsstrahlung is considered for scattering on a hydrogen atom. It is shown that a successive consideration of the atomic electron role, even in the first Born approximation, gives rise to significantly different differential cross sections of electron and positron bremsstrahlung.     

Inner-shell electron capture by impact of positron on the multi-electron atomic targets

The first-order Coulomb-Born approximation has been applied to the study of positronium formation through K-shell electron capture in the collision of positron with multi-electron atomic targets. The single-zeta Roothaan-Hartree-Fock wave functions are used to describe the electron initial bound states. The differential and total cross sections are computed for the impact of positron on helium, carbon, neon, sodium and argon atoms, with the formation of positronium in its ground state. For helium atoms, the calculated total cross sections are compared with the available experimental data and other theoretical calculations. The comparison shows a good agreement between the present calculations and the measurements.     

中低能入射电子与氦离子碰撞的(e,2e)反应三重微分截面

利用Ｂｅｒａｋｄａｒ和Ｂｒｉｇｇｓ对ＢＢＫ波函数Ｓｏｍｍｅｒｆｅｌｄ参数的修正结果（即考虑第三个粒子存在对两个粒子间相互作用的影响），并考虑了入射道的库仑相互作用及出射电子的交换对称性，计算了在共面－等能分享－固定相对角度几何情况下，入射能量分别为:５８．４、６８、８１．６、１０８．８和１７４．４ｅＶ的Ｈｅ＋（ｅ，２ｅ）反应三重微分截面．结果表明，在较低碰撞能量散射中，入射道库仑场对两体碰撞起主要作用． A corrrelated three body continuum wavefunction proposed by Brauner et al has been recently modified further to allow for three body effects by introducing Somerfeld parameters (Berakdar and Briggs). The triple differential cross sections for electron impact ionization of He+ in coplanar equal energy sharing and fixed relative angle geometry are calculated. The initial channel wavefunction involves a Coulomb wavefunction due to the long range Coulomb attraction between the incident...     

Monte Carlo calculations of keV electron and positron slowing down in solids. II

An improved Monte-Carlo simulation technique has been used to investigate positron and electron slowing down in solid matter. Elastic scattering is based on exact cross sections of effective crystalline potentials and inelastic processes are described by Gryzinski's semiempirical expression for each core and valence electron excitation. Calculations with normal and oblique angles of incidence have been made for positrons and electrons impinging on semi-infinite aluminium, copper, tungsten, and gold. Interesting differences have been found between positron and electron penetration and backscattering features.     

Electron-atomic-hydrogen “elastic" scattering in the presence of a laser field

Laser-assisted electron-atomic-hydrogen “elastic" scattering is studied in the first Born approximation. The initial and final states of projectile electron are described by the Volkov wavefunctions; the dressed state of target described by a time-dependent perturbative wavefunction in soft photon approximation. The laser modified cross-sections are calculated in two distinct geometries for laser polarization either parallel or perpendicular to the incident direction of electron. The numerical results shows that the multiphoton cross-sections oscillate by a few orders over the whole scattering angular region. The results for a parallel geometry oscillate more frequently in the intermediate angles; while the results for a perpendicular geometry oscillate more frequently in the forward and backward angles. At large scattering angles, the sum rule of Kroll and Watson is noticeably violated. The laser modification on summed total cross-section increases with field strength, but decreases with field frequency and polarization deviation from the incident direction. Received 18 October 2001 and Received in final form 26 December 2001     

Resonance two-photon pair production on nuclei and on electrons

Expressions are obtained for the cross section of photoproduction of an electron-positron pair on a nucleus and on an electron in the field of a weak electromagnetic wave in the resonance region corresponding to the virtual electron (positron) being on the mass shell. The magnitude of these cross sections relative to ordinary pair production by a photon on a nucleus and on an electron is estimated. It is shown that this mechanism can enhance pair production by a photon on a nucleus and on an electron in the resonance region with simultaneous detection of the electron (positron) at a large angle.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 12–15, February, 1981.We thank A. A. Sokolov for his interest in the work.     

Cq+ 和Oq+与原子碰撞的电子俘获过程的理论研究

在ECPSSR理论的基础上， 利用OBKN近似描述电子俘获过程， 得到了包括电子俘获过程贡献的ECPSSR理论， 编写了相应的计算程序。 采用该程序计算了不同电荷态离子与多种靶原子碰撞的电子俘获截面和相应的X 射线产生截面， 将计算得到的包含电子俘获过程贡献的X 射线产生截面与实验结果进行了比较。 对于具有满K壳层的入射离子碰撞， X 射线产生截面与入射离子电荷态基本无关； 对于以直接电离为主导的碰撞过程， 计算得到的X 射线产生截面与实验数据符合得很好； 对于全裸和单K空穴入射离子的碰撞， 计算高估了X 射线产生截面。 Based on the ECPSSR theory, the contribution of the electron capture is described by the OBKN theory. The cross sections of electron capture and X ray production for the collision of different charge state projectiles with various targets are calculated, and compared with the available experimental data. It is found that the obtained X ray production cross sections are almost independent of the projectile charge states for projectiles without a K vacancy. For the collision processes of direct ionization, the present calculated results agree well with the experimental data. The calculations overestimate the X ray production cross sections for the projectiles with full and a K vacancy.     

Doubly-differential ionization cross sections of positron impact on argon

We report the first measurement of doubly-differential ionization cross sections for positron impact on argon atoms. Energy- and angle-resolved measurements of ejected electrons in time correlation with the produced and detected ions have been performed. Corresponding measurements with incident electrons were made for comparison. With positrons and electrons as primary particles of 100 eV energy and ejected electrons of 15 eV, our measurements were extended over electron-emission angles from 0 to 90. Lacking theoretical predictions for the doubly-differential ionization of argon, we compare our measured data with the theoretical doubly-differential ionization cross sections, calculated for positron and electron impact on hydrogen by Klar and Berakdar (Freiburg) [1]. The angular dependence of positron and electron cross sections for argon agrees well with the theory for hydrogen. In particular, we found that for small angles of electron ejection the cross section for positron impact ionization exceeds that for electron impact by an order of magnitude in accordance with the predictions of Klar and Berakdar.     

Positronium beam technology

A newly constructed magnetically-guided positron beamline is described and investigations of positronium (Ps) interactions are reported. Using a timed beam and a double cell arrangement, the first direct measurements of Ps-Ar total cross sections have been made at intermediate energies. Preliminary results are presented.     

Ionization of ytterbium atoms from an excited state

The cross sections of the photoionization and the electron impact-induced ionization of Yb atoms from the excited 6s6p(³ P ₁) state are numerically calculated. Matrix elements are computed in multielectron relativistic and nonrelativistic approximations with allowance for the superposition of configurations and a relaxation effect. The radial part of the electron wavefunction in a continuous spectrum is calculated using the solutions to one-configuration Hartree-Fock and Dirac-Fock equations. The cross sections calculated by a relativistic method are compared to those for a nonrelativistic approximation. The ratios of the radiation reduced matrix elements and the phase shifts of the wavefunctions of a continuous spectrum calculated for the 6p ɛs and 6p → ɛd transitions are compared to the values obtained by approximating the experimental dependences of the angular distribution of photoelectrons for the photoionization by ultraviolet radiation from an oriented excited state.