Table of contents

The latest, most important developments in (e,2e) studies are reviewed from an experimental point of view. These include ionization processes near threshold, the symmetric energy sharing collisions involving double scattering mechanisms, core states ionization, (e,2e) measurements for autoionizing states and for the simultaneous excitation and ionization of the target. Double ionizing processes with coincidence detection of two or three electrons in the final state are also reviewed, including (e,e'-Auger) experiments and the new born (e,(3-1)e) and (e,3e) domain.

Paul (1989) has shown from an analysis of experimental data that there is a discrepancy between a best cross section for K-shell ionization of atoms by fast protons and a finite Hilbert basis state calculation. In this paper the latter method is shown to give excellent agreement with a basis free calculation using Hartree-Fock continuum states for first Born SCA cross sections. The Paul discrepancy remains.

The first order relativistic corrections to the polarizabilities in hydrogen-like systems are calculated from an effective spinor equation for particles with arbitrary g factors. Spinless particles are included by setting g=0. For this case, the alpha ⁴ correction to the dipole polarizability is also given. The effects are detectable in He Rydberg states and confirm calculations by Drake.

The authors examine the nodal structure of accurate helium wavefunctions calculated by direct diagonalization of the full six-dimensional problem. It is shown that for fixed interelectronic distance R (or hyperspherical radius R) the symmetric doubly-excited resonant states have well defined lambda , mu nodal structure indicating a near separability in prolate spheroidal coordinates. For fixed lambda , however, a clear mixing of R, mu nodes is demonstrated. This corresponds to a breakdown of the adiabatic approximation and can be understood in terms of the classical two-electron motion.

A simple method, based on the sudden approximation, is presented for estimating the probabilities of shake-up, shake-down and shake-off of a spectator Rydberg electron during radiative decay. An analytic model for the relevant matrix elements is developed using quantum defect theory. This model, which depends on a few easily calculable parameters, is needed to extend Hartree-Fock numerical calculations, and is useful in predicting behaviour in the limit of large initial-state excitation. In this limit the probability of shake-up/down is shown to tend to a constant, as is the probability of no-shake. In this same limit, the probability of shake-off is shown to approach zero. The specific case of the Ne(1s)np(¹P) to (2p)xp(¹P) radiative transition is calculated, where x represents any bound (shake-up/down) or continuum (shake-off) final-state orbital.

A high resolution, monochromatized He II alpha , photoelectron spectrum of the 5s correlation satellites in Xe is presented. Approximately 70 new lines, previously not observed in photoelectron spectroscopy, were assigned using optical data. It is found that the dominating lines in the spectrum are associated with levels of high angular momentum.

The authors present the results of ab initio (SCF and CEPA) calculations of the interaction potentials for the C(³P)-H₂(¹ Sigma _g⁺) system. The five potential curves corresponding to collinear (³ Sigma ^-,³ Pi ) and orthogonal (³A₂,³B₁,³B₂) approaches of the C atom to the midpoint of the H₂ molecule have been computed, as well as the curve of ³A' symmetry for an intermediate geometry. These results are analysed and used in quantum mechanical (close-coupling) calculations of the cross sections for excitation of the fine structure transitions in C(³P_J) by ortho- and para-H₂. The rate coefficients for these transitions are evaluated for a range of temperatures encompassing those relevant to studies of the interstellar medium.

A computational study of four new phase rules has been carried out for rotationally inelastic diffraction scattering in sudden-atom-homonuclear-molecule collisions. The new phase rules are obtained from the weak coupling limit of the infinite-order-sudden approximation, when expressed in a semiclassical complex angular momentum form. Calculations have been performed for a deformed inverse power potential of the type in (r( gamma )/r)ⁿ, n>or=3, where r( gamma )=r₀+r₂P₂(cos gamma ). The accuracy of the new phase rules is studied by systematically varying the steepness parameter n and the anisotropy parameter r₂ of the interaction potential. Results are also reported for the Ne-D₂ and He-N₂ collision systems.

L X-ray line production cross sections for proton and ⁴He ion bombardment of elements with atomic number Z ranging from 59 to 70 have been measured in the energy range 0.25-1.80 MeV for protons and 0.40-1.80 MeV for ⁴He ions. An overall good agreement with the few existing data has been observed. The linear dependence on Z of the logarithm of the cross sections has been confirmed also in this region. The agreement of the measured data with the predictions of a modified ECPSSR model, suggested by the authors and based on a recalculation of the binding correction (UA-ECPSSR), is generally good, except for the lowest incident energies.

The authors show that the simple two-amplitude Blair-Anholt formula for ionization in the presence of a nuclear resonance can be generalized to electron capture. This generalization has been obtained from a fully quantum mechanical treatment by performing approximations which are the large-angle counterpart of those used to obtain the standard semiclassical t matrix for forward scattering. The resulting formula involves electronic amplitudes which must be calculated for a prescribed broken straight line nuclear trajectory. Application of this simple two-amplitude formula to different systems shows that it is able to describe the gross features of the electron capture across a nuclear resonance.

Absolute X-ray and VUV emission cross sections have been measured for collisions of H-like C⁵⁺ and N⁶⁺ ions with H₂ and He in the 0.18 au< nu <0.55 au velocity range. From the comparison of both types of measurements it is possible to determine to what extent the population of the 1s2p state is due to single electron capture and to double capture followed by autoionization. The authors' X-ray and VUV data are compared with electron capture cross section data from other authors, obtained either by electron spectroscopy or by charge state analysis. Mostly a good agreement with these data is found.

The authors have measured the cross sections for single electron loss and single electron capture for 0.6-2.2 MeV He⁺ colliding with He, Ne and Ar using the growth rate method and a molecular beam target. The results are compared with other available experimental data. For single electron loss theoretical comparisons are also made with the quantal first Born approximation and some semiclassical calculations. For single electron capture, comparisons are made with the modified classical calculations based on the binary encounter model. To date there seems to be no one theory which accurately describes the data. The theory of Kaneko (1985, 1986) is clearly promising for the neon target at energies greater than 0.15 MeV amu^-1 but this could be fortuitous since the theory does not similarly reproduce the data for helium and argon targets. The theory of Kumar and Roy (1979) reproduces the energy dependence of the capture cross sections in the range 0.1-1.0 MeV amu^-1 but the absolute values of the cross sections are subject to a scaling of the effective charge of the He. Unfortunately no single value of this effective charge applies to all three target gases.

The production of highly excited (24<n*<43) xenon atoms has been investigated in 2-20 keV Xe⁺ collisions with helium, neon, argon and methane using the field ionization technique. Cross sections are found to scale as (n^*)^-3 and display regular oscillatory structure characteristic of quantum mechanical phase interference.

The 0' delta -electron emission was investigated by an electron-projectile coincidence technique as a function of incoming and outgoing projectile charge state for 0.53 MeV u^-1 Cu^q+ on He. The electron emission spectra vary strongly with initial and final projectile charge state. For pure ionization channels the cross sections follow for low electron energy the q²-scaling law whereas in the binary encounter regime the scaling is reversed. CTMC calculations are in fair agreement with the experimental data.

The electron-ion dissociative recombination rate coefficients, alpha _e, have been determined for ten molecular ions, seven of which have not previously been measured. The data were obtained under truly thermal conditions at 300 K using a flowing afterglow/Langmuir probe (FALP) apparatus. Recombination coefficients were obtained for the following ions: CO₂⁺ ( alpha _e=3.1*10^-7 cm³ s^-1), N₂⁺ (2.0), CO⁺ (1.6), KrH⁺ (<0.2), XeH⁺ (<0.4), HSO₂⁺ (2.7), CH₃CNH⁺ (3.3), CH₃CHOH⁺ (3.9), C₂H₅CNH⁺ (4.7) and ((CH₃)₂CO)₂H⁺ (14). The dissociative recombination of many of the ions studied may be important in planetary, cometary and terrestrial atmospheres and be involved in the synthesis of interstellar molecules.

Ground-state capture cross sections in e⁺-H₂ scattering are calculated in the energy region 10-1000 eV using the first Born approximation. The approximation employed by Sural and Mukherjee (1970) in predicting the first Born capture cross section is found to be unjustified. Present capture cross sections for incident positron energies E>or=30 eV are in good agreement with the measured data.