Table of contents

Semi-empirical pseudopotentials can be modified in a simple way so as to reproduce frozen-core Hartree-Fock or Dirac-Fock results, respectively. The influence of relativistic effects is quantitatively discussed (i) for (rⁿ) expectation values and dipole polarisabilities of group IA and IB atoms, and (ii) for the bond lengths of K₂⁺ and Cu₂.

A recent formula due to Dubau and Seaton (1983) for the parametrisation of a Rydberg series of autoionising resonances is discussed and related to expressions given by other authors. A simple method of finding the four parameters is described and illustrated by an application to the autoionising spectrum of Xe I.

The angular asymmetry parameter beta for 5s photoionisation in xenon has been measured in the energy range where the relativistic random-phase approximation (RRPA) including 5s, 5p and 4d interchannel couplings predicts a strong deviation from the nonrelativistic value of two. The authors' data clearly show a deviation from the value two but not as pronounced as predicted by the RRPA calculation. This indicates that the theory has to be further improved in this case.

Absolute excitation cross sections for single-electron capture into selected n, l states have been determined by VUV spectroscopy for impact of 80 keV C⁴⁺ on He, H₂ and Li. For all three collision systems the predominant final ion states belong to one or two electronic shells only. Within such shells a preferential population of higher l sublevels is found.

Electron impact vibrational excitation cross sections (differential, integral and momentum transfer) for the symmetric (100) and the bending (010) modes of the H₂O molecule are calculated in the fixed-nuclei and the impulse approximations from threshold to 10 eV. The normal coordinate dependent K-matrix elements are augmented with the laboratory frame first Born matrix elements for higher angular momenta in order to obtain converged cross sections via a closure formula. A sharp threshold peak and a broad resonance structure is seen around 6-8 eV in the integral cross section in agreement with the recent observations: while the A₁ scattering state plays a not unimportant role, the major contribution to this broad feature is due to the B₂ state. All results compare well with the crossed-beam experimental results of Seng and Linder (1974).

Electron impact inner-shell ionisation followed by autoionisation can dominate the total double-ionisation cross section for certain atomic ions. For Xe⁺ the indirect contribution from the 4d¹⁰ 5s² 5p⁵ to 4d⁹ 5s² 5p⁵ ionisation followed by Auger decay is calculated to be three times the direct 'double-knockout' contribution. The distorted-wave inner-shell ionisation cross section is found to be strongly influenced by both ground-state correlations and term dependence in the ejected-electron continuum. The authors' Xe⁺ results are in reasonable agreement with the recent experimental crossed-beam measurements of Achenbach et al. (1983).

The elastic scattering and excitation cross sections for the (6s6p)³P_0,1,2⁰ states in e-Hg scattering are calculated from 4-7 eV using the relativistic R-matrix method. Resonances corresponding to the 6s6p² configuration are found and their effect on the cross sections is discussed. In most cases good agreement with experiment is obtained, and certain features have been clarified.

A recently published calculation by Drawin and Truong-Bach (ibid., vol.15, p.4477, 1982) concerning the high-frequency Stark effect is shown to be in error. 'Negative intensities' of satellite lines result from an incorrect mathematical treatment of the problem.

Analytical expressions for the momentum representation of the relativistic Hartree-Fock-Roothaan wavefunctions are obtained by means of the Fourier-Bessel transformation. The momentum distributions are calculated by the use of these expressions and the results are compared with the corresponding distributions of the nonrelativistic and relativistic hydrogenic wavefunctions as well as the nonrelativistic Hartree-Fock-Roothaan wavefunctions.

The hyperfine structure of the 6snd Rydberg configurations of barium has been investigated using a two-step excitation process and an atomic beam. Accurate hyperfine energies have been obtained on both the ¹D and ³D levels, which are analysed in terms of singlet-triplet mixing. The mixing coefficients extracted for the ¹D₂ and ³D₂ levels are shown to be internally consistent, to agree with known Lande factors and can well be predicted with existing multichannel quantum defect analyses. Finally the asymptotic behaviour of the singlet-triplet mixing coefficient is discussed in the framework of simple atomic theory.

Photoionisation cross sections of the main ionisation process of neon have been calculated for a wide range of photon energies. The partial cross sections were computed as a sum of a contribution expressible in terms of the Dyson amplitude corresponding to the ionisation processes considered plus a correction term. The numerical results obtained, in satisfactory agreement with the experiment, show, as expected, that for outer-shell ionisation the correction term decreases rapidly with the photon energy. For inner-shell ionisation, however, the correction term appears to be still important at relevant ( approximately 100 eV) energies of the photoelectron. Results given by different choices of the photoelectron waves are also presented.

The emission bands of diatomic mercury bromide molecule lying in the ultraviolet region have been reinvestigated under moderately high resolution. The bands observed in the wavelength range 2770-2700 AA (previously identified as a new D'-X system) simply form an extension (to high mod Delta v mod values) of the well known C-X system. The vibrational assignment of the bands has been confirmed by measuring the well resolved bromine isotope shift. In some bands structure due to mercury isotopes is also resolved. Precise vibrational constants for the X, C, D and E states and approximate rotational constants for the X, C and D states are reported. The E-X transition is shown to consist of two subsystems indicating that the E state is a ² Pi state.

A semiclassical complex angular momentum (CAM) description of low-energy elastic collisions is presented. The scattering amplitude is divided into two parts: the direct part is described by non-orbiting semiclassical trajectories, while the orbiting part is described by a small number of poles of the S matrix. The system H⁺-He was studied in some detail.

Dipole-dipole energy transfer is one of the major mechanisms in the collisional deexcitation of excimers by simultaneous ionisation of excited neutrals. Based on this mechanism, the cross sections for collisional de-excitation of KrF* with several excited species are calculated to be 110-170 AA² at thermal energies.

The authors studied, by energy loss spectrometry, the direct elastic and inelastic processes and measured their differential cross sections (DCS), with varying energy and scattering angle, for the He⁺-He collision; the energies range from 3 to 50 keV (laboratory) and the scattering angles are varied from 0.25 to 3 degrees (laboratory). To explain the elastic DCS behaviour, they elaborated a simple theoretical model which gives a good agreement with the experiment, chiefly at high energies. It also allows them to calibrate the experimental results and gives absolute values of the DCS. Further, by integration of the DCS over all scattering angles, they obtained total cross sections (TCS) for the inelastic processes.

K-shell X-ray production cross sections of ₄₈Cd, ₅₀Sn, ₅₂Te, ₅₃I and ₅₆Ba have been measured for alpha particles with energies between 2.2 and 2.8 MeV. In addition, production cross sections for ₅₂Te have been measured for protons between 1.6 and 3 MeV. The values have been compared with the earlier experiments and with the theoretical predictions given by four different approaches: the perturbed stationary state model corrected for relativistic, projectile Coulomb deflection, and energy-loss effects, the plane-wave Born approximation (PWBA) with corrections for relativistic effects, effective binding and Coulomb deflection, the semiclassical approximation corrected for the same effects as the PWBA in its most recent formulation and, lastly, the binary model corrected for effective energy and relativistic effects. The different degrees of agreement for each theory in the cases examined are discussed.

The Dirac equation for an electron moving in time-dependent electromagnetic fields is solved by the method of finite differences. For simplicity the solution is restricted to problems with a rotational symmetry about an axis. The method is applied to the scattering of Cs²⁰⁺ on a hydrogen-like U⁹¹⁺ ion at relativistic incident energies. Excitation and ionisation probabilities and the K-shell vacancy production probability are calculated.

Total cross sections are presented for the charge transfer reaction X^q++H(1s) to X^(q-1)+H⁺ and ionisation process X^q++H(1s) to X^q++H⁺-e^-. Here X^q+ is a fully or partially stripped ion. The collision energy range lies between 25-200 keV amu^-1. Cross sections are also given for charge transfer into particular ionic n states and n, l states for energies of 25 and 50 keV amu^-1, where X^q+=H⁺, He²⁺, C⁶⁺, O⁸⁺. The calculations were performed by the classical trajectory Monte Carlo method of Olson and Salop (1977), modified to allow for a better representation of the H(1s) electronic radial distribution. Good agreement is seen with previous classical calculations and with quantum calculations performed by Ryufuku and Watanabe (1979) and others. The authors' results exhibit excellent accord (within 20%) with the measurements of Shah and Gilbody (1981).

Systematic experiments on transfer ionisation processes A^q++B to A^(q-k)++Bⁱ⁺(i-k)e+ Delta E have been performed where A and B are rare-gas atoms, q is the charge state of the incident ions (up to 7 for Ne, 9 for Ar, 12 for Kr and 15 for Xe), k is the number of captured electrons found with the projectile long after the collision (k=1, 2 or 3) and i is the charge state of the slow recoil ion (measured by a time-of-flight coincidence technique). The projectile energies range from 3q keV to 15q keV; however, with very few exceptions, the charge-state fractions F_i of recoil ions were found to be independent of the energy in the energy range studied. The ionisation of five additional target electrons in 7% of the two-electron-capture collisions of Xe¹⁵⁺ ions with Xe atoms shows the importance of transfer ionisation in such collisions. The observed charge-state fractions of the recoil ions strongly depend on k, the number of electrons captured, and on q, the charge state of the projectile; the mean charge state (i)= Sigma _jjF_j/ Sigma _jF_j increases strongly with increasing k and q. In general, the investigated processes are exoergic, and the charge states of the recoil ions produced are correlated with the potential energy available in the collision system. When the charge-state fractions F_i observed for a given target atom are plotted as a function of the maximum potential energy Delta E calculated for transitions between ground states they fall on common curves which are only specified by the charge state i and the number of electrons k captured indicating that F_i depend explicitly neither upon the projectile species nor upon the incident charge state.

Cross sections for single charge exchange from Ar⁶⁺ incident on He have been calculated for ion energies in the range 1 to 200 keV (0.025 to 5 keV amu^-1). The adiabatic potential energies and the relevant radial and rotational coupling matrix elements of the system Ar-H⁶⁺ are first calculated using a model potential method. The collision dynamics are then treated in the framework of the semiclassical (impact parameter) approximation. The cross sections for electron capture into a specific excited state of Ar⁵⁺ are compared with experiment. The validity of the theoretical model and the possible influence of translation effects are discussed.

The authors have measured K vacancy production probabilities with decelerated highly-charged Kr^q+ (2.8 MeV u^-1) on Kr and Xe^q+ (3.6 MeV u^-1) on Xe, by particle-photon coincidences. The impact-parameter-dependent probabilities reveal a dramatic change in shape and absolute height as soon as the projectile bears L-shell vacancies. From a comparison with calculations they conclude that K- to L-shell charge transfer can be described, even in the heavy collision system Xe on Xe, by the 2p pi -2p sigma rotational coupling mechanism when the relativistic energy splitting is taken into account. Deviations from a statistical distribution of the projectile L vacancies between the molecular levels are found.

Charge state spectra of ions generated in single collisions of 20 to 50 MeV electrons with rare-gas atoms have been measured by a time-of-flight technique. Relative abundances of ions in charge states up to 2+ for He, 3+ for Ne, 5+ for Ar, 7+ for Kr and 8+ for Xe were deduced from the experiment. Higher charge states, e.g. 12+ for Xe, could also be observed; however their quantitative fractions could not be determined. The relative abundances F_i of the different ion charge states i are constant in the investigated energy range; however they rapidly decrease with increasing i-strongly dependent on the atomic number of the bombarded target. The experimental results are in good agreement with calculations based on inner-shell ionisation cross sections folded with charge state distributions which result from the decay of the corresponding initial vacancies. The capability of fast electrons for efficient production of slow highly charged ions in single collisions with atoms is discussed.

Time histories of the emission of a number of spectrum lines of the ions N⁺ to N⁵⁺ have been measured in a theta-pinch discharge of maximum axial electron temperature 100 eV and electron density 8*10¹⁵ cm^-3. The lines observed in the discharge are subordinate lines in the quartz UV region and the observations were made both end on and side on to the discharge axis. Ionisation rate coefficients have been inferred from comparison with a theoretical model of the transient ionisation. The rates are conveniently expressed as scaling factors of an empirical ionisation rate expression due to Kunze (1971). These factors are 1.5 and 0.55 for N³⁺ and N⁴⁺ and the electron temperatures at the times of maximum abundance of these ions are 32 eV and 68 eV respectively. The result N⁴⁺ agrees well with the measurements of Kunze.

In a recently published paper (Godefroid, 1982) the relations between the radial integrals of the mutual electronic spin-orbit interaction are derived. These and the other required relations were presented in the author's book (Jucys and Savukynas 1973) and in the references cited therein. These relations are also given there for the spin-spin and orbit-orbit interactions.