Table of contents

Absolute cross sections have been measured for the photoionisation of K⁺ ions by VUV radiation in the energy range 31-42.5 eV. The cross section had a maximum value of 3.4*10^-17 cm² (+or-20%) at 32 eV and decreased with increasing energy. No autoionisation resonances were found but there were indications of possible structure at about 35.5 and 39.8 eV.

The photoionisation cross section of the 5s subshell in atomic strontium has been calculated using many-body perturbation theory near threshold. Double-electron resonances have been included by coupling many final-state channels.

The X-ray K alpha single 2l spectator vacancy satellite spectrum of sodium was studied by non-relativistic Hartree-Fock calculations in intermediate coupling. All observed satellites are assigned to within about 1 eV to 2p spectator hole transitions, with a small possible contribution to the alpha ₄ line from the strongest 2s spectator hole ones. The identification of the recently discovered, and hitherto unassigned, C1 line is discussed and the assignment of the C1- alpha ' structure to the (1s2p)^-11P, ³P to (2p)^-21S, ¹D complex is suggested.

The R-matrix method has been employed to study the electron excitation of atomic helium. Nineteen target states (n=1, 2, 3 and 4) have been included in the expansion of the total wavefunction. The results of this calculation are used to obtain the electron-photon coincidence parameters lambda and chi for the excitation 1¹S-2¹P at 22, 24, 26.5 and 29.6 eV. The calculated parameters lambda and chi are compared with values obtained from experiments and with earlier 5- and 11-state R-matrix calculations.

Measurements are reported of mutual neutralisation of Li⁺ and H^- ions for interaction energies between 33.3 and 2400 eV. When the results are compared with similar measurements for H⁺/H^-, He⁺/H^- and H⁺₂/H^- they provide further evidence that cross sections for neutralisation of singly charged positive ions by H^-, when plotted against relative speed, fall close to a common curve for speeds greater than 3*10⁵ m s^-1. Neutralisation of Na⁺ by O^- has been measured between 27 and 2470 eV and the results compared with earlier measurements taken at lower energies by Moseley et al. (1975) and Weiner et al (1971).

New theoretical results for positions and lifetimes of N⁵⁺(3l3l') singlet states made possible a fit of the 3l3l' experimental electron line observed in N⁷⁺-He collision at 70 keV, 10'. It has been found that two-electron capture occurring in the highest angular momentum ¹G^e(2, 0) state represents about 50% of the capture into 3l3l'. Because of the post-collision interaction, this fitting procedure allows the first experimental check of 3l3l' ¹L theoretical positions and lifetimes.

A quantum mechanical formulation of the distorted-wave Born theory is presented, taking full account of the internuclear potential. Upon expansion in terms of one of the two electronic potentials, the strong-potential Born theory is rederived. Thereby an additional term emerges which had not been considered previously and which has considerable influence on the capture probability at large scattering angles.

High-resolution electron impact excitation studies of helium in the energy region just below the first ionisation threshold provide further evidence for the existence of a Rydberg series of doubly excited resonances. These measurements extended the work of Buckman et al (1983) to states in which both excited electrons have values of the principal quantum number n>or=8. The energies of these states show good agreement with the values predicted by a number of semi-empirical double-Rydberg formulae and with a recent quantum mechanical calculation.

For pt.I., see ibid., vol.20, p.L49 (1987). First-order many-body theory (FOMBT) is used to predict the differential cross section (DCS) and electron spin polarisation function S_p(k, k') associated with electron impact excitation of the 3³P levels of the Mg atom, for incident electron energies of 10, 20 and 40 eV. The theoretical DCS agrees well with the measured DCS except at large scattering angles. By performing calculations with both single- and two-configuration representations of the Mg ground state it has been determined that configuration mixing is not very important in describing the electron scattering characteristics of the Mg atom. A comparison between comparable results for He, Mg and Hg appears to support the premise that the structure present in the electron spin polarisation function (and hence (L_{perpendicular to} )) is due to interference among partial waves associated with the electron scattering process.

The probe method for the measurement of the electron-atom collision integral in a plasma is described. The method is applied to the plasma of the positive column of an electrical discharge in helium. The collision integrals S₀, S₁ and the transport cross section of the elastic electron-atom collision are measured.

Electron correlation is examined in momentum space for the individual intra- and inter-shell electron pairs in the ground state of Li(²S). It is established that the radial and angular components of the correlation have opposing influences on the inter-particle momentum distributions f(p₁₂) and g(p₁₂;p₁), where p₁₂= mod p₁-p₂ mod and p₁= mod p₁ mod . By examining the Coulomb shifts Delta f(p₁₂) and, in particular, the partial Coulomb shifts Delta g(p₁₂;P₁) for each electron pair, such a feature enables one to identify the major component of correlation in different regions of momentum space. This represents a distinct advantage over the more traditional position space approach. For K alpha K beta , not only was it found that intra-shell correlation effects were influenced by the occupation of the 2s orbital but, more interestingly, the nature of the influence was seen to change markedly throughout different regions of space. For the K alpha L alpha and K beta L alpha inter-shells, the importance of angular correlation was highlighted and, for K alpha L alpha , the consequence of the Fermi effect was observed. Comparisons were made with a previous, and complementary, position-based study of Li(²S) and with the correlation effects found in momentum space for closed shell Li⁺(¹S) and Be(¹S).

The self-consistent charge (SCC) X alpha method has been applied to investigate the electronic structure of molecules which contain heavy, Mossbauer-active elements (Sn and I). For a series of halogen-containing compounds, with various oxidation states of the heavy elements, the authors have calculated orbital energies and dipole moments, which compare well with corresponding experimental data. In the case of hypervalent ions the inclusion of d besides s and p orbitals is sensitive with respective to dipole moments only and leaves orbital energies practically unaffected. For tin- and iodine-containing molecules they have calculated electron charge densities rho (0) and electric field gradients (EFG) at the nuclear site of the Mossbauer atom. Comparing these results with corresponding isomer shifts and quadrupole splittings (coupling constants), which were partly measured by one of them, they derive as calibration factors the fractional change of the nuclear radius Delta R/R and the quadrupole moment Q for ¹¹⁹Sn, ¹²⁷I and ¹²⁹I: Delta R/R(¹¹⁹Sn)=1.61*10^-4, Delta R/R(¹²⁷I)=-0.345 Delta R/R(¹²⁹I), Delta R/R(¹²⁹I)=4.18*10^-4; Q(¹¹⁹Sn)=-0.061 b, Q(¹²⁷I)=-0.60+or-0.03 b, Q(¹²⁹I)=0.701Q(¹²⁷I).

The spin-polarised SCC-X alpha method is used for the evaluation of electronic- and magnetic-structure properties (i.e., electron densities at the iron nuclei, electric-field gradients, g factors and magnetic hyperfine tensors) of oxidised and reduced (2Fe-2S) ferredoxin models, represented by the structure formula ((SR)₂FeS₂Fe(SR)₂) (n=-2, -3), with R=H, CH₃ and (C₆H₄(CH₂)₂)₁2/. A strong geometry dependence of charge and spin densities, and their related properties, is found for minimal cluster size (R=H) on the position of the terminal hydrogen atoms. A Fe-S-H bond angle of about 90' turned out to be preferable to 180'. Experimental hyperfine data of synthetic analogues and of ferredoxins are in reasonable agreement with corresponding calculated properties obtained for the (2Fe-2S) ferredoxin models mentioned above. Comparing experimental and calculated g factors of reduced (2Fe-2S) centres indicates that the model clusters used are of g_av approximately 1.91 type. Finally, the exchange-coupling constant J between the two iron ions in the oxidised (2Fe-2S) ferredoxin model is estimated from the difference in the total energy between the high-spin and broken-spin symmetry states of the model.

The cross sections for the production of L_{l+ alpha} , L_beta and L_gamma X-rays in Nd, Sm, Eu Gd, Dy, Er, Yb and Lu by photons with energies of 10.005, 17.781, 25.77 and 32.89 keV have been determined. Fairly good agreement is found between the experimental and calculated values. Contrary to the findings of Garg et al (1986) that the experimental values are 5-20% higher than the calculated values, our results do not show any systematic regular trends which would indicate any departure from theory. Plausible reasons for the discrepancy reported by Garg et al are discussed.

L alpha /L_l X-ray intensity ratios are measured in 19 elements in the region 55<or=Z<or=80. A 30 mCi ²³⁸Pu source is used to provide the excitation while a high-resolution Si(Li) detector connected to a 4 kbyte multichannel analyser and a microprocessor is used for the detection and measurement of X-ray intensities. The results show agreement with the theoretical predictions of Scofield (1969).

The authors have investigated the magneto-optical rotation in caesium vapour near the 6S_1/2 to 6P_3/2 resonance (Macaluso-Corbino effect) at 40 and 50 G, using a single-mode laser. The measurements were performed at low light intensities in order to avoid non-linear effects due to saturation or optical pumping. They find excellent agreement between their results and a complete theoretical treatment with energies and wavefunctions obtained by diagonalising a Hamiltonian that includes the hyperfine interaction. Allowance for wavefunction perturbation, neglected in most treatments, is found to be essential.

The authors report a sequence of configuration interaction calculations of the oscillator strength for the 2s²2p⁵²P^o-2s2p⁶²S^e transition in Ne II and Mg IV. They obtain 0.096+or-0.005 for Ne II, which is 25% larger than the experimental values, and 0.089+or-0.005 for Mg IV. The good agreement between some previous calculations and experiment for Ne II was due to the neglect of external correlation in the earlier work. They have also calculated oscillator strengths for the 2s²2p⁵-2s²2p⁴3s ²P^e, ²D^e and ²S^e transitions in Ne II, obtaining 0.11, 0.063 and 0.0058 respectively. The experimental values for the ²P^o-2s²2p⁴3s ²S^e line are almost a factor of two larger than theory.

The time-resolved response of a gas absorber exposed to two independent sources of microwave radiation, one steady and the other pulsed, is described using a formalism based on the Bloch equations for a three-level system. This predicts that a series of experiments can be made on the resonant and off-resonant recoveries of the absorptive and dispersive properties of a single sample by which the individual collisional relaxation parameters that characterise its rotational states may be separately measured. Some of these can be determined only by double-resonance experiments. The construction and operation of amicrowave double-resonance bridge spectrometer capable of carrying out these experiments is described, and results obtained for the (J=2 from 1)-(J=1 from 0) system of OCS are discussed in detail. The polarisation relaxation rate for the 2 from 1 transition is found to be 38.2(2) mu s^-1 Torr^-1 and its population relaxation rate, including cross-relaxation effects, is found to be 38.9(1.9) mu s^-1 Torr^-2. Values for the cross-relaxation parameters are obtained for the first time.

For pt.I, see ibid., vol.20, p.5675 (1987). Pulsed Fourier transform techniques for the analysis of time-resolved absorption signals have been applied to microwave-microwave double-resonance gas spectroscopy. The resonant frequencies of connected transitions have been identified by a two-dimensional Fourier transform plot, and a novel two-pulse technique which displays spectra over a band exceeding 100 MHz has been introduced. Sample results are presented for the species OCS and ClCN. New spectral features caused by the simultaneous excitation of two pump transitions have been observed and are discussed.

For the three-body Coulomb problem a hyperspherical parametrisation of independent variables is given on a five-dimensional sphere S⁵ with a hyperradius R_H, the first linear invariant of the inertia tensor. The hyperspherical adiabatic basis is defined as a complete set of eigenfunctions and eigenvalues of the Hamiltonian on the sphere S⁵ for every fixed value of the slow variable R_H. The partial wave analysis in the total momentum J representation allows the authors to separate three Euler angles and to reduce the hyperspherical problem on S⁵ to a system of (J+1) two-dimensional problems. Classification is given of the hyperspherical adiabatic basis for small and large values of the hyperradius R_H. The logarithmic Fock singularity at the point of triple collision (R_H=0) is explicitly shown. The approach is assigned to computing the cross sections of mesic atomic processes in the muon catalysis problem.

The low-energy elastic and inelastic scattering of C⁺(²P) by H(²S) is used as a model system to probe the dynamical ramifications of non-adiabatic couplings and resonances in open-shell-atom-open-shell-atom scattering. The authors briefly review the background theory of the influence of non-adiabatic interactions on open-shell-open-shell atomic collisions. The underlying dynamics and role of resonances are illustrated by presenting contributions from individual partial waves to the total cross section. This enables them to compare the resonance structure of the scattering cross sections with that for the photodissociation of CH⁺, both of which involve identical continua. The presence of phase differences between the multiple molecular states, correlating to a single elastic channel, is shown to produce the analogue of Stuckelberg-type oscillations in the partial elastic scattering cross sections. As expected, averaging over partial waves tends to wash out much of the interesting non-adiabatic resonance structure predicted for the CH⁺ photodissociation cross section.

The rate constant for dissociative recombination of NO⁺ was measured in the translational temperature range from 2800 to 4500 K using a shock tube technique. The rate was found to vary as alpha (T)=4.3(+or-0.8)*10^-7(T/300)^{-0.7(+or-0.1)} cm³ s^-1. This measurement supports the view that there is no strong dependence on vibrational quantum number in the dissociative recombination of NO⁺.

The author has calculated collision strengths for electron excitation of the 1s²2s²2p⁵²P^o_3/2-²P^o_1/2 transition in Ne II and Mg IV using the R-matrix method. The author has included the 1s²2s²2p⁵²P^o and 1s²2s2p⁶²S^e states, which were represented by sophisticated configuration interaction wavefunctions. Reactance matrices were obtained in LS coupling and transformed using JAJOM to give the fine-structure collision strengths. These have been averaged over a Maxwellian electron velocity distribution to give the effective collision strengths, which we have fitted as analytic functions of temperature between 0 and 40000 K. To estimate the accuracy of the results the author has calculated the positions and widths for the lower members of the 1s²2s2p⁶ns and 1s²2s2p⁶np resonance series. The results are in good agreement with experiment and with previous theory for both Ne II and Mg IV.

Coincident energy gain spectroscopy is applied to the study of single- and double-electron capture by multiply charged ions on different targets. Attention has been focused on the determination of branching ratios between the two processes. The influence of collision energy and the charge state of the projectile on the relative state population is discussed.

Dielectronic recombination into the singly excited M-shell states of neon-like selenium is studied using results from a multiconfigurational relativistic distorted-wave Auger model. The relativistic calculations are based on the use of a continuous radial grid which is logarithmic at small r and linear at large r. Detailed results are presented for recombination coefficients on a state by state basis, in a manner compatible with detailed level population kinetics modelling of highly-stripped ions in a plasma. The theoretical partial recombination coefficients for recombination through the 3l3l' manifold yield peak recombination rates of 1.58*10^-11, 1.37*10^-11 and 1.56*10^-11 cm³ s^-1 for recombination from the fluorine-like ²P_3/2, ²P_1/2 and ²S_1/2 states respectively.

Absolute total scattering cross sections have been measured for electron scattering from krypton in the energy range 0.175-20 eV. The measurements have been performed with a linear time of flight electron transmission spectrometer. Comparisons are made with the results of a number of other experiments and theoretical calculations and the agreement is generally good. However, at low energies, particularly in the region of the Ramsauer minimum, the present results and the recent measurements of Jost et. al. differ by more than 30%. A momentum transfer cross section is derived from the data by the use of the modified effective-range theory to enable direct comparison with swarm-derived cross sections.

Employing two different experimental techniques, absolute total cross sections for e^--CO₂ scattering have been measured over a very wide energy range between 0.5 and 3000 eV. The cross section function exhibits a sharp increase below 1.5 eV and a resonant maximum around 3.8 eV. It then rises again up to a broad maximum near 30 eV and then decreases slowly. A very broad feature was observed centred at an energy of about 300 eV and extending from about 200 to 600 eV. Comparison is made with other available measurements and calculations.

First-order models have been extensively applied to the study of the interaction mechanism in electron impact ionisation. While first Born theories have been successfully applied in the dipolar regime (small momentum transfer), the impulse approximation has been used in the interpretation of (e, 2e) triple differential cross sections in the binary regime (high momentum transfer and symmetric geometry). This work investigates the intermediate regime where neither the simple first Born nor the impulsive approximation is correct. For this purpose (e, 2e) cross sections of He have been measured in asymmetric conditions at 1024.6 eV incident energy, for ejected electron energies ranging from 4 to 50 eV. Depending on the scattering angle, the momentum transfer K varied from 0.5 to 2 au. The experimental results have been compared with the theoretical predictions of the plane-wave impulse approximation (PWIA), the distorted-wave impulse approximation (DWIA) and the first Born approximation including an approximate post-collision interaction correction (BA-PCI). The results can be summarised as follows. (i) At low ejected electron energy correlation effects due to long-range Coulomb interactions are important. The BA-PCI calculation, which takes into account these effects, reproduces both the absolute value and the angular dependence of the experimental data at the smallest momentum transfer. (ii) Provided the kinematical conditions are confined to the Bethe ridge, where the recoil lobe becomes negligible, the cross section is well described by impulsive models even for momentum transfers as low as 2 au. The last result is of particular interest because it allows asymmetric conditions to be used for spectroscopic purposes.

Distorted-wave Born approximation results for the differential cross sections and various angular correlation parameters for electron impact excitation of the rare gases Ne, Ar, Kr and Xe are presented and compared with recent experimental data and other theoretical calculations. The sensitivity of the results to different static and optical potentials in the calculation of the distorted waves is analysed and the importance of relativistic effects both in the description of the target states (intermediate coupling) as well as in the wavefunction for the continuum electron is investigated. The overall agreement with the available experimental data is very satisfactory over a wide range of incident-electron energies.

The crossed electron-ion beams technique has been used to measure the absolute cross sections for the single ionisation of Ne⁺, Ar⁺, Kr⁺ and Xe⁺ ions at electron energies ranging from the ionisation threshold to 2000 eV. The total error in the cross sections at a 90% confidence level is estimated to be +or-5% or less at energies more than a few eV above threshold. In contrast to some previous measurements, the metastable contents of the ion beams are negligible except in the case of Xe⁺. The cross section curves for Ar⁺, Kr⁺ and Xe⁺ show significant contributions from excitation-autoionisation and possibly direct ionisation of inner-shell electrons. The present results for Ne⁺ and Ar⁺ are in excellent agreement with those previously measured in the laboratory. Good agreement is also obtained with the more recent data from the Giessen group for Ne⁺, Ar⁺ and Xe⁺. Comparisons with theoretical calculations show that the scaled Born approximation of McGuire tends to underestimate the cross sections at all energies, but especially at low energies. The semi-empirical formula of Lotz also underestimates the cross sections at low energies. Comparisons are made with other available experimental and theoretical calculations.

Distorted-wave calculations are performed for electron (and positron) impact excitation of lowest-lying autoionising levels in alkalis (Li, Na, K, Rb and Cs). Appropriate initial- and final-state static potentials have been used for distorting the incident and scattered projectile electron (and positron) waves, respectively. In the case of lithium the effect of the polarisation potential (in addition to static potentials) is also considered for the distorting potential of the projectile electron. Results for differential and total cross sections, anisotropy parameter ( beta ), polarisation fraction ( pi ) as well as for lambda and chi parameters are reported. Comparison of the present results with earlier available experimental and theoretical results is presented.

Multiple ionisation of Ar atoms by single-electron impact is investigated in the energy range from threshold up to about 600 eV. Partial cross sections for the production of Ar^q+ with 3<or=q<or=6 are measured. In the near-threshold region the ionisation curves can be fitted according to power laws sigma varies as (E_el-E_thr)^K, where the exponent K is found to be smaller than q for high charge states. At higher electron energies the ionisation cross sections show a significant influence of inner-shell ionisation, which becomes the dominant mechanism for the production of high charge states.

The cross section for antihydrogen formation in excited 2s, 3s and 2p states is estimated using the Born approximation and compared with the cross section for antihydrogen formation in the ground state. These excited state cross sections are shown to be of comparable size to the ground state ones, thus increasing substantially the feasibility of antihydrogen formation in the laboratory. Partial s, p and d cross sections in the Born approximation are also given for the various processes.

Previous measurements of the total scattering cross section of positrons in molecular nitrogen are compared with theoretical predictions. The inelastic cross section is estimated by subtracting calculated values of the elastic cross section, and the results are found to be consistent with the first Born approximation at high energies. The sum rule is applied to the integral of the total cross section over all momenta and predicts a value for the zero-energy scattering length of -(2.65+or-0.33) a₀ (where a₀ is the Bohr radius). This lies within the rather wide range of experimental and theoretical values obtained by other methods.

The 0-0 and 1-1 bands of the a ⁴ Sigma ^--X ² Pi system have been recorded at high resolution and analysed. Rotational structure of the B ² Sigma ⁺-X ² Pi band system has been reanalysed. The a ⁴ Sigma ^- and X ² Pi state Hamiltonians and a direct approach have been used to determine rotational constants. Band by band fitting and merge procedure have been employed to derive the rotational and fine-structure constants from the a ⁴ Sigma ^--X ² Pi 0-0 and 1-1 bands and the B ² Sigma ⁺-X ² Pi 0-0 and 0-1 bands.

The forward scattering of a resonant intense and broad-band (of the order of 1 GHz) laser light on sodium atoms is described. The experimental results for the D₁ and D₂ sodium lines (the 3²S_1/2-3²P_1/2 and 3²S_1/2-3²P_3/2 transitions) are compared with those of the scattering of the light from a conventional spectral lamp and from a monochromatic laser. The observed differences are explained by taking into account the Zeeman coherences created in the irradiated atoms and the concept of the 'hyperfine uncoupling' is proposed for describing the interaction of atoms with the broad-band laser light.