Table of contents

By employing a many-electron approach and the Breit-Pauli Hamiltonian, the authors have computed the energy levels and autoionisation widths of the 2p², 2p3p, 2s3d ³D and ¹D states and the fine structure of the 2p², 2p3p ³P, and 2p3p, 2s3d ³D states of He. The fine structure of 2p3p ³D (3.9 cm^-1) is apparently not large enough to explain the recent puzzling findings on the width of 2s2p ³P⁰ by Cederquist et al. (1983). Comparison with the previous large scale calculations shows general agreement for the energies and widths, except for the 2p3p ³D state.

Accurate results for the photoionisation of the Si⁺ (3s²3p ²P⁰) ground state are reported for the photon energy region between the first two ionisation thresholds, Si²⁺ (3s²¹S^e, 3s3p ³P⁰), at 750 and 540 AA. The cross section is dominated by the autoionised ³P⁰ np, nf (n>or=4) Rydberg series, whose shapes and oscillator strengths are drastically perturbed by the ¹P⁰ 4p ²S^e, ²D^e interlopers. These results are the first outcome of an elaborate calculation, based on a consistent LS multichannel description of the initial bound state and the final continua. The ejection of one valence electron is allowed for, together with flexible relaxation within the remaining valence shell, and possible monoexcitation into a near-spectroscopic orbital (3d, 4s, 4p). This extensive calculation (12 ionic states, up to 22 channels) is the first application of a new numerical approach combining the advantages of the R-matrix and the multichannel quantum defect (MQDT) theories.

The 'giant resonances' observed in atomic photoionisation are discussed in the context of scattering by a short-range attractive well. It is argued that they correspond to a scattering regime of the virtual states not usually probed in high-energy physics. In the shape-independent approximation, a general formula for the profile of a 'giant resonance' is derived and compared with experiment. A 'universal curve', connecting the breadth of a 'giant resonance' to its energy above threshold is also demonstrated from available data for free atoms.

New data are reported which are believed to be the first observations of the 4d to f 'giant resonances' in Gd and GdF₃ vapours. The profiles of the resonances are compared with a simple two-parameter formula recently derived from the shape-independent approximation.

The differential cross section (DCS) for electron capture at asymptotically high, but non-relativistic, energy is investigated within the second-order distorted-wave approximation given by second-order continuum distorted waves (CDW2) and is compared with second-order Oppenheimer-Brinkmann-Kramers plane waves (OBK2). It is concluded that the inclusion of explicit intermediate states in both theories ensures a uniquely determined value for the high energy 1s-1s DCS at the forward peak and indeed another uniquely determined value at the Thomas peak. In particular the authors find a resolution of the anomalous behaviour of the CDW1 approximation of the Thomas peak in that the catastrophic dip arising from the accidental degeneracy in CDW1 at the real Thomas angle is precisely eradicated by inclusion of CDW2. Moreover, they note that the magnitude of the 'interference minimum', which occurs near 0.6 times the Thomas angle, depends upon the theory adopted.

Ratios of elastic differential cross sections of CO₂ to those of N₂ have been measured at electron impact energies of 500, 800 and 1000 eV in the angular range of 5 degrees to 120 degrees , using a crossed electron-beam-molecular-beam geometry and the relative flow technique. These ratios have been multiplied by previously known N₂ elastic differential cross sections to obtain elastic differential cross sections for CO₂. At 500 eV, the authors' experimental results agree quite well with the absolute values of Bromberg (1974). Comparison has also been made with the theoretical results obtained in the renormalised multicentre potential model approach of Botelho et al. and shows an agreement within 20% in the entire angular range. The elastic integral and momentum transfer cross sections have also been determined from the differential cross sections.

The relativistic R-matrix method is used to calculate elastic and inelastic cross sections for electrons incident on caesium atoms with energies from 0-3 eV. These cross sections reveal a wealth of resonance structure in this energy range. The differential cross sections as well as the spin polarisation function S_P( theta ) and the left-right asymmetry function S_A( theta ) are calculated and enable conclusions to be drawn on the importance of spin-dependent interactions.

For pt.III see ibid., vol.16, p.L433-6 (1983). The variety of approximations used in estimation of the wavelengths of 1s2s ³S-1s2p ³P lines studied in this series reveals the need for a status report. Two new features are discussed: the need for basis set extrapolation at the low-Z end of the isoelectronic sequence, and an estimate of two-body corrections to the electron self-energy. The recommended theoretical values lie mostly within experimental error estimates for Z<10. For higher values of Z the authors propose to change to the values given in the second paper (1983) of this series.

Extensive calculations of energy levels and transition probabilities for the E1 transitions between levels of 2s²2p⁴ and 2s2p⁵ of the oxygen-like ions have been performed for 25 ions up to Z=42. The multiconfiguration Hartree-Fock method was used for determining the radial orbitals. The total wavefunction includes configurations that contribute significantly either to electron correlation in the n=2 shell or to relativistic effects through the Breit-Pauli approximation. The authors' results are compared with other existing theoretical values and, in the case of energy levels, also with observation.

The highly excited 4f₇2/¹³6s₁2/(J')np_j(J) Tm states lying near the first two ionisation limits of the atom were investigated. The automatic laser setup used contained three dye lasers pumped by a nitrogen one. The states' energies were measured with an accuracy of +or-0.1 cm^-1. Two excitation schemes were realised. In the first one, the states with J'=3 were excited; the states of this type with n>or approximately=28 are autoionisation states. In the second excitation scheme, the bound Rydberg states with J'=4 were excited. Identification of the observed levels was performed using the calculation of the spectrum made using relativistic perturbation theory with zero-order model approximation. Unambiguous classification was obtained for the following three series: 4f₇2/¹³6s₁2/(3)np₃2/(⁹/₂) (n=26-65), 4f₇2/¹³6s₁2/(4)np₃2/(¹¹/₂) (n=29-57) and 4f₇2/¹³6s₁2/(4)np₁2/(⁷/₂) (n=28-36). For n>27, three shorter series were observed too, for which alternative classifications are possible at the present stage. The experimental results are in very good agreement with the theoretical results, particularly for large values of the principal quantum number n. A peculiar grouping of the states belonging to the different series was observed varying with the variation of n. This confirms the non-Coulombic properties of the highly excited spectrum of the atom considered.

The authors give a theoretical description of the spectrally resolved intensity correlations of an electromagnetic field, in analogy with the definition of the physical time-dependent spectrum. The results are applied to evaluate the time correlations between the three lines in the spectrum of resonance fluorescence of a two-level atom. The authors include the possibility of a finite bandwidth of the incident radiation, and they allow for collisions with perturber atoms. Photons in the central Rayleigh line are emitted in a fully random fashion, without any correlation with previous or subsequent emissions. Two photons from the same sideband display antibunching in time, whereas two photons from opposite sidebands tend to bunch with a strong asymmetry in time. The effect of collisions and of a finite bandwidth is to diminish this asymmetry.

Photon-induced, post-collisional Auger decay processes are studied near to excitation threshold situation. The Gell-Mann-Goldberger two-potential formalism is applied for deriving the transition amplitudes which are shown to be dominated by the second-order terms. The combination of the two-potential formalism with the spectral analysis following from the complex coordinate method has been shown to be applicable in the treatment of the final-state interactions. Expressions for the scattering amplitude are derived without sudden impulse arguments. As the result of the complex coordinate treatment, the scattering amplitude distinctly separates into direct, background and resonant parts.

Photoionisation cross sections for excited nl states (n<or=20, l<or=4) of Rb and Sr⁺ have been computed in the framework of a single-electron model by the use of a central potential. The evolution of non-hydrogenic behaviour of photoionisation near threshold is studied along a given series. Moreover, isoelectronic variations from Rb to Sr⁺ as well as systematic trends along the sequence of alkali atoms Li through Cs are discussed. The non-hydrogenic traits, mainly the occurrence of minima in photoionisation cross sections, are analysed in terms of quantum defect theory.

Fluorescence series excited in BiS by the argon-ion laser lines 4880, 4889 and 5145 AA have been recorded at moderate resolution and used to derive constants for the ground state X Omega =¹/₂(² Pi ₁2/). Dye-laser absorption experiments have been used to record hyperfine structure in a number of lines of the A¹/₂-X¹/₂ system at Doppler-limited resolution. Under these conditions the lines are resolved into ten components in Lande patterns, regular or inverted according to whether the line arises from an e or an f parity level in the lower state. At values of J in the range 40.5 to 75.7 the dominant contribution in the hyperfine structure is the so-called hyperfine doubling.

A direct fully numerical method for the generation of molecular continuum wavefunctions is presented. The general approach represents a major extension of procedures which are well established for atomic systems. The 1 sigma _g photoionisation cross section for the hydrogen molecule is reported using a relaxed Hartree-Fock potential for the molecular ion core determined from a (15s, 10p) uncontracted Gaussian bound-state calculation. Good agreement with experiment is reported to within 6 eV of threshold.

Using a classical description for the laser field, previous formulations of the low-frequency laser-assisted potential scattering theory are generalised to account for arbitrary polarisation and for spatial variations of the field amplitude. Attention is paid to providing a relatively simple derivation of the basic results, and to establishing contact with similar or more restricted results obtained within other treatments. Restricted to the first Born approximation and pure Coulomb potential, it is found that the field momentum introduces new divergences, whenever K_f-K_i+NK=0, K_f, K_i and K being, respectively, the final and initial particle wavevectors and the field wavevector. The role of the laser momentum is found to be critically important also in forward and small-angle scattering, when the transferred momentum is typically very small. Finally, the present derivation adds a new limitation to the validity of a well known formal low-frequency result.

For pt.I see ibid., vol.16, p.1393 (1983). The relative emission cross sections for direct excitation of five Ne 2p⁵3p levels have been measured in the low-energy centre-of-mass range (70 eV-2 keV). They all show a single series of large amplitude Rosenthal-type oscillations which-except 2p₁-seem to have the same characteristics. The 2p₁ differs also by its general form since it decreases at high energy while all the others are increasing. These features are discussed within the framework of the diabatic correlation diagram for the (He-Ne)⁺ system.

The amplitude for simultaneous electron transfer and excitation in an ion-atom collision is examined to first order in the electron-electron interaction. The amplitude is shown to separate into a sum of two parts, one corresponding to a resonant process (RTE) and the other to a non-resonant term (NTE). The NTE term, which assumes the motion of the two electrons to be uncorrelated in space and time, can be factored into the separate amplitudes for capture and excitation. Using a strong-potential Born wavefunction for the transferred electron, the two terms are seen to peak at different energies and with different widths. Possible interference effects of the two processes are discussed.

Electron capture by protons from the L and K shells of Li atoms is studied in a wide energy range 0.6-400 keV (lab) using two-centre atomic orbital expansions with translational factors within the impact parameter formalism and the effective potential approximation to the electric field in the lithium atom. A review of available theoretical and experimental data given in the paper indicates that cross sections for capture from the L shell are generally in good agreement with calculations by methods employing different types of expansions. It is confirmed that capture into the n=2 shell of hydrogen dominates at energies up to 10 keV (lab). Cross sections for capture from the K shell are found to be relatively small for energies lower than 150-200 keV (lab). The latter conclusion is in agreement with an interpretation of the old measurements of Il'in et al. but disagrees with recent classical Monte-Carlo estimates of Olson.

Cross sections for single-electron capture by He²⁺ from the L- and K-shells of Li atoms are computed in a wide range of energies between 1.9 keV and 1.6 MeV (lab) using the same atomic orbital expansion method and numerical techniques as those described in the companion paper by Ermolaev. It is confirmed that the preferential capture of the 2s electron from Li into the n=3 states of He⁺ persists for energies as high as 40-65 keV (lab). The present calculations also confirm that an experimentally observed high-energy shoulder in the total capture cross sections is due to an increasing importance, for energies of 100 keV and higher, of the capture of the K-shell electrons from Li. For low energies (1.9-5 keV) the computed cross sections are considerably lower than those recently calculated by Fritsch and Lin. This disagreement highlights the necessity of further theoretical and experimental studies of the reaction in this low-energy region.

Symmetric resonant charge transfer in atomic collisions in the presence of a homogeneous constant magnetic field is investigated within the impact parameter method and the atomic two-state expansion. The novelties brought about by the presence of the magnetic field are discussed in detail. The most peculiar new feature is the appearance of a magnetic-field-dependent phase factor in the process wavefunction, which is responsible for severe reductions of the total cross section at moderately strong fields. Numerical evaluation of the transition probability and the total cross section is performed for the Rb⁺+Rb to Rb+Rb⁺ reaction. Reduction of the cross section up to 43% are found at relative velocity nu =6.8*10⁶ cm s^-1 and magnetic field B=0.05 B₀ (B₀=2.35*10⁵ T being the atomic unit of magnetic strength). The magnetic field also enters the problem through a 'dressing' of the atomic states. With the values of the magnetic field strength considered, which affect the atomic energies only moderately, the dressing is found to have only secondary importance in determining the absolute values of the total cross section, which are mainly controlled by large impact parameters. Moreover the dressing is found to be important in modifying the oscillatory structure of the total cross section, which originates at small impact parameters.

Polarised electrons have been scattered elastically from mercury and xenon atoms and the change of the electron polarisation caused by the scattering process has been measured at the fixed scattering angles theta =90 degrees and 75 degrees (mercury) and theta =80 degrees (xenon) in an energy range 18-360 eV. Together with the absolute differential cross section these measurements yield the maximum possible information on the scattering process.

The modified Glauber approximation is used to investigate the 1s-2s excitation of He⁺ by electron impact at intermediate scattering energies. Formulae of the relevant amplitudes in electron-hydrogen-like ion scattering to be used in the calculations are derived and put in forms which are easily accessible for numerical computations. Especially in the case of the 1s-2s transition considered, the formulae are shown to be reducible to closed forms. The differential and integrated cross sections of e^--He⁺ collision (1s-2s transition) are calculated at several energies and the results are compared with experimental data and with those obtained in other methods of approximation. In general, the modified Glauber method fares reasonably well at medium scattering energies in comparison with other methods of approximation.

Measurements of the resonant attachment frequency of excess electrons to O₂ molecules in dense helium gas have been performed in the temperature range 50-100K. The attachment frequency exhibits a sharp peak at a density N_P, which has an unexpected temperature dependence. The peak shape has been simply related to the density dependence of F(E_R), the value of the energy distribution function at the resonance energy E_R. The results of the experiment indicate that resonant capture by molecular impurities can be used to get unique information on the density of states of a disordered system.

The attachment of very slow electrons (<0.2 eV) to polyatomic molecules is discussed with a model in which the s wave of the incident electron dominates; the mechanism of capture is a non-adiabatic coupling due to the velocities of the nuclei. The attachment may be made permanent by the dissociation of the transient negative ion formed, or by collisions in the gas. The model accounts for cases where the rate of attachment is large at room temperature (e.g. SF₆) by the assumption of a virtual state of the incident electron. The model can also account for cases where the rate constant is small at room temperature, but becomes large when the target molecules are heated to a few thousand K.

Electron impact excitation has been used to observe inner-shell and outer-shell excitation in HCl, HBr and Br₂. Inner-shell energy-loss spectra have been obtained corresponding to excitation of a 2p electron in HCl and a 3d electron in HBr and Br₂, at an incident energy of 1500 eV and with a resolution of typically 75 meV. The spectra are characterised by broad continua corresponding to excitation to unbound valence orbitals, and higher energy discrete structure corresponding to excitation into Rydberg orbitals. Assignments of these structures have been made and the binding energies of the inner-shell electrons have been deduced. Outer-shell, energy-loss spectra have been obtained in HBr and HCl under similar conditions.

The authors consider the Doppler-broadened index of refraction of a two-level molecule of a gas for a weak travelling electromagnetic plane wave in the presence of a strong plane wave travelling in the opposite direction at the same frequency. The authors use the FVH vector model for a two-level system to calculate the real part of the non-linear susceptibility of the gas. The optical dispersion curve exhibits a wiggle at the centre line, that broadens and deepens as the gas is increasingly saturated by the strong incident wave.