Table of contents

Multipole polarisabilities and shielding factors for helium, neon and argon have been calculated accurately in the coupled Hartree-Fock approximation. A shell by shell analysis is given for the first four multipoles.

Analytic results for the fluorescent spectrum arising in the cooperative interaction of a population of two-level atoms, driven by resonant laser radiation with the electromagnetic field are presented. The bistability and hysteresis characterising single-time atomic averages are evidenced explicitly as this spectrum is viewed throughout a hysteresis cycle. In the region of bistability full support is given to the designation of a cooperative and a non-cooperative branch. Spectral features reveal linewidths associated with cooperative and non-cooperative atomic decay rates respectively. A dominant feature is strong coherent scattering throughout the cooperative region, fully supporting an identification of this problem with super-radiant phenomena. Linewidth narrowing is predicted in the vicinity of both instabilities.

The cross section for double photoionisation has been calculated for Be. Effects of core polarisation are included in the calculations.

The programming of the R-matrix method recently used to calculate total and partial atomic photoionisation cross sections has been extended to enable photoelectron angular-distribution beta parameters to be studied for a general atomic system. The program has been used to calculate the beta -parameters for photoelectrons ejected from the outer p subshell of both neon and argon atoms. Good agreement with absolute experiments has been obtained in non-resonant regions. In resonant regions the results are the first to be obtained and have been parametrised according to the prescription of Kabachnik and Sazhina (1976).

The 304 AA photoelectron spectrum of Xe has been obtained with high resolution (50 meV or better). In addition to 5p-5s main lines, 8 satellites having intensities of 2-32% have been observed and compared with X-ray data in the binding-energy region of 23.5-28.3 eV. A straightforward assignment with the optically known levels of Xe II was not possible. Various configurations-mixing mechanisms are discussed and attention is drawn to the possibility of misassignments in the optical levels of Xe II.

Theoretical estimates of the absorption coefficients of the negative molecular fluorine ion are given. The strong shape resonance found in e-F₂ scattering also appears asa resonance feature in absorption coefficients.

An alternative derivation of the Bates distortion approximation is given within the impact-parameter formulation. In this new derivation use is made of a bound on the transition amplitude.

Electron spectra from 5-10 keV Li⁺-Ne, Li-Ne, Na⁺-Ne and Na-Ne collisions have been measured. The Wigner spin-conservation rule causes marked differences between the spectra from neutral and ionic systems. Fifteen lines arising from doubly excited neon levels have been observed. The experimentally determined level energies are found to be in good agreement with recent calculations by Sichel and Langlois (1977).

Cross sections for one-electron loss by ⁴He⁺ ions in both atomic and molecular hydrogen have been measured in the range 35-1000 keV using a furnace target technique. The results are compared with recent calculations based on the first Born approximation.

The eikonal-Born series approximation is used to study the excitation of the 2¹P state of helium by 200-500 eV electron impact. The differential cross sections are in excellent agreement with recent absolute measurements, particularly at the higher energies. In contrast to this, further work is required to obtain satisfactory values of the angular correlation parameters.

The results of distorted-wave theory are applied to the excitation of the 6s6p ¹P₁ state of mercury for electron impact energies of 100, 180 and 300 eV. The atomic wavefunctions calculated by the Coulomb approximation provide reasonable differential cross sections and spin polarisations at backward scattering angles and also the total cross section. However, at small scattering angles the calculated results depart from the experimental results throughout the range of electron impact energies examined. On the other hand, atomic wavefunctions calculated as the eigenfunction of the Hartree field give reasonable profiles for the differential cross section and the spin polarisation for all scattering angles, however, the absolute values of the cross sections are several times larger than the experimental ones. In both calculations the spin polarisation behaves abnormally when the electron impact energy is 300 eV and the scattering angle is about 75 degrees .

The strong threshold structures in the vibrational excitation of H₂O molecules by electron impact have been studied in a crossed-beam experiment. Results for the widths, energetic positions, cross sections and angular dependences have been obtained for the fundamentals of the nu _1.3 (stretching) and the nu ₂ (bending) modes. The experiments indicate that the threshold peak in the nu ₂ mode is strongly influenced by the direct dipole excitation while for the nu _1.3 modes a resonant excitation mechanism corresponding to that proposed for the hydrogen halides should be dominating.

Taylor et al. (1977) have recently computed resonance states of HCl^- by the stabilisation method and interpreted observed e^--HCl scattering phenomena in terms of these states. Their results are shown to be compatible with an alternative interpretation, suggesting that a virtual state occurs at each vibrational excitation threshold.

The expansion of plasmas produced by focusing a CO₂ laser pulse onto solid planar targets is discussed. The plasmas are studied using an extreme ultraviolet spectroheliograph. With titanium and iron targets the plasma blow-off observed in transitions within highly ionised species (e.g. Fe XVI) occurs parallel to the target normal. The plasma is tightly confined to narrow cylindrical structures about 0.7 mm in diameter, and is observed as far as 1 cm from the target surface. The electron density is about 2.8*10¹⁸ cm^-3 at a distance of 0.7 mm from the target surface and decreases to approximately 6.5*10¹⁷ cm^-3 at a distance of 2.9 mm from the surface.

The extension to coupled eigenvalue differential equations of the standard method of solving single equations is used to solve the coupled equations of the random-phase approximation (RPA) appropriate to the (Isnp) ¹P, (1snd)¹D and (1snf)¹F states of some two-electron systems. Excitation frequencies, dipole and electric quadrupole oscillator strengths and octupole matrix elements are presented. It is demonstrated that the RPA excited states are not sensitive to the ground state.

The screening of the nuclear charge by other electrons in the atom is customarily allowed for by introducing two screening parameters sigma ₁ and sigma ₂ in the Sommerfeld relativistic two-body energy equation. If the internal screening parameter sigma ₂ is known, the total screening parameter can be calculated for any (n,l) shell whose term value is known from experiment. Values of sigma ₁(Z), with probable errors, for the levels L₂L₃, M₂M₃, M₄M₅, N₂N₃, N₄N₅, N₆N₇, O₂O₃ and O₄O₅ have been calculated for the range of atomic numbers Z=13 to Z=103 wherever data were available. The method used for the calculation is superior to the original Sommerfeld method in as much as it involves no series expansion and consequent truncation errors. A study of the variation of sigma ₁ as a function of Z gives clear evidence of shell fillings and shell closures in the periodic system of elements. The values of sigma ₁ for the various shells in the atomic number ranges Z=13 to 46 and 93 to 103 have been calculated for the first time.

Using a pulsed tunable dye laser pumped by a nitrogen laser, the 4s4p ¹P₁⁰ level in Ca I was selectively populated from the 4s²¹S₀ ground state. Transitions from the 4s4p¹P₁⁰ level to 4sns¹S₀ and 4snd¹D₂ and, for n approximately=20, also to 4snp¹P₁⁰ and 4snf¹F₃⁰ were observed. The latter transitions arise because of Stark mixing of Rydberg states in the field of laser-produced ions and electrons. By applying a voltage to electrodes in the calcium vapour, Stark shifts were measured for levels belonging to the 4snf¹F₃⁰ and 4s(n+1)d¹D₂ series in a region where the series cross. Diagonalisation of the energy matrix for interacting states with hydrogenic dipole matrix elements gave theoretical Stark shifts in reasonable agreement with those measured. The 4snf¹F₃⁰ series has been extended from n=12 to n=28 by absorption from 4s3d¹D₂. The latter level was found to be populated by stimulated emission from 4s4p¹P₁⁰.

The fine structure of the 3s3p ³P⁰ term in Mg I was calculated using multiconfiguration Hartree-Fock wavefunctions. The influence of the polarisation of the 2p shell and of relativistic effects other than the spin-orbit interaction were investigated. In this approximation the oscillator strength of the intercombination line 3s²¹S₀-3s3p ³P₁⁰ is f=2.67*10^-6, in close agreement with other results.

A theoretical study of quantum-beat intensity profiles is presented for the case of zero-field hyperfine-structure beats. By means of the graphical methods of angular-momentum theory, an explicit expression for the quantum-beat profiles, in the form of a multipole expansion is obtained.

The ionisation rates of lithium-like nitrogen (N V) and oxygen (O VI) were measured using a 60 kJ theta pinch as a plasma light source. The time evolutions of strong VUV lines were observed with a 2.2 m grazing-incidence monochromator. Two different plasmas were used (2.5 mT N₂ and 20 mT (He+1% O₂)) and diagnosed with 90 degrees Thomson scattering (T_e) and holographic interferometry (N_e). The plasma was modelled using the coupled rate equations with the semiempirical formula of Kunze (1971) for the ionisation rates. The measured ionisation rates for the lithium-like ions were found to be consistent with those calculated by Kunze's semiempirical formula to within 10%. Corrections were made in the model for end losses, temperature dependence on the excitation rates, radial electron density and temperature distributions, lowering of the ionisation limit due to collisions, and overlap of the 2s and 2p levels in the ground-state population.

Computations of width and shift parameters of rotational spectral lines in the microwave region have been performed using the Frost extension (1976) of the Anderson-Tsao-Curnutte theory (1962), a modification of this theory, the theory of Murphy and Boggs (1967) and the Cattani modification (1972) of the Murphy and Boggs theory. The parameters calculated are for the lines OCS J=2 to 3; CH₃Cl J,K=0,0 to 1,0; CH₃I J,K=1,0 to 2,0 (8a foregin to foreign and CH₃CN J,K=5,2 to 6,2 both self-perturbed and perturbed by selected foreign gases. The results are compared with experimental data. The two modified theories agree closely with one another and offer better overall agreement with the experimental widths and shifts. Differences between the calculations and the experimental results are a measure of the inadequacies of theories of this type in the region of intermediate collision impact parameter.

In a hollow-cathode discharge, molecules such as diatomic platinum hydride and platinum oxide have been obtained. The cathode was made of aluminium lined with a platinum foil which was sputtered by a discharge in argon gas. By studying the emission spectrum from the hollow cathode, it was found that, with oxygen in the argon, PtO was easily formed and that the experiments with PtO were easily reproducible. However, the formation of PtH and the isotopic molecule PtD was more sensitive to the experimental conditions. A new band spectrum was also observed, probably originating from the hitherto unknown molecule PtAl.

Expressions for the potential matrix elements are derived in the case where the particles are either linear molecules in Sigma states or atoms with an active outer electron.

For pt.I see ibid., vol.10, no.18, p.3665 (1977). Quantum-mechanical calculations of collisions between C⁺ ions and para- and ortho-H₂ have been performed in order to determine the cross section for the C⁺ 2p¹P₁2/⁰-²P₃2/⁰ fine-structure transition at thermal energies. The interaction potential, at long range, is derived from the usual perturbation theory expansions and, at short range, from available computations of the potential energy curves. The effects of coupling to excited rotational states of the H₂ molecule are investigated. The corresponding fine-structure cooling rate coefficients are tabulated over the temperature range 10<or=T<or=250K for application to studies of the thermal balance of the interstellar medium.

The drift velocity of atomic helium ions in helium has been measured at temperatures between 294 and 338K in the range 10 Td<or=E/N<or=1200 Td. The high-field theory of Skullerud (1973) was used to analyse the drift-velocity data and a cross section for symmetric charge exchange in collisions He⁺+He has been obtained for energies in the range 0.3 eV<or= epsilon _lab<or=8 eV. An investigation of the validity of the analysis indicated that this cross section is in error by less than +or-6% for energies greater than 1 eV and less than +or-8% for energies less than 1 eV. The cross section obtained is compared with recent theoretical calculations and the results from single-beam experiments.

The electron-capture cross sections in inert-gas targets for ground and metastable states of N⁺ ions have been measured in the energy range of 2-30 keV for ground-state, and 5-20 keV for metastable-state ions. Results obtained by three different experimental methods (initial growth, beam attenuation and condenser measurements) coincide above 15 keV, showing a negligible influence due to scattering. An analysis of the correlation between electron-capture cross sections and the corresponding energy defects for the three possible metastable states of N⁺, ¹D, ¹S and ⁵S, suggests that the metastable beam component observed in the present experiments is the ⁵S state. Collisionally induced de-excitation of the metastable ion has been measured. The de-excitation cross section has been found to be of the same order of magnitude as the electron-capture cross section of the metastable ion.

A semiclassical Blankenbecler and Goldberger wavefunction (1962) is used in place of the usual eikonal wavefunction in the optical-model approach to obtain the differential cross sections for e-He elastic scattering at 50, 100, 200 and 500 eV. The calculated results are in better agreement with the available experimental data than the results of the optical eikonal theory.

An intermediate-energy approximation based on the two-potential treatment of the first-order static and the second-order polarisation-absorption parts of the optical model potential has been developed. The approximation yields with only moderate computation results for e-He elastic scattering in good agreement with experiment over the entire angular range for incident energies in the range 100-500 eV and is easily extendable to more complex target atoms.

The differential cross section and the spin polarisation of electrons scattered elastically from mercury have been measured as a function of collision energy in the range 4.0 to 6.0 eV for several selected scattering angles. The observed data can be understood if one assumes that resonances are formed at 4.55 eV in the ²S_1/2, at 4.71 eV in the ²D_3/2 and at 4.94 eV and 5.51 eV respectively in the ²D_5/2 scattering state.

A set of total cross sections for scattering of electrons by He has been evaluated over the energy range of zero to 3000 eV by means of the analysis of experiments and theories on total cross sections for elastic scattering, ionisation and excitation, and on differential cross sections for elastic and inelastic scattering. Between 0 and 19.8 eV, where no inelastic processes occur, the total cross sections for scattering are equal to those for elastic scattering. Above 19.8 eV the authors have evaluated total cross sections for scattering of electrons by adding those for ionisation, excitation and elastic scattering. The total cross sections thus obtained are probably accurate to about 5% over a large part of the energy range. They appear to be in good agreement with the experimental results of Blaauw et al. (1977).

A unitarised distorted-wave polarised-orbital model is used to investigate electron impact excitation of the Li, Na and K resonance transitions at low and intermediate energies. Results are presented for integral (total) cross sections, differential cross sections, coincidence parameters, and, for K, spin-flip differential cross sections. They are compared where possible with experiment and other theoretical models. The evaluation of the adiabatic polarisation interaction is discussed. Core-exchange effects are investigated for the case of Na. Tables of T-matrix elements are available for all three cases to aid in interpretation of photon-electron coincidence experiments.

The excitation by electron impact of levels of neon, argon, krypton and xenon atoms that decay by emission of ultraviolet light has been studied as a function of incident energy up to a few eV above the energies of the lowest excited levels. The width (FWHM) of the energy spread of the incident beam was typically less than 30 meV. A crossed-beam interaction region was used and UV photons emitted approximately normally to the electron and gas beam directions were detected by a channel electron multiplier. Precautions were taken to avoid detection of scattered electrons and metastable atoms by the photon detector. The onset positions of the lowest excited levels were used in calibrating the incident energy scales. The energies of resonance structures in the excitation functions could therefore be obtained, and are compared with the results of previous experiments.

A merged-beam experiment designed to study collisions between electrons and molecular ions at very low energies and with high energy resolution is described. The theory behind the technique is reviewed and a comparison is made with other intersecting-beam techniques. Cross sections for dissociative recombination of H₂⁺ and H₃⁺ with electrons have been measured through the energy range 0.01-4 eV with an estimated energy resolution of approximately 0.04 eV. The structure found in both curves reflects the formation of Rydberg states of the molecules which in some cases choose to decay through the autoionisation and pair-production channels.

Vibrational excitation of the CO₂ ground state by electron impact through the ² Pi _u resonance between 3 and 5 eV has been studied using an electron-impact spectrometer. Further experimental evidence is obtained on the 'boomerang effect' occurring in the symmetric-stretch vibrational mode of the compound state. A one-dimensional model has been used to describe the observations and the complex symmetric-stretch potential for the linear configuration of the CO₂^- state has been obtained.

By setting on resonance a polar gas at low pressure with an inhomogeneously saturating coherent wave, an oscillating transient nutation is observed, the fast damping of which may be principally related to the electromagnetic field inhomogeneity. The authors report the first experimental evidence of 'rotary' photon echoes allowing a partial restoration of the macroscopic polarisation. These 'rotary' echoes are the electric analogues of Solomon's 'rotary spin echoes' in NMR (1959) and must be carefully distinguished from the usual photon echoes related to the free induction decay (FID). The experiments have been performed in the microwave domain and show a dramatic reduction of the expected echo amplitude due to the molecular motion. Eventual applications, alternative methods of detection and the extension to the optical domain of these 'rotary' echoes are discussed.

New data for the mobility of Ar⁺ ions in argon are reported. An influence of spin-orbit coupling on the mobility of atomic argon ions in the ²P state could not be detected. The new data disagree slightly with those reported by Beaty (1962).