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The use of anomalies in emitted intensities due to cancellations of the transition integral is suggested as a probe of small model-dependent effects. Calculations of predicted anomalies in both the total intensity and in the relative intensities of fine-structure components are presented for specific transitions in certain ions of the K I isoelectronic sequence, which indicate the experimental feasibility of this approach.

In an earlier measurement of the He II 4686 AA line (1977) (at electron densities of (1-3)*10¹⁸ cm^-3 and electron temperatures of 9.4 to 19.9 eV) the authors compared the measured profile with the only available calculation. Good agreement was found between the electron densities predicted by the Stark-broadening theory and those measured in the experiment. In this letter they present the comparison of their experimental data with a Stark-broadening profile using the unified theory. Both the theories describe the experimentally obtained profile, but the densities obtained by fitting to the unified theory line profile are in strong disagreement with the experiment.

For the first time an optical transition between bound states in a negative ion of an atom has been established experimentally. The transition at 3490 AA is observed after beam-foil excitation. By applying an electric field it is shown to belong to the Li^- spectrum. According to Bunge (1980) it connects two core-excited quintet states in the negative lithium ion.

Using the distorted-wave approximation the authors have calculated the degree of alignment of the 2p_3/2 subshell vacancy in Ar resulting from the ionisation of Ar atoms by electron impact. The use of the distorted-wave approximation allows one to widen the range of agreement between theory and experiment significantly.

The authors demonstrate numerically the convergence, in the sense of minimum variance, of the stabilisation method for several basis sets, using as an example the 1s resonance of the perturbed hydrogen atom. This convergence permits choosing between conflicting results for very high fields from different calculations. Using the maximum stabilisation criterion in a definition of convergence, they show that the agreement between Hehenberger and co-workers (1974) and Froelich and Brandas (1975) is due to the fact that the results of the latter authors have effectively converged.

Measurements of the attachment rate coefficient nu _a/N² for thermal electrons in O₂ and O₂-N₂ mixtures using the Cavalleri electron sampling technique (1969) show an unexpected pressure dependence. This dependence is explained in terms of a perturbation to the expected Maxwellian energy distribution function caused by the selective loss of electrons by attachment at the resonance energy for the process. Theoretical analysis of the problem predicts the general features of the experimental observations but overestimates the effect when commonly accepted values of the cross sections are used.

The full transverse interaction generally has not been computed in atomic structure calculations, though relevant formulae have appeared in the literature. The paper describes an economical algorithm which requires little more effort than the simpler algorithm for the Breit interaction. Examples are given to validate the method.

On the basis of a local model potential a semiquantitative MO method is suggested combining some basic ideas of the X alpha approximation and the iterative extended Huckel theory. The molecular potential is represented alternatively by the superposition of simplified atomic charge densities or atomic X alpha potentials, and the non-spherical neighbour contributions to the potential are taken into account explicitly. The iteration is a self-consistent charge procedure reducing the computational effort considerably compared to the SCF theories; the method should, therefore, be applicable to large molecules. The approximations are discussed and directions are described by which improvements are possible.

For pt.I see ibid., vol.13, no.14, p.2683 (1980). The self-consistent charge X alpha method, described in the preceding paper, has been applied to thirteen H, C and O containing small molecules. The calculated molecular ionisation potentials, dipole moments, XPS intensities and equilibrium geometries show the same order of agreement with experiment as ab initio or other model potential calculations. The method is fast enough to be applied to larger systems.

Foil excitation of fast ion beams has been used to produce few-electron ions of magnesium and aluminium. The emission spectra have been analysed with a 10 cm curved crystal X-ray spectrometer. The spectra show series of transitions in H- and He-like ions and numerous satellite lines. By comparison with Hartree-XR calculations, these lines are seen to arise from core-excited two- to five-electron ions.

Emission lines of krypton IV and V belonging to the 4s²4p^q-4s²4p^q-14d and 4s²4p^q-4s4p^q+1 transition arrays are classified with the aid of a theoretical model developed for analogous Fe XII and Fe XIII transitions in a different atomic shell. Xenon VI lines are also identified. The observational material consisted of archived Zeta spectrograms.

The spectrum of light scattered by a two-level atom in an intense laser beam with amplitude modulated light is calculated. The spectrum is obtained by evaluating the Fourier transform of the autocorrelation function of the dipole moment. The modulation introduces new components in both the coherent and incoherent parts of the frequency distribution. The total intensity when plotted as a function of the average Rabi frequency exhibits parametric resonances.

The authors present a weak light field, scattering theoretical description of the intensity autocorrelation function for the scattered light from a many-atom (molecule) system. The theory considers both resonant and near-resonant scattering for atoms with many sublevels in both the ground and electronically excited states, and it incorporates the effects of the finite coherence time of the incident radiation. The theory is derived using incident two-photon wave packets with spatial dispersion among the target atoms. Purely anti-bunching contributions arise from single-atom-two-photon scattering and from successive two-atom-two-photon scattering events. However, there are interleaved two-atom-two-photon scattering processes which provide purely bunching contributions since both atoms may be scattering at the same time. Path length differences in the successive two-atom scatterings can also induce some bunching character to the correlation function. The dependence of the risetime and decay of the intensity correlation function upon the atomic excited state lifetimes and the coherence time of the incident light emphasises the correlation between photons which have been coherently and/or incoherently scattered.

The broadening and the shift of the cadmium 5s5p³P₁-5s²¹S₀, lambda =3261 AA line caused by the presence of helium and argon were determined using excitation with the frequency-doubled output of a nitrogen laser pumped dye laser and detection of the fluorescent light of the two hyperfine structure components of ¹¹¹Cd by a photomultiplier. For frequency calibration the Zeeman splitting of the resonance line of ¹¹⁴Cd in a magnetic field was observed. The measurements were carried out for temperatures ranging from 230 to 390 degrees C, and the foreign gas pressure was varied from 0 to 760 Torr. For the cadmium-helium system, the shift is beta =-0.22(10) MHz Torr^-1 and the broadening gamma =2.52(17) MHz Torr^-1. For the cadmium-argon system, the shift is beta =-1.86(80) MHz Torr^-1 and the broadening gamma =2.05(34) MHz Torr^-1. The broadening gamma is quoted for the half half-width (HWHM). The minus sign of beta refers to a red shift. No temperature variation of the shift and broadening parameters was observed. The dependence of broadening and shift on the perturbing gas pressure is interpreted in terms of the impact theory of collision broadening, developed by Lindholm and Foley (1945), assuming a Lennard-Jones (6-12) interaction between radiating and perturbing atoms.

An experiment is described in which the absorption spectrum of the Rydberg series of sodium (principal quantum number, n>or=4) is studied in a strong non-resonant laser field. The field is generated from a PTM laser oscillator/amplifier system operating at 1.06 mu m, producing 250 MW in a 10 ns pulse. After splitting, most of the beam is focused into a sodium heat pipe oven where field strengths up to 8*10⁷ V m^-1 are generated. The other fraction of the beam is used to form a plasma from a solid lead target which is used as a continuum source for absorption spectroscopic measurements. Quenching of high-lying state (n>11) has been studied, the lowest quenched state being found to be dependent on laser intensity I_l in the form I_l varies as n^{-(3.9+or-0.8)}. Such quenching is significantly different from that observed in a DC field. Additionally, line broadening, profile asymmetry and intensity dependent shifts are observed.

Ab initio molecular orbital calculations suggest that the A ² Pi state of BeF is regular, with Lambda doubling constants p and q of the same sign, implying a molecular orbital configuration with a single unpaired pi electron. This is in contrast to the published spectroscopic analysis of the A-X spectrum, which gives p and q of opposite signs, normally associated with inverted Pi electronic states.

The energy shifts of K_beta X-rays and the K_beta /K_alpha intensity ratios by 2.8 MeV N⁺-ion bombardments have been measured for eight elements from Ti to Ge. The K_beta /K_alpha ratios are expressed as relative values to the ratios for 2 MeV proton impact. The experimental results indicate single K- plus multiple L- and M-shell ionisation, and strongly reflect the relative importance of M-shell vacancies in the K_beta /K_alpha ratios at the time of K X-ray emission. These data are compared with the Hartree-Fock-Slater calculations and used to deduce information about the vacancy configurations of ionised atoms. The present results can be qualitatively explained within the framework of the direct Coulomb ionisation theory.

The rate coefficients and product ions for the reactions Ne²⁺, Ar²⁺, Kr²⁺ and Xe²⁺ ions with He, Ne, Ar, Kr and Xe have been determined in the Birmingham selected-ion flow tube (SIFT) at 300K, and the energy variation of several of these reactions has been studied in the Innsbruck drift tube (IDT). In the case of Xe²⁺, the reactions of the ground state (³P) and the metastable electronic states (¹D₂ and ¹S₀), were studied whereas for Ar²⁺ a study of only the ³P and ¹S₀ state reactions was possible. In the case of Ne²⁺ no state selection was achieved and for Kr²⁺ state selection was only achieved in the reaction with Ne. Single charge transfer was the only observed mechanism in all of the reactions except the Ne²⁺ reactions with Kr and Xe in which double charge transfer channels were also evident. The widely varying rate coefficients are interpreted in terms of a model involving the internuclear separation, R_x, at which the ion-induced dipole attractive and the Coulombic repulsive potential curves cross, and which indicates that a 'reaction window' exists for R_x between 2 and 6 AA. The drift tube data are totally consistent with the model and the increasing rate coefficients with increasing energy for the Kr²⁺(¹D₂)+Ne and Xe²⁺(³P)+Ar reactions clearly indicate an increasing efficiency for the production of the ²P_1/2 states of Kr⁺ and Xe⁺ rather than the ²P_3/2 states which are the favoured products at thermal energies.

The polarisation of atomic spectral lines, excited by electron impact, shows in some cases an anomalous behaviour near threshold. It is suggested that this anomaly may be caused by the influence of electron correlation effects similar to those which give rise to specific threshold laws for electron impact ionisation. Both model calculations and experiments have been performed which tend to support the proposed model.

Using a modified trapped-electron technique in a crossed-beam geometry, the authors have measured the total relative cross sections for electron impact excitation of the (2s²2p²3s)⁴P and (2s2p⁴)⁴P states of atomic nitrogen in the region near threshold. The (2s²2P²3s)⁴P cross section exhibits a sharp resonance at threshold, within their resolution of about 100 meV, and decreases to half the peak value 0.45 eV above threshold. The cross section for the (2s2p⁴)⁴P state increases monotonically from threshold in agreement with theory. The significance of their results to recent in situ measurements of 1200 AA and 1134 AA emission in the airglow is discussed.

The electron impact excitation of the configurations 2s2p⁶3s and 2s2p⁶3p in neon has been observed in the energy-loss mode over a range of impact energies and scattering angles. The spectra of the 3p configuration have been analysed to obtained the variation with angle and energy of the line shape parameters and intensities for the ¹P and ³P terms. Spectra of the 3s configuration show predominantly the ¹S term but a detailed analysis has enabled a value to be obtained for the energy of the ³S term which is in disagreement with previous results. The variation of line shape and intensity for the ¹S term is given. A very broad feature at an excitation energy of about 44 eV has been observed for the first time and its behaviour is discussed.

Resonant scattering of electrons from molecules is discussed in the context of an exactly solvable model. There is a strong non-adiabatic entanglement of electronic and nuclear motions for resonances near threshold. The calculations show that the interaction of the electronic and vibrational motions can lead to a dramatic reduction of the resonance width. The limits of applicability of the so-called compound state formula, which has been used so far to analyse low-lying shape resonances in molecules, are pointed out.

Absolute total electron scattering sections for H₂ have been measured from exponential attenuation in the energy range 0.2-100 eV with 0.07 eV FWHM energy resolution. Resonant structures in the 11-15 eV energy range are presented for the first time with their absolute size. The overall absolute error on the cross section was estimated to be from +or-1.7 to +or-5% in different energy regions. Comparison with previous experimental and theoretical results are given. The authors' measurements are higher than the previous ones although not further than the combined error. A fair agreement can be found with the calculations of Hara (1968).

Electron energy-loss spectra have been obtained for electronic states of acetylene in the range 7-11 eV with resolution as high as 0.015 eV. Measurements taken at high scattered electron energies and low scattering angles enable quadrupole allowed singlet states to be observed, and the authors have assigned three such transitions as involving ¹ Sigma _g⁺ and ¹ Delta _g or ¹ Pi _g states. At low scattered electron energies and high scattering angles a substantial number of transitions involving triplet states are observed, all but one of these for the first time. By energy comparison with nearby singlet states the authors have tentatively assigned most of these to electronic states theoretically predicted to lie in this region.