Table of contents

Eleven new Ca I lines observed in emission have been identified with transitions which confirm the new 4snd ¹D levels (7<or=n<or=13) found by Armstrong et al. (1977) and give improved energy values for these levels. In addition, the unidentified satellite lines observed in an absorption experiment by McIlrath (1974) on selectively excited calcium have been interpreted as transitions due to the Stark effect.

Spectra taken with a lead stearate pseudocrystal show no evidence for the KL satellite of boron at 191 eV or the KL³ satellite of carbon, and very weak emission in the carbon KL² satellite at 287 eV. Experimental details and two possible interpretations are presented. L vacancies may be filled before X-ray emission, thereby decreasing satellite fluorescence yield. This may be the case in boron, where the response time of the electron gas is much smaller than the lifetime of the boron ion with a K vacancy, while in carbon these times are comparable. The possibility that the fast ions produce mostly K ionisation cannot be ruled out by the experiment.

Recent experiments suggest that a highly stripped ion can remove up to 10 electrons in a single collision with a neutral atom or molecule. It is shown that this process can take place through the coherent superposition of the heavy ion's electric field at different times, in a way analogous to multiphoton ionisation by intense radiation fields. For relatively fast ions, a simple theory is developed and results are presented for a variety of cases. The theory is in reasonable agreement with measured cross sections for 1s-vacancy production in Ne by Cl^Z+ ions at 50 MeV (2<or=Z<or=15).

By means of absolute light intensity measurements between 2000 and 6500 AA cross sections have been determined for electron capture into excited projectile states during collisions between 100 keV Ne^z+ (z=1,2,3,4) and different target gases. Total single-electron-capture cross sections have been obtained by measuring ion-current attenuation. There is direct experimental evidence that capture into excited states is related to the energy defect of the respective reaction and furthermore, that for higher projectile charge states an important part of the single-electron-capture process is caused by capture into excited states with moderate exothermic energy defects.

The angular distribution of O^- ions produced by dissociative attachment of electrons in N₂O around 2.3 eV has been studied for scattering angles from 30 to 130 degrees. The angular distribution does not depend on the incident energy between 1.9 and 2.9 eV, and is almost symmetrical with respect to 90 degrees . Analysis of the data shows that the contribution of the s wave to the process is important.

Hanle effect studies are reported of several vibration-rotation sublevels of the state B ³ Pi _0u⁺ of the two homonuclear isotopes of bromine, yielding mod g_J mod and sigma ⁽²⁾ values. The states were populated by exploiting the accidental coincidence of the 5145 AA line of the Ar⁺ laser with the electronic transition x ¹ Sigma _g⁺ to B ³ Pi _0u⁺.

The relative sizes of the neutral from hydrogen to xenon are discussed in terms of several properties based on the density distribution. Of the various indices of atomic size considered, a density contour approach is shown to be most consistent with the periodic behaviour of chemical and physical properties and to allow satisfactorily for the arbitrariness inherent in any theoretical approach to atomic dimensions. By means of an empirical scaling procedure, the theoretical radii are scaled to the range of values common to empirical radii.

It has been shown earlier by Mande and Damle (1965) that screening constants for orbital energies can be calculated from Dirac's relativistic equation using X-ray spectroscopic data. This work is now extended to calculate the effective nuclear charge for the valence states of atoms of a number of elements. Using the values of the effective nuclear charge, a new scale of electronegativity, consistent with Pauling's original definition, has been constructed. This method has the advantage that it does not use any arbitrary method of determining the screening constants. The merits of the new scale are discussed.

Equations are derived for the polarisation of light emitted by impulsively excited atoms. A spontaneous electric dipole transition between Russell-Saunders terms is considered. The density matrix of the excited term is described by a double tensor expansion which allows for anisotropy in the directional distribution of the spin as well as the orbital angular momentum. Conditions are discussed under which one may test for the presence of spin anisotropy by polarisation measurements or zero-field quantum beats.

A dressed-atom approach is generalised for studying several effects which could be observed on an atomic beam interacting with two monochromatic resonant laser beams saturating two atomic transitions sharing a common level, a situation which occurs frequently in stepwise excitation experiments. The problem of the optimisation of the population of the upper state is investigated. Analytical expressions are derived for the positions, the widths and the weights of the various components of the fluorescence and absorption spectra. The results obtained at the limit where one of the two lasers has a much weaker intensity than the other one are interpreted perturbatively by treating to lowest orders the scattering of the weak laser beam by the atom dressed by the intense one.

A technique is described by which an atomic beam can be used to measure relative changes in microwave power down to the 100 ppm level. Its relevance to the measurements of the n=2 hydrogen Lamb shift is discussed in detail.

A simple and practical arrangement of coupling lenses for viewing a fast beam light source with a grating spectrometer is shown to provide very efficient light collection with good spectral and spatial resolution together with a simple procedure for calibrating spectra.

Lifetimes of He I states (n=3, 4 and 5) have been measured by the beam-foil technique. For most states the experimental accuracy is of the same order as that of the theory. Some lifetimes differ significantly from theoretical predictions. The lifetimes of the states 4^1,3F have been measured for the first time.

Comparison of theoretical and experimental lifetimes has been performed for excited levels in the Ga I and In I isoelectronic sequences in order to test the applicability of radiative lifetimes based upon semiempirical calculation methods. The theoretical lifetimes are obtained from (i) calculations using a single-parameter numerical Coulomb approximation, for which a consistent cut-off criterion has been developed, and (ii) published data based upon a parametrised Dirac radial equation. Twenty-five experimental lifetimes for Ge II, As III, Se IV and Sn II, Sb III, Te IV have been measured by means of the beam-foil technique. Good agreement is obtained between theoretical and experimental lifetimes for higher-lying levels, whereas deviations are present particularly for low-lying nd ²D levels.

Investigates experimentally the broadening and the shift of the sodium 3S-4D and 3S-5S two-photon transitions without Doppler broadening by collisions with the noble gases. The broadening cross section is larger for the 3S-5S transition than for the 3S-4D transition. The experimental values for the 3S-5S transition are compared with semiclassical calculations using the van der Waals potential and the potentials calculated by Pascale and Vandeplanque (1974).

Stark width and shift parameters of Li, Na, K and Rb resonance lines have been experimentally determined in an electromagnetic T-tube plasma. The plasma parameters were varied in a wide range, electron temperature from 15000K to 26000K and electron number density from 1.5*10¹⁷ cm^-3 to 5*10¹⁷ cm^-3. Measured values of Stark parameters were compared with those calculated theoretically by Griem (1974) and fair agreement was found. Some regularities in behaviour of Stark parameters were noticed.

The influence of spectral-line blending, line-wing loss and self-absorption on the experimental determination of the integrated intensity of various Ar I lines emitted by a homogeneous LTE plasma have been investigated using a computer-based model of the spectrum. Correction factors have been determined for the 714.7, 560.67, 641.63 and 430.01 nm lines as a function of electron density and as expected have been found to be largest for lines originating from the most highly excited levels. These results appear to explain some recent experimental observations more simply than explanations in terms of the high-density corrections to the Boltzmann distribution of number densities.

The ejected-electron spectrum of magnesium vapour has been observed at 75 degrees with respect to an incident electron beam of 62 to 400 eV incident energy. Ejected electrons corresponding to both Mg I autoionising levels and Auger transitions are reported. Comparison is made with photoabsorption data, ejected-electron spectra from ion impact and high-energy electron impact experiments and available theoretical calculations.

The time-dependent Schrodinger equation for a particle in one dimension in the field of two colliding delta potentials can be solved exactly. Assuming an initial state which is bound about the left delta potential for large negative times, the solution has been used to calculate the probabilities of finding a state bound about one of the potentials long after the collision. These probabilities are oscillating functions of the reciprocal velocity. The ionisation probability goes to zero for high collision velocities and reaches ¹/₂ for small velocities.

Absolute differential elastic cross sections have been measured for K, Rb-He, Ne, Ar, Kr, Xe as a function of collision energy between 0.5 and 50 eV at selected scattering angles between 5 and 15 degrees . The repulsive part of the intermolecular ground-state potential is determined at internuclear distances of about 0.3r_m<r<0.9r_m and 0.3r_m<r<0.7r_m for K, Rb-noble-gases respectively, where r_m is the position of the potential minimum.

Measurements are reported of the cross section for the non-symmetric charge transfer ³He²⁺+H* to ³He⁺+H⁺ in the relative energy range 0.25 to 478 eV for excited hydrogen atoms with principal quantum numbers n ranging from 8 to 24. The experimental method involves merged beams and time-correlation analysis of the reaction products. Field ionisation is used to modify and specify the population of the excited states. The results are presented as cross sections sigma (n₁, n₂) averaged over consecutive values of n ranging from n₁ to n₂, namely sigma (8, 9), sigma (10, 14), sigma (15, 19) and sigma (20, 24). The cross sections are large, up to 10^-10 cm², and appear to be roughly independent of energy below 50 eV.

For pt.II see ibid., vol.9, p.269 (1976). The He⁺-Ar collision is studied experimentally in the 100-2000 eV laboratory energy range. Differential cross sections sigma ( theta ) are determined for the main inelastic processes. Direct excitations of Ar are identified by the electrostatic energy analysis of the scattered ions and charge-exchange processes are investigated by a time-of-flight technique. The He⁺-Ar system differs markedly from the He⁺-He and He⁺-Ne collisions previously studied by the importance of the charge-exchange processes into excited states of Ar⁺ and by the weakness of the charge exchange in the ground state. Ab initio potential energy curves and differential cross section calculations confirm the proposed excitation mechanism, at least for moderate-energy collisions.

A variational formulation of the e^--H scattering problem is presented in which both direct and exchange effects are described by a purely local potential. The theory is illustrated by calculations of ¹S, ³S, ¹P and ³P elastic scattering phaseshifts and by a calculation of the affinity of H^-.

General expressions are derived for the partial transition matrix elements for a transition from any initial state to any final state of the hydrogen atom by electron impact using an approximate form of the Faddeev equations, including exchange and taking into account the coupling of all the intermediate bound states.

The alignment of ions with an inner-shell vacancy resulting from ionisation by a fast charged particle is considered. The alignment has been calculated in the Born approximation using the Herman-Skillman model. The angular distribution and polarisation of characteristic X-radiation and Auger electrons from decay of aligned ions are discussed. It is shown that the theoretical anisotropy of the angular distribution is sensitive to the wavefunctions used. The results of calculations using the Herman-Skillman wavefunctions are in much better agreement with experiment than the previous calculations involving the hydrogen-like functions.

The authors have investigated structure near the L_2,3 edges of argon, the M_4,5 edges of krypton and the N_4,5 edges of xenon, using electron impact at an incident energy of 1.5 keV and with an energy resolution of typically 70 meV. The structure observed corresponds to the promotion, via an optically allowed or optically forbidden transition, of an electron from an inner shell of the atom to an unoccupied Rydberg orbital. The energies of these excited states have been obtained, enabling the accurate determination of the Rydberg series limits. The effective quantum numbers of the excited states have been found to be closely related to those of the 'equivalent-core' atoms K I, Rb I and Cs I.

A general R-matrix theory of low-energy electron scattering by diatomic molecules is developed. The theory is based on the frame-transformation approach of Chang and Fano (1972) in which different representations are used for different ranges of the electron-molecule distance coordinate. Particular emphasis is given to short distances where exchange and correlation effects between the electron and the molecule are strong, and it is shown in detail how the R matrix on the surface of this region can be expressed in terms of a multi-centre configuration-interaction wavefunction. Finally, a description is given of the numerical procedures which are used in the calculation of electron collision cross sections for an arbitrary diatomic molecule.

Absolute oscillator strengths for absorption near the K edge in N₂ and the carbon K edge in CO have been derived from 8 keV electron energy-loss measurements. The resonance structure in the spectra is compared to results of the multiple-scattering model as applied by Dehmer and Dill (1976).

The authors report relative abundances for the formation of CO⁺, O⁺, CO²⁺, C⁺, O²⁺ and C²⁺ ions from CO following dipole transitions in the carbon K shell (287-310 eV). A complete collection is made of all dissociation fragments and their kinetic energy is estimated. They find evidence for post-collision interaction, which transfers some oscillator strength from the double- (and triple-) ionisation channels to that of single ionisation. The corresponding excited CO⁺ states are found to be completely dissociative.