Table of contents

State- and isotope-selective perpendicular excitation of a thermal energy, mixed metastable Ne(2p⁵3s ³P₂, ³P₀) atom beam with a polarised 640.2 nm laser beam yields a well defined sample of ²⁰Ne(2p⁵3p ³D₃) atoms, which are ionised by UV photons (wavelength range 331-412 nm). Core-switching processes are found to be negligible: autoionisation features in the region between the two Ne⁺(²P_3/2,1/2) fine-structure thresholds and photoelectrons corresponding to Ne⁺(²P_1/2) ions could not be detected. The application of these results for the production of state-selected, polarised Ne⁺(2p^{5 2}P_3/2) ions is briefly discussed.

A novel technique of measuring small pressure shifts in spectrally broad CO₂ laser transitions is described. A very small shift-to-width ratio is observed for the P(20) transition in the 10.6 mu m band in a one atmosphere TEA laser.

The impact parameter dependence of Kr L vacancy production in 1.4 MeV Kr-Xe and Kr-Kr collisions has been studied by means of ion-photon coincidence experiments. A theoretical analysis shows that rotationally induced vacancy transfer from the quasimolecular 4f phi orbital into the 4f sigma orbital via 4f phi -4f delta -4f pi -4f sigma rotational coupling is the dominant mechanism for KrL excitation in both collision systems.

Angular correlations between scattered electrons which have excited the 2¹P state of helium from its ground state and the 58.4 nm photons from its subsequent decay have been measured for an incident electron energy of 26.5 eV and for electron scattering angles up to 120 degrees . The variations of both the lambda and chi parameters with electron scattering angle differ significantly from previous measurements at higher incident electron energies. There is good agreement between experiment and the R-matrix calculations of Fon et al. (1980) for lambda over the whole angular range.

Reports theoretical cross sections for the elastic and inelastic scattering of low-energy electrons (10.8-13.4 eV) from the ground state of argon atoms. The calculations are carried out in the LS coupling scheme and include nine target states in the expansion of the total wavefunction. The energies and shapes of the resonances which the authors find agree well with the experiments of Brunt et al. (1976). In most cases the authors also confirm their grandparent model although some of our resonant assignments are different.

Recently, experimental isotope shifts have become available for individual levels of Li and K. The authors have used many-body perturbation theory to calculate all first- and second-order contributions to the specific mass shifts. The correlation effects were obtained by numerical solution of inhomogeneous radial 'pair equations'. For the lowest s, p and d states of Li the authors results were 962, -3273 and -2 MHz compared with the experimental results of 1111(6), -3608(11) and 0(6) MHz, respectively. As in the authors earlier hyperfine structure calculations, the need to include higher-order effects is more pronounced for K, especially for the strongly perturbed d states. Adding the lowest-order correlation effects to the results obtained with a self-consistent treatment of the change in the core orbitals due to the valence electron gave satisfactory agreement for the 4s and 4p states in K, -53 and -30 MHz, respectively, compared with the experimental data, -61(2) and -28(2) MHz. The 5d state shows a larger discrepancy, the theoretical shift being -22 MHz compared with the experimental result-53(8)MHz, emphasising the need to include higher-order correlation effects.

High-resolution selective field ionisation (SFI) spectra of laser-excited Na(ns, nd) atoms with 28<or=n<or=36 reveal structure that is strongly dependent upon the slew rate of the ionising field. Two mechanisms, which may lead to this structure, are identified and discussed.

Transition probabilities for 11 Kr I and 9 Kr II lines between 366.5 and 599.3 nm were obtained from measurements with a wall-stabilised arc at atmospheric pressure in pure krypton. The population densities of the excited krypton levels were calculated under the assumption of LTE from electron densities measured by laser interferometery. The uncertainties for the Kr I and the Kr II data are 15 and 25% respectively.

The profile of the 2p^-2(³P)3p¹P resonance in the photoionisation of neon has been calculated in the close-coupling approximation with two open and two closed channels. An integral method of calculating the atomic continuum wavefunction in the vicinity of resonances is used, which is based on Bloch's formulation of the unified nuclear reaction theory. Good agreement with experiment has been obtained for the characteristics of this resonance in the approximation which simulates the violation of the LS coupling for the classification of such states.

Calculations of the triple differential cross section (TDCS) of the laser-assisted Compton effect in hydrogen-like atoms are reported. The theoretical model is a generalisation of the field-free exact treatment given years ago by Eisenberger and Platzman (1970), without resorting to the impulse approximation. The laser field is taken in the dipole approximation, homogeneous and single mode. The incident and the outgoing X-ray photon energy are taken to be 20 and 19.95 keV, respectively, and the X-ray photon scattering angle is taken as 5 degrees . The laser electric field is considered to be much smaller than the characteristic intra-atomic field, while the laser frequency is off-resonant with any pair of levels of the target atom. Significant changes in the shape of the TDCS are found when laser photons are exchanged. The origin of the modifications to the TDCs is discussed in detail. The considerations given here for modifications of the Compton electron angular distribution also largely hold true for other laser-assisted processes, such as, for instance, the X-ray photoeffect and electron-atom ionisation.

A detailed rotational analysis of the A¹ Pi -X¹ Sigma ⁺ transition in As ¹⁴N and As¹⁵N molecules was performed in order to explain the numerous perturbations observed in the ( sigma pi *) A¹ Pi state. A deperturbation of the ( nu =0 to 4) levels of this state shows that several levels of the ( sigma pi *) a³ Pi state are involved in the observed interactions. Spectroscopic constants deduced for both these states are as follows: A¹ Pi : T_e=35999 cm^-1, omega _e=869 cm^-1; omega _ex_e=5.1 cm^-1, B_e=0.5006 cm^-1, alpha _e=0.0068 cm^-1; a³ Pi : T_e=29632 cm^-1, omega _e=897 cm^-1, omega _ex_e=6.9 cm^-1' B_e=0.5045 cm^-1, alpha _e=0.0061 cm^-1. The effective spin-orbit constant A for a³TGP is found to be 420 cm^-1. A discussion is made about the nature of some states perturbing A¹EGP.

A semiclassical approximation for the momentum space matrix elements of the transition operator T(E) for elastic potential scattering is derived. The derivation is based on a previously discussed semiclassical approximation of the matrix elements t_l(p', p; E) in the angular momentum decomposition. The final expression for the full matrix elements (p' mod T(E) mod p) between plane-wave states can be written in a compact form which agrees with a formula recently obtained by Gerber (1981) using a different approach. The general features of the T-matrix elements as well as the underlying physical picture are discussed and illustrated by numerical results for the Coulomb, the r^-2 and the exponential potential.

Target K-shell ionisation and K-shell capture cross sections differential in projectile scattering angle have been measured for selected combinations of accelerated ¹H, ⁴He and ⁷Li ions and gaseous targets of neon and argon. The K-shell vacancy producing events were identified by scattered-particle, KLL Auger electron coincidence techniques. The choice of projectile-target combinations has resulted in a set of data illustrating for fully stripped projectiles, the dependence of the K-shell processes on projectile atomic number Z₁ and target atomic number Z₂ at a fixed collision velocity nu , measured relative to the orbital velocity of the K-shell electron nu _K ( nu / nu _K approximately=0.46). Further, in view of theoretical predictions, the selection is aimed in particular at forming groups of data with approximately constant Z₁/Z₂ ratios. In one case, He to Ne, the authors also studied the K-shell ionisation process (i) for projectiles carrying electrons, to test for a possible charge state effect, and (ii) for faster, fully stripped projectiles ( nu / nu _K approximately=0.8). The authors also present measurements of impact-parameter-dependent, total capture probabilities.

The authors study electron capture from the K-shell of a hydrogenic target to the K-shell of a bare ion at the asymptotic high-energy limit. The agreement between the continuum distorted-wave (CDW) and the second-order Brinkman-Kramers (B2) approximations is theoretically proved not only for total cross sections but also for angular distributions. The shape of both approximations near the double scattering critical angle is carefully analysed in terms of the transverse impulse transfer. Numerical results for the CDW, exact and peaking B2 approximations are compared for proton-hydrogen resonant capture.

The optical attenuation method (OAM) developed recently by Matsumoto et al. (1980) has been extended to photon emission in the VUV spectral region to investigate initial- to final-state selective electron capture in Ne²⁺-Xe collisions at 40 keV. One-electron-capture cross sections for Ne²⁺ ions in their ³P ground state were found to be only one half of that for the metastable ¹D ions which is explained by a larger contribution from transfer ionisation for the latter. With these results initial- to final-state selective excitation cross sections for the most important electron capture channels were derived and found to be consistent with the corresponding total one-electron-capture cross section.

The non-iterative integral formalism is utilised for the calculation of total and differential cross sections, both elastic and inelastic, for electron-hydrogen-atom scattering. The energy range extends from the ionisation threshold upwards 1.00<or=k²<or=4.00 Ryd (13.6 to 60 eV). In this range a large number of partial waves (L>25) is required. The influence of more accessible coupled channels is examined by including n=3 levels. Comparison with other close-coupling calculations and with recent experiments shows this method to be really competitive in the numerical sense.

Presents original experimental and theoretical data for the autoionisation of helium by electron impact at 100 eV energy. The results concern the ejected electron angular distributions, consequent on the decay of the (2s²)¹S, (2s2p)³P, (2p²)¹D and (2s2p)¹P autoionising levels, for a scattering angle of 15 degrees and an average over scattered electron directions. Generally good agreement is obtained between theory and experiment, except at large ejection angles for the double differential cross sections.

Studies the excitation of various krypton states by electrons for impact energies in the vicinity of the first ionisation threshold. In some of the excitation curves broad oscillatory structures are observed which extend beyond the ionisation threshold. The occurrence of these structures is associated with the occurrence and decay of correlations in the motion of two slow electrons in the field of the ionic core during the excitation process. An anomalous threshold behaviour is observed in the excitation curves of high angular momentum states. A tentative explanation for this behaviour is formulated by considering the angular momentum dependence of the decay probability of the two-electron correlation.

The Coulomb-Born-Bely approximation, which is an extension of the Ochkur-Rudge approximation to the excitation of the positive ion by Bely (1966) has been used to obtain the cross sections for electron impact excitation of helium-like ions (3<or=Z<or=10, Z=14, 20 and 26). The present cross sections for 1¹S to 2¹S and 1¹S to 2¹P transitions have been compared with those of the Coulomb-Born, Coulomb-Born-Oppenheimer and distorted-wave approximation.

TEA CO₂ laser induced perturbation of shape and intensity of the first Balmer lines emitted by a hydrogen low-pressure glow discharge is investigated for radiation intensity I_L in the 10 MW cm^-2 range. Because of the AC Stark effect, shifts of levels n_a=4, 5, 6, 7 and 8 are measured (i.e. for I_L=8 MW cm^-2, Delta E₅=-0.59 cm^-1, Delta E₆=1.81 cm^-1, Delta E₇=0.55 cm^-1). In addition, splitting and optical satellites originating from high orbital quantum numbers are clearly observed on the H_delta line. The laser one- and two-photon ionisation processes almost completely depopulate the excited states n_a>or=8. Because of collisional radiative processes the states n_a=4, 5, 6 and 7 which do not undergo photoionisation are also strongly depopulated. From a kinetic model, an approximate calculation of the collisional excitation transfer rate coefficient shows that the infrared laser perturbation method can be a powerful tool for investigating collisional excitation transfers connecting excited state population in plasmas.