Table of contents

Calculations are performed for the photoelectric cross sections and angular distributions for the CO molecule. The method used is the single-centre coupled partial-wave method, in which the potential is the sum of the static potential and a local exchange potential for an electron in the field of CO⁺. Comparisons are made with recent experimental data.

A calculation of the photoionisation cross section of CO₂ reveals that the sigma _u shape resonance at approximately 19 eV, accessed by the 1 pi _g and 4 sigma _g valence levels, is reduced and broadened by coupling to the ground-state vibrational motion. Off resonance the effect of nuclear motion is negligible. Results are compared with experimental branching ratios.

The absorption spectrum of UF₄ in the vapour phase has been recorded and is compared with the absorption spectrum of solid uranium in the energy range corresponding to excitation of a 5d electron. The close similarity of the two spectra, which are dominated by strong peaks above each of the 5d thresholds, provides experimental evidence for the existence of a resonantly localised 5, epsilon f continuum state of essentially atomic character in uranium.

A time-of-flight system developed to study positron scattering has been adapted for electrons. Total scattering cross sections have been measured for helium and argon in the energy range 2-50 eV to absolute accuracies of better than 4%.

The authors have observed the He I 2³P-3¹D intercombination line by using laser absorption spectroscopy over a 140 m optical path in a laboratory plasma. Whilst electric-field-induced forbidden transitions are well known in laboratory plasmas, this is the first observation of a collision-induced intercombination line. Discussion is given of the likely underlying mechanisms.

The partial level density in a many fermion system is studied for widely varying values of the range of the interaction. The effect of the range is examined through matrix calculations and scalar averages. With increasing range, the results show an increasing departure from the Gaussian form as well as a slower rate (with respect to the number of particles) of approach to normality. The distribution of the two-body matrix elements for the different ranges of the interaction is also studied.

The development of the tunable dye laser as well as other more advanced experimental techniques has recently made possible the high-resolution study of the fine structure of the highly excited Rydberg states of alkali atoms. By recognising analytically the n dependence of the energy eigenvalue of the highly excited state based on its quasi-hydrogenic character, the authors were able to analyse quantitatively all the existing experimental measurements. The qualitative aspects concerning the fine structure splitting will also be discussed.

The procedure adopted by Deodhar and Rai for sorting out spin doublets in X-ray satellite spectra has been critically examined. It has been demonstrated that though spin doublets follow the two rules regarding the constancy of wavelength separation and linearity of ( Delta nu/R)^1/4 against Z, the converse is not always true. It is pointed out that some of the spin doublets reported by Deodhar and Rai are spurious and it is suggested that efforts should be made to find other physically meaningful criteria for establishing the existence of spin doublets in X-ray satellites which, hopefully, may throw light on the origin of X-ray satellites.

Antibunching phenomena have been extensively discussed previously (1979) in the case of an intense driving field. Previously the intensity correlation function has been obtained in the so-called weak-field case by a limiting procedure. The author gives a description of weak light beams from a general light scattering point of view which has not been given previously. He shows that some specific phenomena like the indistinguishability of the photons or the wavepacket structure of the weak light beam have been neglected in earlier works by the use of their limiting procedure. He describes the scatterer rigorously, so the dynamical evolution is not restricted to the particular case of two-level atoms as in all previous work. Even with weak fields and a two-level system his result exhibits a supplementary contribution associated with the boson character of the photon. In addition the theory is extended to cover near-resonant phenomena.

Energy levels are calculated for fourteen low-lying states of atomic hydrogen in a static uniform magnetic field using a basis of simple separable functions of cylindrical symmetry. Results are presented for field strengths 0<or= gamma <or=1, where gamma =h(cross) omega _c/2 Ryd, (0<or= mod B mod <or=2.35*10⁵ T). The eigenfunctions corresponding to bound states are used to obtain the electric dipole transition probabilities. For strong transitions (A_nm>0.1*10⁸ s^-1) the authors' transition probabilities at 10³ T and 10⁴ T agree with those of Brandi et al. (1976). Previous results for fields B<or=10⁴ T are extended to B<or=2.35*10⁵ T. The simple basis functions used allow bound-free transition probabilities (to the unperturbed Landau continuum) to be evaluated in closed form.

The plasmas under investigation have been generated in a 4 mm bore wall-stabilised arc operating at atmospheric pressure with currents in the range 20 to 100 A in argon as well as in a mixture of argon and hydrogen. The electron density was determined with a statistical error of less than 1% by means of 2 lambda interferometry at lambda 632.8 nm and lambda 1152.3 nm. Possible systematic uncertainty caused by excited atoms was estimated to be less than 3% for the electron density range 1.89*10¹⁶ cm^-3<or=N_e<or=1.64*10¹⁷ cm^-3. Under the assumption of local thermodynamical equilibrium, experimental transition probabilities were obtained for the Ar I 430.0 nm line (A_nm=3.2*10⁵ s^-1+or-7%) and the Ar II 480.6 nm line (A_nm=7*10⁷ s^-1+or-14%). The values are not line-wing corrected. For the considered electron density range, the experimental half and quarter widths (in the reduced wavelength scale) of the H_beta line were found to be constant within the limits of the experimental accuracy. In addition, it is shown that the experimental data of the Ar I 1430.0 nm line transition probabilities given in the literature can be brought to a common value by considering the present reduced widths of the H_beta line profile.

The angular distribution of electrons from autoionising states is derived, taking into account fine and hyperfine structure. Partially unresolved structures of the decaying state lead to a depolarisation of the anisotropy of the angular distribution. A detailed investigation of this effect is presented and applied to various coupling schemes important for the analysis of experimental data.

It is shown that the absorption of photons by electrons above the ionisation threshold can take place through one- and two-step processes. These competing mechanisms show quite different features. Diagnostic methods based on the intensity and pressure dependences and on the magnitude and the angular distribution of the probabilities are proposed.

The authors give the theory of optical Faraday rotation and circular dichroism for a transition of mixed magnetic-dipole and electric-quadrupole character in the presence of hyperfine structure. The results are applied to two transitions of atomic bismuth, lambda 647.6 nm (J_j=³/₂ to J_j=⁵/₂) and lambda 875.5 nm (J_j=³/₂ to J_j=³/₂). They have studied the Faraday rotation profiles of these lines under high resolution; the results confirm the theoretical predictions and for each transition give a value for the ratio, chi , of the electric-quadrupole and magnetic-dipole reduced matrix elements, (J_j//( omega /4 square root 3)(-2er²C⁽²⁾)//J_i) and (J_j// mu //J_i). They obtain chi =-0.60(2) for lambda 647.6 nm and chi =+0.13(7) for lambda 875.5 nm, to be compared with the theoretical values -0.65 and +0.11 respectively.

Energy spectra of 15-35 eV electrons ejected from 15-70 keV Li⁺-Ne collisions have been measured. The lines are assigned to neon configurations of the type K 2s²2p⁴ nln'l', K 2s2p⁵ nln'l', K 2s2p⁶ nl and K 2s2p⁵ nl. Several lines which have not been observed or identified before are classified. The spectral pattern changes drastically with increasing energy. At low energies lines from levels with two 2p vacancies dominate. At high energies only lines with a 2s vacancy are observed. Analysis of this strong velocity dependence of the n=2 neon subshell correlation shows that the correlation at the 2 sigma -3 sigma pseudocrossing obeys the Barat-Lichten rule at low velocity and confirms the prescription of Eichler et al. at high velocity. A corresponding critical velocity of nu _c approximately=0.3-0.4 au is derived.

A low resolution survey comparison of the population distribution of the CH A² Delta -X² Pi system formed by the dissociative excitation of acetylene, acetone and butane by the impact of 21 keV Kr⁺ and of butane by 43 keV Ne⁺ ion bombardment is reported. Little sensitivity of the rotational level population distribution to the target molecule from which the radical was ejected, or to the environment or the latter-whether gas phase, or adsorbate layer on a silicon substrate, or solid (frozen)-was observed. Apparent non-equilibrium of the rotational level distribution in the (0,0) band for 21 keV Kr⁺ ion induced dissociation contrasts with the equilibrium-like distribution for 43 keV Ne⁺ bombardment, despite only a factor of three increase in projectile velocity. A particular sensitivity of the rotational level excitation mechanism to the electronic structure of the projectile is suggested, rather than to the collision time, or molecule, or molecular environment from which the radical was derived. The vibrational level population distribution is found to exhibit a strong parent molecule and environment dependence.

The K_beta /K_{alpha 12} X-ray intensity ratio of the Si K spectrum was measured for proton impact on SiO₂ in the energy range 300-800 keV. An energy dependence of the intensity ratio was found and an explanation is given in terms of multiple ionisation.

A charge-exchange optical-pumping experiment to measure the K+K⁺ charge-exchange cross section in a He buffer gas has been performed. K⁺ ions are polarised by charge-exchange collisions with optically pumped K atoms. The value of the charge-exchange cross section sigma _ce(K⁺-K) at thermal energy is measured to be (5.4+or-0.8)*10^-14 cm².

Photon emission between 20 and 250 nm, and slow electron and ion production cross sections have been measured for collisions between Ar⁶⁺ and H₂ at impact energies from 200 to 1200 keV. These studies indicate that single electron charge transfer into excited states of the product ion is the most important inelastic process. The capture occurs mainly into n=4 levels with the excitation of the higher angular momentum states dominating over most of the projectile energy range. Slow electrons are apparently produced primarily by the capture ionisation process in which the projectile captures an electron and an additional electron is emitted from the target. The capture ionisation cross section is appreciable, amounting to 30 to 40% of the total excitation cross section.

The optical model is applied to the elastic scattering of electrons from the metastable 2s state of atomic hydrogen. Polarisation effects are found to be very small compared with those arising from absorption. Calculations are performed at 200 and 400 eV, and demonstrate that the optical model can predict reliable results at all values of the momentum transfer, in contrast to perturbative calculations.

The authors have modified the generalised Coulomb-projected Born approximation proposed by Stauffer and Morgan (1975) by introducing a screening parameter which varies with the coordinates of the incident particle. Total and differential cross sections have been calculated at various medium and high incident energies for 1s to ns (n=2,3,4) transitions in electron-hydrogen collisions. Elastic scattering results are also included for electron-hydrogen collisions.

Electron-impact excitation may occur either directly, or by a resonant process involving a doubly excited autoionising compound state. The contribution of the resonances is computed here by a perturbation method involving the principle of detailed balance and the calculation of autoionisation and radiative-decay transition probabilities. In ions of the Be I isoelectronic sequence, resonances are found to be of negligible importance for the strong 2s²¹S-2s2p ¹P degrees excitation, but to provide 25 to 50% of the total excitation rate for the weak ¹S-³P degrees excitation. For atoms more than 3 or 4 times ionised, this method-combined with distorted-wave calculation of the direct excitation process-should, in most cases, give total rates comparable in accuracy to detailed close-coupling calculations, and would appear to be particularly useful when departures from pure LS coupling are great or when radiative decay of the compound state is important.

Absolute cross sections were determined from transmission measurements made with an electron time-of-flight spectrometer in the energy range of 0.02-2 eV. The total cross section for molecular hydrogen was found to increase monotonically with energy; no resonance-like structures were observed. The isotopes H₂, D₂ and HD have identical cross sections within the accuracy of the measurements (+or-2.5%). Helium cross section measurements were also made in order to check the spectrometer.

A simple model is found for the anomalous density dependence of electron mobilities by applying the energy shift and broadening of multiple scattering theory plus the uncertainty principle to the transport equations. The shift alone when properly applied immediately predicts the observed positive and negative effects in Ar. The exponential decrease observed in H₂, CO₂, and He is found to result from a combination of their negative energy shift plus the broadening, which together push the electron distribution function down to localised negative energy states. Possible application of the model to amorphous semiconductors is also discussed.