Table of contents

A procedure for directly prescribing a term of arbitrary order in an alpha Z expansion of the Dirac energy of a one-electron atom is presented, and utilised to obtain higher-order corrections to the Dirac fine-structure formula. These can then be combined with terms not included in the Dirac formalism and applied, for example, to semi-empirical charge-screening parametrisations of multi-electron atoms.

Ab initio SCF calculations in the Hartree-Fock approximation of static parallel and perpendicular polarisabilities and their first derivatives for CO₂ and N₂ are reported. For N₂, results for values of the internuclear separation from 1.7 to 2.4 a₀ are presented. For CO₂, only the linear symmetric-stretch geometries are considered, and polarisabilities are given for values of the oxygen-carbon separation from 2.0 to 2.4 a₀.

The influence of autoionisation resonances in Tl atoms on the photoionization cross section angular distribution and spin polarisation of photoelectrons is considered using the random-phase approximation with exchange. A new effective method of producing the spin-polarised electrons is proposed. The method consists of irradiating Tl atoms by a circularly polarised light source of wavelength and width equal, respectively, to the wavelength and width of the autoionisation resonance with total angular momentum J=¹/₂.

An expression is derived for the amount of two-photon ionisation produced by sub-nanosecond laser pulses under resonance conditions. On this time scale damping effects can be neglected and the ionisation probability becomes proportional to the cube of the pulse duration. A similar prediction for resonance multiphoton processes of higher order leads to a qualitative agreement with recent experimental observations. Finally, it is demonstrated that the same result can be obtained from a simple analysis of a two-step model.

Reports a series of self-broadening and self-shifting measurements of some rotational transitions of CH₃Br and CH₃I as a function of temperature and rotational quantum numbers.

Single-electron-loss cross sections for ground-state hydrogen atoms and ground-state and metastable helium atoms colliding with xenon atoms have been measured in the energy range from 50 keV to 4 MeV. A pronounced double-maximum structure has been observed. The maxima occur at approximately the same impact velocities in the three cases studied.

It is shown that Coulomb-Born-Oppenheimer cross sections are in close agreement with distorted-wave results for 1s to np transitions in hydrogenic ions with Z>or=6 at all energies. Results are given for n=2, 3, 4, 5, 6 in Fe XXVI and compared with Coulomb-Born values. Results are also given for the 1s-2s transition at selected values of Z(2<or=Z<or=26). The results are intended for use in planned solar-maximum mission experiments.

Expressions for the angular distributions of electrons in the excitation of O₂ to the a ¹ Delta _g and to the b ¹ Sigma _g⁺ states are given. Assuming that both processes proceed via the O₂^-2 Pi _g state, theory agrees very well with experiment. Further, the mechanism for the parity unfavoured ³ Sigma _g^- to ¹ Sigma _g⁺ transition is demonstrated to be rotational-electronic coupling.

A semi-empirical definition method of the dependence of electronic moment on internuclear separation in diatomic molecules is presented. The calculation has two stages. In the first stage the matrix element of the electronic transition moment over vibrational wavefunctions is calculated analytically by the stationary-phase method. As a result, the band strength is expressed as a sum taken over the vibrational quantum number of the electronic transition moment. In the second stage the coefficients of the expansion of the electronic transition moment in real internuclear separations are obtained by the least-squares method. The suggested method avoids the weaknesses of the r-centroid method. As an example of this method, the calculation of the electronic transition moment of the Meinel band system of N₂⁺ is given.

Accurate bound-state wavefunctions obtained by the close-coupling method in the frozen-cores approximation are used to calculate oscillator strengths for neutral oxygen. Results include transition probabilities for all terms of O I (excited and ground states) obtained by adding an s, p or d electron to the ground-configuration terms of O II. The authors consider Rydberg series up to n=6. Many of the transitions are parentage forbidden and configuration mixing provides the driving mechanism. Results agree with experiment to within 10%. Strong intercombination lines are observed in oxygen; the effects of deviations from LS coupling on the allowed transitions presented are discussed by using the unpublished results from an independent investigation by Zeippen (1977). Calculations include the continuum region, and the photoionisation cross section for oxygen is obtained. These results are in very satisfactory agreement with close-coupling calculations by Taylor and Burke 1976.

The continuous absorption coefficient of N^- is calculated in a three-configuration target approximation. It is found that absorption coefficients are very sensitive to the number of states included in wavefunction expansions; results for three to eight states are given.

The sixth spectrum of Mn emitted from a vacuum sliding-spark light source has been observed. A method of distinguishing Mn VI from other spectra is described. 43 lines of the transitions 3d4s-3d4p and 3d4p-3d4d have been classified in the 800-1550 AA range, whereby the four levels of 3d4s and 15 of the 18 levels of 3d4d have been established and the accuracy of the 3d4p levels has been improved significantly.

The intercombination lines 1s3p ³P₁ to 1s²¹S₀ of the ions Mg XI, Al XII, P XIV, S XV and Cl XVI have been observed for the first time in the spectra of laser-produced plasmas. The wavelengths and relative intensities of such lines were measured. The identification of the observed lines was carried out by means of a comparison of the experimental wavelengths and intensities with the theoretical ones. The dependence of the relative intensities of such lines on the plasma parameters was investigated.

The vacuum-ultraviolet fluorescence of Kr and Xe and its dependence on gas pressure (10 to 500 Torr) was studied under monochromatic excitation in the vicinity of the first (³P₁) and second (¹P₁) resonance states. For excitation synchrotron radiation was used. Excitation in the long-wavelength wing of the ³P₁ states directly results in excited molecules (e.g. 2Kr+h(cross) omega to Kr₂*). The radiative decay of these molecules results in the first (Kr: 1250 AA, Xe: 1500 AA) and second (Kr: 1470 AA, Xe: 1700 AA) continua. Under resonant ³P₁ excitation, molecule formation via three-body collisions (e.g. Kr*+2Kr to Kr₂*+Kr) is established. Excitation of ¹P₁ states results in ¹P₁ emission and the continua. The ¹P₁ emission contains molecular components. In Xe, it is strongly quenched by collisional interaction between the ¹P₁ state and a near-lying atomic p state.

Mercury atoms are excited in a weak gas discharge to the metastable 6³P₂ state, forming a longitudinal alignment, which is observed in the absorption of the linearly polarised green Hg line ( lambda =5461 AA). By a pulsed RF field, transverse components of the alignment are produced additionally, giving rise to modulated absorption. During the free precession the signals decay due to depolarising collisions with mercury atoms in the ground state. The cross sections for the destruction of the alignment have been determined to be sigma ₀=(262.9+or-5.2) AA² for the longitudinal component, sigma ₁=(257.8+or-7.5) AA² for the first transverse component (modulated with the Larmor frequency) and sigma ₂=(273.1+or-9.0) AA² for the second transverse component (modulated with twice the Larmor frequency). It is emphasised that these values refer to the alignment.

It is demonstrated that a specially designed wall-stabilised arc combined with a Michelson interferometer and a stigmatic spectrograph is suitable for hook measurements. For a first test of the system relative f values of 24 resonance lines of Ti I have been determined and converted to an absolute scale by means of literature data. The results are in agreement to within better than 25% with published data which were obtained by entirely different methods.

Starting from the exact unified expression of the line profile, a quantum theory is developed making exact allowance for the long-range dipole interaction. The knowledge of the positions of the bound states below the n=2 threshold enables approximation of the contribution of short-range interactions. Good agreement with exact quantum calculations is obtained.

The Stark broadening of some Al I and Al II lines emitted in an argon plasma jet in which the average electron density is 1.28*10¹⁷ cm^-3 is investigated for the Al I lines and 1.54*10¹⁷ cm^-3 for the Al II lines. The average temperatures are about 13300K and 14000K respectively. The experimental values of lineshifts and linewidths are compared with the semi-classical theoretical results of Sahal-Brechot (1969) and Griem (1975). Good agreement, within an error of 20%, is observed. For Al II lines, the measured and calculated widths are narrower by a factor of two to five those measured in any previous experiment.

Saturated-absorption profiles of the lines at lambda =557 nm involving the lowest metastable level of krypton show intense and broad Gaussian backgrounds due to thermalising collisions. These backgrounds are greatly reduced by the addition of xenon atoms which quench the metastable level. The variation of the amplitude of the background relative to the amplitude of the narrow absorption resonance versus pressure is quadratic in pure krypton and linear in mixtures of krypton and xenon. An interpretation of the results is given taking into account transit-time effects, while making use of published results for the quenching cross section.

The semiclassical approximation of the off-shell t-matrix elements t_l(k',k;E) for potential scattering (Korsch 1976) is studied for the case of purely repulsive potentials. The general features of the semiclassical approximation are discussed. Numerical calculations are presented for a Hulthen potential describing the triplet H-H interaction. A comparison of the semiclassical approximation with exact results shows very good agreement. The dependence of the t_l(k',k;E) on the momenta, the angular momentum, and the energy is investigated.

For the example of collisions of alkali metals with mercury the potentials for the ground state and the lowest excited states are determined by a comparison of a pseudopotential calculation with experiments. The potentials for Na, K, Rb and Cs are described by one common pair of parameters in the pseudopotential. From this example a new method for the evaluation of scattering data is derived, which should allow a uniform description of the potentials of a class of collision systems.

A charge-exchange optical-pumping experiment to study the temperature dependence of the Cs-Cs⁺ charge-exchange cross section sigma has been performed. Cs⁺ ions are polarised by resonant charge-exchange collisions with optically pumped Cs atoms. The Cs⁺ ion resonance linewidths are measured over the temperature range 20-26 degrees C. They are used to study the charge-exchange cross section as a function of temperature. The cross section sigma (Cs⁺ to Cs) varies inversely as the square root of the temperature. The charge-exchange cross section sigma (Cs⁺ to Cs) has also been re-measured and found to be (6.6+or-0.5)*10^-14 cm², in good agreement with theoretical calculations.

A second-order method for charge exchange is introduced. Using the impact-parameter method, a transition amplitude for the ground-state capture from atomic hydrogen by fast protons is obtained in closed analytical form. At vanishingly small impact parameters rho , the Brinkman-Kramers (BK) transition probability (behaving as nu ^-10 with increasing incident velocity nu ) is found, but reduced by the factor 0.2857. At large impact parameters, the dominant contribution arises from the second-order effects in the second-order in the electron-proton interaction. Total cross sections for the capture into 2s and 3s states of atomic hydrogen in H⁺+H(1s) collisions as well as for the capture summed over all final discrete states of H are in excellent agreement with experimental data.

The authors calculate the cross sections for the process of charge exchange between ion and atoms in the case of one-electron systems using a molecular description of the collision. Results show that the charge-exchange process selectively populates one excited state of the ion. Extension of this conclusion to systems with more than one electron is discussed.

An uncoupled distorted-wave formalism is presented. Source parameters necessary for obtaining the coincidence rate of Ly- alpha photons and scattered electrons for electron excitation of the 2p state of hydrogen at 54, 100, and 200 eV are calculated in this formalism. These results are compared with source parameters obtained in other theoretical models.

The ejected-electron spectrum of lithium vapour has been observed at 75 degrees with respect to an incident electron beam of 80 eV, 200 eV and 500 eV kinetic energy. A total of 39 autoionising levels of Li I are observed in the energy range 56 eV to 65 eV. Comparison is made with experimental and theoretical data.

Cross sections for the formation of positronium in the scattering of positrons by neon and argon atoms have been calculated using the Born and a distorted-wave approximation. The Born approximation values are greater than the experimental estimates of Coleman et al. (1975), although they are in closer agreement with the results of Kauppila et al. (1976) (argon only). However, the distorted-wave approximation values are smaller than the experimental ones except for the energy range between the positronium and excitation thresholds.

The cross sections for vibrational and rotational excitation of H₂, Li₂, Na₂ and K₂ by slow electrons are calculated in the adiabatic approximation using a zero range potential model to obtain the amplitude of electron scattering for a 'frozen' molecule. The advantage of the zero range potential model is the possibility of taking into account explicitly the effect of multiple scattering. The results are compared with the available experimental data and other calculations.

The polarisations of four excited levels in ⁴⁰Ar⁺ ions have been measured at different times after excitation in a beam-tilted-foil experiment. Within the uncertainties typical of present beam-foil work, the possible transfer of polarisation resulting from optical cascades has no observable effect on time-resolved quantum-beat measurements.

Both phenomenological and rigorous kinetic theories are used to describe transient effects on the drift velocity and mean energy of ions in drift tubes, assuming that spatial variations are negligible and that ion diffusion is much slower than other processes. The theories are accurate by comparison with Monte Carlo calculations for special cases. Experimental implications are discussed.

The authors have studied the excitation of the B² Sigma _u⁺ state of N₂⁺ by energy transfer from neon in Ne-N₂ mixtures at total pressures of several hundred torr. The decay of the light pulse corresponding to the transition nu '=O, nu "=O( lambda =391.4 nm) was reconstituted by means of the time-amplitude conversion method. The curves decrease exponentially. The variations of the decay time constant with the partial pressures of Ne and N₂ enabled the kinetic scheme of the transfer process which uses Ne⁺ ions to be deduced.