Table of contents

Generalized oscillator strengths and their first derivatives for zero momentum transfer (i.e. in the optical limit) are calculated for the helium atom in the framework of the random phase approximation with exchange. A good agreement with the experimental data of Backx and colleagues (1975) is obtained.

Beam-foil mean-life measurements have been made for two levels in Sc II in an attempt to resolve a discrepancy between earlier beam-foil and beam-laser measurements. The measurements yield the mean-lives tau (3d4p ¹P)=8.6+or-0.6 ns and tau (3d4p ³F)=6.5+or-0.4 ns. The discrepancy between the beam-foil measurement of Buchta and colleagues (1971), and the beam-laser measurement of Stoner and colleagues (1976) for the ¹P mean-life is explained as a result of a blend between Sc II and Sc III.

Intercombination transitions 2s²2p⁴³P-2s2p⁵¹P⁰ in the oxygen-like ions from Fe to Zn are obtained in laser-produced plasma spectra. The relative intensities and the wavelengths of these transitions are in good agreement with calculated ones. The laboratory data obtained may be used for predicting the wavelengths of forbidden lines in solar flares.

The L emission spectrum of tungsten has been reinvestigated using a newly constructed curved crystal spectrograph. Three new lines at 1074.5, 1075.6 and 1076.3 XU corresponding to the transitions L_IIN_VII, L_IN_I and L_IIO_II have been found. Previously reported transitions L_IO_I, L_IIN_II, L_IIM_V, L_IM_I and L_IIM_III have been confirmed.

Using a pulsed dye laser and the method of delayed coincidence the lifetime of the 7, 8 and 9²P_1/2 and ²P_3/2 levels were measured in pure Cs vapour and in a mixture of Cs and Xe (Xe pressure: 3.3, 50, 200 and 600 Torr). Under the influence of collisions with gas atoms the lifetimes of these levels are shortened. This effect is interpreted as energy transfer by collisions between levels within the range of the thermal energy.

Laser-induced fluorescence has been used to study the metastable content of argon and xenon ion beams extracted from an electron bombardment source. The first direct evidence for the production of the 3d'²G_7/2 and 3d'²G_9/2 levels of Ar⁺ has been obtained, and detailed measurements of the energy dependence of the excitation cross sections for the 3d'²G_9/2Ar⁺ and 5d⁴D_7/2 Xe⁺ levels have been carried out.

The spectra of ejected electrons following the excitation or ionization by 2 keV electrons in the outermost np shell of Ca (n=3) and Sr (n=4) have been measured with high resolution. Binding energies of the np_1/2 and np_3/2 electrons as well as the excitation energies of np⁵(n+1)s²nd¹P levels of Ca and Sr have been determined.

Comparative measurements of differential cross sections for 2³S and 2¹S excitations of helium in He⁺ on He collisions at an incident energy of 150 eV are presented. These processes are isolated from the other n=2 excitations by detecting the scattered ions in coincidence with the recoil metastable atoms. A simple three-state model is proposed to interpret semi-quantitatively these excitation processes.

Protons with kinetic energies up to 4.4 eV have been observed in the dissociative photoionization of H₂ using Ne II 26.9 eV line radiation. The most energetic protons must result from the dissociative autoionization of doubly excited H₂, and it seems most probable that the lowest state of ¹ Sigma _u⁺ symmetry is involved. It is estimated from the integrated kinetic energy spectrum that at 26.9 eV the photoionization cross section for the production of 2 eV to 4.4 eV protons from H₂ is 0.012_-0.004^+0.007 Mb.

The vibrational state distributions of fast N₂, CO, O₂ and NO molecules produced by (symmetric) charge transfer reactions involving 0.004 to 2.2 keV incident ions have been determined via a multistate impact-parameter treatment which closely couples many vibrational channels. The product N₂ and CO molecules are primarily formed in the ground vibrational level at all impact energies, while the O₂ and NO products occupy a range of vibrational levels with these distributions broadening as the kinetic energy of the incident ion is increased.

Using a semiclassical method, the author has derived a formula to give the transition probability between two electronic states of a colliding two-atom or molecule system. He has not imposed severe restrictions on either the form of the diabatic potentials or the time dependence of the relative motion of the particles. The difficulties of the Landau-Zener-Stueckelberg formula are removed.

The optical Autler-Townes effect (dynamic Stark splitting) induced by a single-mode CW dye laser has been observed in neon gas with a cascade level scheme and a He-Ne probe beam. The splitting is found to be proportional to the square root of the saturating laser power and independent of the detuning. The existence of two Rabi nutation frequencies due to level degeneracy is clearly shown by changing the polarization axis of the probe beam.

Methods of implementing ab initio relativistic self-consistent field techniques for atomic configurations containing open shells are discussed with a view to economical calculation of a whole group of atomic states. The low-lying energy spectrum of Hf III has been studied in terms of wavefunctions including the 5d², 5d6s, 6s² even-parity manifolds and the 5d6p, 6s6p odd-parity manifolds. The results show that the valence orbitals are substantially modified by direct and indirect relativistic effects, so that non-relativistic models must require substantially longer and more complex calculations to give results of comparable precision.

The 4s²4p⁴4d+4s²4p⁴5s+4s4p⁶ configurations of Kr II have been investigated theoretically, including configuration-interaction effects. The investigation has been carried out by fitting the observed energy levels to theoretical energy expressions using the least-squares technique. The calculated energy levels show much better agreement with the experimental values than those calculated without configuration interaction. The 4s²4p⁴(¹D)4d²S_1/2 level is found to interact strongly with the 4s4p⁶²S_1.2 level, and to a lesser degree with the 4s²4p⁴(¹S)5s ²S_1/2 level. The energy parameters obtained from the least-squares fit are in agreement with the results of parametrized central-field calculations.

Oscillator strengths are presented for the transitions 2s2p ³P⁰ to 2s3l³L^e(l=0,2), 2s3s³S^e to 2s3p ³P⁰ and 2s3p ³P⁰ to 2s3d ³D^e for members of the beryllium sequence up to Ne VII. Configuration-interaction wavefunctions are used and results are expected to be accurate to within about 5%. Comparison with recent beam-foil experimental data is generally good, though in some cases cascade effects may be larger than had been expected.

The intensity ratio of the K²-KL hypersatellites K_{alpha 1}^h and K_{alpha 2}^h is calculated in intermediate coupling. Agreement is found with present and previous experimental data.

The authors have investigated the two-step process 6²S_1/2 to 6²P_3/2 to (epsilon ²S_1/2, epsilon ²D_3/2, epsilon ²D_5/2) in Cs for circular polarization of the radiation in both steps. Theoretical expressions are presented for the photoelectron spin polarization P_z and the ion count-rate 'asymmetry' A_i (ratio of ion rates for right or left circular polarization in the second step), based on the hyperfine-coupling scheme for the first step. These expressions contain ratios of the three radial matrix elements involved in the ionizing step. The experimental determination of P_z and A_i, for different groups of hyperfine transitions, permits an evaluation of these ratios. The experiment was performed on a Cs atomic beam using a pulsed GaAs laser at 8521 AA for the first step and a pulsed Ar-ion laser at 5017-4545 AA for the second step. Effects of saturation in the first transition are studied and optimal conditions for use of the process in a spin-polarized electron source are discussed.

The 4¹D level in a helium flowing afterglow was populated by two-stage excitation: resonance radiation trapping in a flowing afterglow populated the 2¹P level which was then pumped with a tunable dye laser. Fluorescence decay of the other n=4 levels was observed, and this excitation transfer is ascribed to neutral collisions. The data is fitted by a simple model giving excitation transfer and associative ionization cross sections. This model suggests that excitation to the triplet levels only occurs via the F states and that the selection rule Delta L=+or-1 is preferred for all transitions.

The excitation-transfer process between S- and P-state atoms is investigated theoretically by the impact-parameter method. With respect to the P states of an atom, all three degenerate states of mutually orthogonal polarization are taken into account. This yields coupled differential equations which can be solved numerically by the Runge-Kutta-Gill method. As a typical example, the cross section for the excitation transfer between H and D is calculated numerically. The cross section is found to have its maximum 420 pi a₀² at a relative velocity of about 1.5 v₀, where a₀ is the Bohr radius and v₀ is the thermal velocity at 300K. In the high-energy region, the decrease of the cross section is inversely proportional to the relative velocity.

Excitation cross sections sigma and rate coefficients (v sigma ) are studied for transitions with a small resonance defect omega in electron-atom and ion-atom collisions. The universal curves for sigma and (v sigma ) are obtained in the Born approximation for dipole allowed transitions using a model potential of the type V(R)= lambda R/(R²+R₀²)³2/. The choice of effective radius R₀ is suggested. The condition of validity of the Born approximation for transitions with a small resonance defect is obtained in the form ( lambda omega /v²)²<or approximately= omega R₀/v<or approximately=1 where v is the velocity of the incident charged particle.

Calculations have been performed for the P-wave scattering of positrons by hydrogen atoms for energies up to the first excitation threshold of hydrogen. The trial wavefunction for the complete system consists of a two-state expansion corresponding to the two possible open channels (elastic scattering and positronium formation) in this energy region plus 50 short-range correlation terms. The authors present results for the elastic and positronium formation cross sections. Values for the P-wave contribution to the annihilation rate parameter Z_eff are also given.

Analyses the difficulties created in the molecular approximation to atomic collisions from the so-called 'momentum-transfer' problem. It is shown that, except in some very special cases, no satisfactory solution to the problem exists at the present time. Furthermore, contrary to previous assertions, there is, in principle, no reason why the effect of momentum transfer should decrease with decreasing energy.

The molecular constants of twenty electro-vibrational levels in the spectrum of the LuF molecule are obtained through direct adjustment of the wavenumbers. The global fit of the nineteen bands involving these levels leads to a considerable improvement in the accuracy and consistency.

Recent experiments on the dissociative recombination of H₂⁺ and D₂⁺ are summarized and the projection operator formalism of resonance scattering processes in molecules is reviewed. The application of the theory to specific systems requires the calculation of electronic energies and matrix elements, and of vibrational wavefunctions; comprehensive results are presented for H₂⁺ and D₂⁺. To obtain further insight the author develops a semiclassical theory which is surprisingly successful; in particular the cross sections for producing atoms in high Rydberg states are predicted to be large, in agreement with experiment. The rate of recombination at thermal energies (<300K) requires special consideration: it is shown that the so-called indirect mechanism is significant and that the total thermal rate is reasonably fast.

Close-coupling calculations have been performed to investigate the formation of three-particle bound states in a model collinear system. A generalization of Levinson's theorem (1966) is employed to determine the critical potential strengths necessary for the existence of the triatomic states. The system consists of an atom interacting through a Morse potential with a diatomic molecule bound by a harmonic potential. Comparisons are made with classical phase-space and S-matrix theory and with quantum-mechanical variational and perturbation calculations. When only a few bound states are involved it is shown that the quantal approximations are more accurate than the classical results.

The mobilities of atomic ions of krypton and xenon in their parent gas are shown to depend on the spin state of the ion. In both gases the mobility of the ground-state ion (²P_3/2) was found to be lower than that of the metastable ion (²P_1/2). This difference originates in the different interatomic potentials which arise from the interactions of (²P_3/2)+(¹S₀) and (²P_1/2)+(¹S₀) states respectively. The results may be related to a recent theoretical investigation of a difference in the charge-exchange cross section of both spin states at low ion energies (Sinha and Bardsley 1976). The zero-field mobilities were obtained by extrapolation at 294K in krypton and at 273, 293 and 345K in xenon.

The DC conductivity of a thermally produced plasma in a plasma diode has been measured in pure potassium vapour. A simple theory for determining the influence of the plasma sheaths on the measured conductivity was developed and experimental conditions were established under which the potential drops across the sheaths could be neglected. By measuring the DC conductivity of the plasma at temperatures ranging from 1250 to 1600K the potassium momentum-transfer cross section has been obtained. The measured cross section (2.37+or-0.24)*10^-14 cm² at 1500K is in good agreement with the recent experimental results of Beynon and Brooker (1972). Good agreement with the two-state exchange close-coupling calculation by Karule (1965) was found.

Conditions for obtaining spontaneous population inversion between electronic levels in atomic or molecular ions as a result of selective velocity distributions in fast-moving plasmas are discussed. Experimental evidence from a plasma accelerator is presented which suggests that these conditions, which can occur between widely separated energy levels, are realizable, and the suitability of particular devices for use as a laser is also considered.