Table of contents

Some of the atomic levels of Cs I and the Cs-Xe molecular terms were excited using a pulsed tunable dye laser in a Cs-Xe gas mixture. The time-resolved fluorescence from the directly populated levels or the states populated by collisional and radiative transfer were detected employing the delayed-coincidence technique. On analysing the shapes of the fluorescence curves, the lifetimes of the molecular states were found to be shorter than 1 ns and to have no significant influence on the lifetime measurement of the atomic states within the range of Xe pressures applied (3.3 up to 600 Torr). The lifetime for the 7²S, 5²D and 4²F atomic states of Cs could be determined.

Exploratory calculations of atomic photoionisation cross sections including both relativistic and correlation effects are undertaken using the relativistic random-phase approximation (RRPA). In these preliminary calculations the authors examine the photoionisation of He and Be atoms. The cross sections and asymmetry parameters for these light elements agree precisely with non-relativistic RPA values. Elastic phaseshifts for the e^-+He⁺ and e^-+Be⁺ systems are obtained and agree with non-relativistic calculations in the ¹P eigenchannel; the ³P phaseshifts have no counterpart in non-relativistic theory.

A model assuming simple analytical forms for potential curves and coupling matrix elements is used to interpret the experimental results on preionisation and predissociation of NO via high-lying molecular states of NO. Information regarding the potential energy curves for the high-lying molecular states are obtained from the model.

The impact-parameter dependence of atomic excitation in collisions with charged heavy projectiles is obtained from the full treatment of inelastic scattering processes in a semiclassical approximation. It is demonstrated that the semiclassical result obtained from the plane-wave Born approximation is identical to the first-order perturbation result based on classical straight-line trajectories for the motion of the projectile relative to the target nucleus.

The photon-energy-dependent anisotropies of molecular K X-ray spectra have been investigated for systems with a combined atomic number Z_u=Z₁+Z₂ between 26 (Al-Al) and 94 (Ag-Ag) at beam energies between 3 and 64 MeV. The X-ray energy E_d at the inflection point of the positive slope in the observed anisotropy peak is proportional to the united-atom K_alpha transition energy E_K^u. The measured ratio between these two energies is f_d=1.08+or-0.03. This result is valid for symmetric (Z₁=Z₂) as well as for asymmetric systems (Z₁ not=Z₂).

The effect of the ² Pi _g resonance of N₂^- upon the rotational excitation of molecular nitrogen by electron impact is considered. The calculations follow on from the recent ab initio investigations of Buckley and Burke (1977). In order to produce results in a form comparable with experiment, it is necessary (a) to consider excitations from all the rotational levels of N₂ between 0 and approximately 20, and (b) to make allowance for the energy resolution possible with present-day detectors. It is predicted that the very large enhancement to the Delta j=+or-2 and +or-4 cross sections around 2.4 eV will give rise to observable structure in energy-loss measurements if resolutions of FWHM <or approximately=10 meV attained.

A simple treatment on the time development of a super-radiant system is presented. In this treatment, each atom in the system is considered to decay independently of the others and yet a good approximation for the time behaviour of the system is obtained.

Calculations for K-L transitions, with 20<or=Z<or=90, indicate that many K alpha ₂ X-ray lines arise from intrinsically inverted transitions.

Approximations for scattering lengths are calculated using variational methods. In most cases bounds for the scattering are obtained. The calculations extend the usual theory to deal with indefinite operators and potentials and in all cases good approximations are achieved.

The lowest five energies of the XeH⁺ system have been calculated using the valence-bond method. The effects of spin-orbit coupling are introduced by a semi-empirical procedure. These results serve to interpret qualitatively processes involved in collisions between protons and xenon atoms and, in particular, the polarization of spin on the hydrogen atoms formed by charge exchange is discussed.

The Fermi coupling constant a_1s has been measured in helium-like ¹³C(¹³C V) using the zero-field quantum-beat technique in beam-foil observations. Observing the 1s2p ³P level this coupling constant is found to be a_1s=71.5+or-1.0 GHz which is consistent with the experimental results obtained in the 1s6h ³H level, where the fine-structure splitting is small compared to the hyperfine structure. These results are in good agreement with previous hyperfine studies performed in the He isoelectronic sequence. The fine-structure splitting in the 1s6h ³H level is also measured. The results agree with the hydrogenic value of the fine-structure coupling constant.

The coupled equations were solved for the collision of an electron with Be⁺. The Be⁺ levels included were 1s²2s and 1s²2p in solving for the 1s²2s² ground state and 1s2s²²S, 1s2s(¹S)2p ²P, 1s2s(³S)2p ²P and 1s2p²²D, ²P, ²S for the Be⁺ (inner shell)+continuum states. The solutions were used to calculate the photoionization cross section. The fluorescence yield of Be was calculated with the assumption that the photon radiation created the inner-shell vacancy. Comparisons with experiments were made for both the inner-shell photoionization and the fluorescence yield.

Measurements are reported on the photoelectron spin polarization and ratio of the ion count rates for linearly and circularly polarized light obtained in two-photon ionization of caesium atoms via the 7²P_1/2 and 7²P_3/2 intermediate resonance states. Depending on the pulse length of the radiation used the transitions have to be described either in the fine-structure or in the hyperfine-structure coupling scheme. A simple model is used to describe intermediate cases. From these measurements ratios are obtained for the various bound-free matrix elements.

It is shown that the Dyson equation gives finite values for multiphoton resonance absorption rates when an appropriate diagram summation technique is applied. A simple recurrence formula connects the n-photon absorption probability per unit time with lower-order absorption probabilities per unit time.

A high-resolution electron spectrometer is used to measure the angular dependence ( theta =0-20 deg) of scattered electrons which have excited the 3²P state of atomic sodium for incident electron energies of 54.4, 100, 150 and 250 eV. The results are expressed as absolute differential cross sections and generalized oscillator strengths. Comparisons are made with the results of calculations in which Born, distorted-wave polarized-orbital and Glauber approximations have been applied.

A total of 66 autoionising states of Rb I have been observed between 15 and 21 eV excited-state energy, excitation being by electron impact at kinetic energies of 27, 40, 100 and 400 eV. Of the observed states, 40 have been reported in ultraviolet absorption experiments. The remaining 26 states do not appear in any previous data. Spectra are presented for electrons ejected at an angle of 75 degrees with respect to the incident electron beam.

For pt.I see ibid., vol.9, p.2073 (1976). Differential measurements on charge-processes in H⁺+Kr, Ar collisions have been performed using a time-of-flight technique. Oscillating cross sections for the well resolved H+Kr⁺(4p⁵²P₁2/) and H+Kr⁺(4p⁵²P₃2/) processes are observed in the angular (0.2<or= theta <or= 3 deg) and energy (100<or=E<or=350 eV) ranges covered by experiments. A theoretical investigation of the near-resonant charge-transfer processes, based on a previous molecular study of the H⁺-Kr collisional system, is presented. The calculations involving the diabatic 'projected valence-bond' basis are found to agree satisfactorily with experiment.

A modulated crossed-beam technique has been used to determine cross sections for the formation of H(2p) and H(2s) atoms in collisions of He⁺ with H and H₂ within the energy range 5-26 keV. Measurements were based on observations of the Lyman- alpha radiation emitted from the crossed-beam region both in the presence of and in the absence of electric-field quenching of the 2s state. The polarization of the Lyman- alpha radiation has also been studied. H(2p) formation cross sections are considerably larger than those for H(2s) formation for targets of H₂ and H and a general understanding of the He⁺-H case is possible in terms of a quasi-molecular model.

Calculations are reported of matrix elements of the Baylis-Gombas model potential for Li-He and Li-Ne interactions. Included are comparisons of two computational approaches, one based upon a numerical double-integration procedure, the other an analytical, approximative method. The special emphasis of the calculation is the sensitivity of matrix elements that are important for collision-induced electronic transitions to the precise choice of the Baylis model parameter r₀. Also reported and compared to the Baylis results are calculations of Li-He/Ne matrix elements for a Hartree-Fock frozen-core model potential and for two Bottcher-type model potentials.

Cross sections and polarizations of impact radiation have been calculated for 2s to 2p excitation of Li in collisions with He and Ne, and for 3s to 3p excitation of Na in collisions with He and Ne, for the (centre-of-mass) energy ranges E_CM=0.2-100 keV (Li/Na-He), E_CM=0.7-100 keV (Li-Ne) and E_CM=1-100 keV (Na-Ne). The calculations have been based on three-state one-centre expansions of the electronic states, 2s, 2p₀ and 2p_+or-1 in the case of Li-He/Ne collisions, and 3s, 3p₀ and 3p_+or-1 in the case of Na-He/Ne collisions. For Li-He/Ne collisions a study of excitation cross sections and of polarizations of emissions calculated for two Baylis model potentials, two Bottcher-type model potentials and a Hartree-Fock frozen-core potential is reported. For Na-He/Ne collisions the results obtained by the Baylis model potential are reported.

Total emission cross sections and polarizations have been measured for the 2²S-2²P, 2²P-3²D and 2²P-3²S Li I multiplets in Li-He, Ne collisions, and the 3²S-3²P and 3²P-3²D Na I multiplets in Na-He, Ne collisions in the 0.6-60 keV energy range. The excitation of the alkali resonance multiplet is found to be the dominant inelastic process for all four collision systems. The results are compared with recent calculations by Manique et al. (1977) based on a quasi-one-electron description using various model potentials. Good agreement is found for Li-He with a simple Hartree-Fock frozen-core potential. The agreement is fair for Li, Na-He calculations with a Baylis-type potential but poor for calculations based on Bottcher-model potentials. For Ne as target all three potentials overestimate by far the size of the cross section.

In microwave spectroscopy it is generally accepted that the line shape of the rotational absorption lines is Lorentzian. In 1972 Parsons et al. confirmed experimentally that this assumption was extremely accurate. A study is made of the influence of the absolute velocity distribution of absorbing molecules on the line shape of a microwave absorption line broadened by collisions only. It will be shown that the transition rate due to collisions depends on the absolute velocity of the absorbing molecule. As a consequence, the line shape is given by a super-position of Lorentzian line shapes, each of which corresponds to a class of absorbing molecules with a specific absolute velocity.

For pt.I see ibid., vol.10, no.9, p.1735 (1977). A novel method for the determination of line widths and line shifts in the microwave region is described. Experimental results for the line width obtained with rotational transitions of OCS, CH₃Cl and C₃H₄ (propyne) for temperatures ranging from about 140 up to 420K are compared with calculations using Anderson's (1949) as well as Murphy and Bogs' theory (1967), which are modified as in part I. As far as the temperature dependence is concerned, the agreement is satisfactory. However, significant differences between theoretical and experimental results are found with regard to the absolute value of the line width. Deviations from a Lorentzian line shape are observed.

The total photoionization cross section of N₂ has been obtained from the ionization threshold to 100 AA with an accuracy of +or-3%. In addition, partial photoionization cross sections for transitions into the x² Sigma _g⁺, A² Pi _u and B² Sigma _u⁺ electronic states have been measured using the technique of photoelectron spectroscopy over the wavelength range from 745 to 304 AA. Transitions to higher electronic states are also observed at 304 AA.

Experimental data for the temperature dependence of the drift velocity of thermal electrons travelling through polar gases were treated using effective range theory for electron scattering by polar molecules. The free parameter of the theory was chosen so that the theoretical temperature dependence of the drift velocity fitted the experimental data. Diffusion cross sections averaged over a Maxwellian distribution were calculated. The theoretical dependence of ( sigma _d) on the dipole moment D is more monotonic than that given by Christophorou and Pittman (1970) and does not show the jump near D_cr=0.6395 au.

Population densities of excited and ground states of hydrogen-like ions have been computed on a time-dependent basis for an initially steady-state plasma in which the electron temperature subsequently decays exponentially with a half-life sufficiently short ( approximately 10^-13s) to produce population inversions with respect to the ground state. The principal results are for Z=6 (carbon), initial electron temperature 200 eV and electron density 10²¹ cm^-3, though variations in these parameters are considered. The results are related to cooling rates in expanding plasmas as predicted by (i) the one-dimensional numerical code MEDUSA, and (ii) a simple model of cooling at the centre of an expanding plasma.

A parallel-plate retarding-potential-difference technique has been used to measure electron energy distribution functions in non-self-sustaining Townsend discharges in argon. The experiments were performed for values of E/p₀ equal to 150 and 200 V cm^-1 Torr^-1 (where E is the electric field and p₀ the gas pressure reduced to 0 degrees C). The energy distribution functions were evaluated from the measured retarding characteristics using the analysis derived here for a parallel-plate energy analyser.

A model is developed for laser-induced selective low-level excitation of a molecular species which phenomenologically incorporates coherent resonant energy transfer and collision damping. The molecules are represented as harmonic oscillators, perturbed by resonant and non-resonant collisions and driven by an external monochromatic electromagnetic field (laser). The non-resonant collisions are treated as a thermal bath. The laser field is shown to be able to drive the molecules far from thermal equilibrium, even in the presence of collisions. The resonant exchange rate Omega _R renormalizes the resonant condition, whereas the non-resonant collision rate Omega _D causes damping.