Table of contents

Fast ion beam bombardment was used to collisionally prepare a target gas in excited states, to which conventional laser spectroscopy was then applied. The versatility of this method is demonstrated with atomic targets of caesium, for a state of Cs⁺ that is 16 eV above the ground state, as well as for a short-lived state (38 ns) of the neutral atom. The local temperature in the caesium oven is also obtained.

A two-potential localised exchange approach is used to study the dynamics of the atomic collision process in electron impact excitation of the resonant transition in sodium at three intermediate energies: 12.1, 22.1 and 30 eV. Results are presented for the angular variation of the components of polarisation (P₁, P₂ and P₃) of the radiation emitted normal to the scattering plane. From these results the coherence of excitation and the alignment of the charge cloud ( gamma ) are investigated. Differential and total spin asymmetry in the scattering of spin-polarised electrons with spin-polarised sodium are also studied.

Coherent anti-Raman scattering in quartz has been observed using two copropagating high-power laser beams at 1.064 mu m and 1.053 mu m from a neodymium glass laser. This is due to the beat frequency at 98.2 cm^-1 being within the broad-band Raman gain of quartz which extends from about 20 to 550 cm^-1. The beams were each of intensity 3*10⁹ W cm^-2 and had an interaction length of approximately=5 cm. The measured gain is 9.5*10^-10 cm W^-1 in agreement with the established value.

The accuracy of electronic structure calculations made within the algebraic approximation (that is by making finite basis set expansions) is compared with the results of previously reported fully numerical calculations for diatomic molecules. Sequences of even tempered basis sets of atom-centred Gaussian-type functions are employed to explore the accuracy that can be achieved in matrix calculations for the H₂⁺, HeH⁺ and HeH²⁺ molecular ions and the H₂ molecule. It is demonstrated that sub- mu Hartree accuracy can be achieved in energy calculations. The relevance of these results to the accuracy that can be achieved in studies of polyatomic studies is emphasised.

A formula for the energy of the lower edge of doubly excited states of a many-electron atom is derived using the approximate solution of the Schrodinger equation. This formula is a generalisation of Rydberg's formula for single-electron excitations and correctly takes into account the interaction of the electrons and between themselves and with the core. Comparison with the experimental data shows that the generalised Rydberg formula well reproduces the energies for ground states of the type (core)(n s²¹S) as well as those for highly excited resonances and autoionising states of atoms and negative and positive ions. The accuracy of the estimate increases with n. For example, the energy for negative-ion He^- resonances at n<or=9 is reproduced with a maximum error of 18 meV. It is shown that at large n the energy for the lower edge of double excitations is given by the expression E(n, Z)=-Z-0.18169/n².

The authors present experimental results for the ionisation of excited hydrogen atoms, with initial principal quantum number n₀ in the range 32<or=n₀<or=48, by low frequency microwaves in the scaled frequency range 0.05( Omega (0.2. They compare these with results of a classical Monte Carlo calculation showing that there is structure in the experimental ionisation curves of quantal origin. A further comparison with a simple one-dimensional quantal theory, using an adiabatic basis, shows that the observed structure can be explained in terms of resonances between a few adiabatic states. A very simple two-state model is shown to possess all the relevant features of the system. Further analysis of this one-dimensional model shows that when the field frequency tends to zero the static field ionisation limit is approached through infinitely many such resonances, at Omega approximately=1/p, of width O(e^-p), p being a large integer.

The three-photon detachment cross sections of the ions F^-, Br^- and I^- are measured, using the fundamental 1064.2 nm wavelength of a single-mode Nd:YAG laser. The detachment signal is measured by detecting the neutral atoms produced from an ionic beam of kinetic energy 1.2 keV. The data analysis takes into account the laser pulse time profile, the actual spatial profile of the laser beam in the interaction region, and the fast motion of the ions through the laser beam during the laser pulse. Saturation of the three-photon process is observed and used to normalise the detachment signal. The authors obtain sigma (F-)=6.1^$+^2.6_-1.8*10^-95 s² m⁶, sigma (Br-)=1.6^+0.8_-0.6*10^-94 m⁶ and sigma (I-)=3.3⁺¹⁶_-1.1*10^-94 s² m⁶. These measured cross sections are compared with theoretical predictions. The result obtained on F^- agrees with a very recent measurement performed with a different technique.

A simple method for estimating cross sections for atom multielectron ionisation by a fast multicharged ion is proposed. This method is applied to estimation of cross sections for multielectron ionisation of Ar atoms.

Three different distorted-wave models are formulated for the ionisation of a bound relativistic electron from an atom by the impact of bare ions at relativistic energies. In all models the initial bound state is distorted by an eikonal phase factor. For the final continuum state the authors treat the cases of no distortion, an eikonal phase factor and a Coulomb wave distortion, respectively. The importance of correct asymptotic Coulomb boundary conditions is stressed by comparing the ultrarelativistic versions with the undistorted first Born result.

An investigation of several alternative representations of modified effective-range theory (MERT) has been carried out for low-energy electron scattering from the rare gases argon, neon and krypton in an effort to determine the energy range over which they can be applied to the analysis of scattering cross sections. One purpose of the MERT analysis is to provide a bridge from a total cross section to a momentum transfer cross section in order to compare the results of beam experiments with those of electron swarm experiments. Simulations using published (theoretical) phaseshifts indicate that extended versions of the standard effective-range theory with five adjustable parameters are required to give an adequate description of the phaseshifts for argon. A five-parameter MERT fit gives a good representation of a recent e^--Ar total cross section experiment at energies less than 1.0 eV. The resulting momentum transfer cross section is in agreement with a momentum transfer cross section obtained from a swarm experiment at energies below 0.3 eV, but at energies greater than 0.5 eV there are differences of up to 20%. The application of MERT to neon and krypton total cross section data has also been investigated. The derivation of momentum transfer cross sections (via MERT) for both these atoms involves much larger uncertainties than was the case for argon.

Differential cross sections (DCS) have been measured by the crossed-beam method. The angular range is 5-135' and the electron energies are 75, 100, 150, 200, 300, 500 and 700 eV. The overall energy resolution is not sufficient to discriminate elastic scatterings from rotational and vibrational excitations. The absolute DCS are determined by using a gas chamber on the basis of the known absolute DCS for He. The experimental DCS are extrapolated to 0' and 180' scattering angles by fitting the square of the Legendre polynomials to the measured values. The integral and momentum transfer cross sections are then obtained. The errors are about 10% for all experimental results. The present data for CH₄ agree well with the previous measurements and calculations. For CF₄ and SF₆, the DCS increase steeply at the forward angles. The integral cross sections obtained are consistent with the previous results for total and inelastic cross sections.

Techniques for analysing resonance structures in photoionisation cross sections by means of many-channel quantum defect theory developed by Dubau and Seaton (1984) are extended to distorted-wave electron impact ionisation calculations. The theory is applied to the analysis of resonances in electron impact ionisation of neon-like selenium ions.

The authors analyse the validity of the approximation of neglecting the 2²S_1/2-2²P_3/2 coupling in the description of the fluorescence from the n=2 states of hydrogen atoms decaying in the presence of an external electric field. The results show the inadequacy of such an approach and point to the need for a re-interpretation of some previously reported experimental values of the state multipoles of the collisionally excited n=2 hydrogen atoms.

A polarisation correlation technique has been used to study electron impact excitation of the 3¹P state of helium. Values of the three relative Stokes parameters P₁, P₂ and P₃ are reported over an angular range of 35' to 120' and at energies of 29.6 and 26.5 eV. The data are compared with recent angular correlation measurements from this laboratory and with the predictions of a 19-state R-matrix calculation.

The authors present triply differential cross sections for electron impact ionisation of helium for a coplanar asymmetric geometry at incident energies of 256 and 600 eV. The calculation is based on the Glauber approximation (GA). The effect of post collision interaction (PCI) is incorporated in the GA using a classical method. The angular distributions of the ejected electrons predicted by the present method are in reasonably good agreement with experiment. The inclusion of PCI improves the recoil distributions considerably at 256 eV.

Electron impact excitation of the 3¹P state in magnesium has been studied by electron-photon coincidence techniques. Two types of experiments have been performed. An extensive set of measurements have been undertaken in which the three components of the Stokes vector were measured at incident energies of 20 and 40 eV for electron scattering angles between 5 and 20'. These measurements not only provide data on the coherence of the excitation process but also they form a basis by which approximations in scattering theory can be tested. In addition, an angular correlation function has been measured at 40 eV and at a scattering angle of 7'. The scattering parameters derived from this measurement agree with those measured in the polarisation study. The scattering parameters derived from the present work are in very good agreement with the recent close-coupling calculation of Mitroy and McCarthy (1989).

Differential cross section for vibrationally elastic and inelastic ( nu =0 to 1,2,3,4) scattering from N₂ have been measured at incident energies of 2.1, 2.4 and 3.0 eV over an angular range 20-90'. The measurements cover the energy range encompassing the well known ² Pi _g resonance and enable, for the first time, a comparison of measured cross sections for high-order vibrations with theoretical calculations in this energy region.

Differential cross sections for the electron impact excitation of HCl and HF have been measured in a crossed-beam experiment. Results on rotational and rovibrational state-to-state transitions are obtained by a deconvolution procedure of the energy loss peaks at primary energies between 0.5 and 10 eV and in the angular range from 15 to 135'. In the case of HCl the angular dependences and the excitation functions demonstrate the interplay of direct and resonant interactions. For HF only direct interaction has been observed. The results are compared with different theories.

Measurements of the decay rate of orthopositronium in dilute ethane and methane gas are interpreted within the context of three different models. In the first it is assumed THAT THE O-Ps atom becomes trapped in a bubble which it creates. In the second it is assumed that the o-Ps atom passively samples 'frozen' density fluctuations in the host fluid in the absence of self-trapping. In the third, due to McNutt and Sharma (1978), it is assumed that the o-Ps atom only samples regions of reduced density. A number of criteria are developed to test each hypothesis. Rough estimates based on sample data for ethane favour the second approach.

A self-consistent model of a self-sustained discharge XeCl laser with He buffer gas is described. The validity of the model is confirmed by comparing the results with the measured time dependences of electrical variables, the lasing pulse, the densities of XeCl*, Cl^-, Xe* as well as with the dependences of the laser output energy on the charging voltage and on the gas composition. For a gas pressure of 2.2 atm model optimisation of the energy density deposition and the length of the laser active medium (for a fixed stored energy of 7 J) is carried out. The quenching of XeCl* molecules by electrons is found to be the main factor limiting the enhancement of laser efficiency and output energy when the deposited energy density increases.