Table of contents

The quadrupole and exchange interactions representing the short-range electron-ion scattering process have been observed and determined via the measurements of p⁵s and p⁵d, J=0 and 1 highly excited Rydberg states in Ne. The quantitative determinations were made possible by the combination of high precision (better than 100 MHz) perturbation-free measurements and multichannel quantum defect theory. A new precise value for the fine-structure splitting of the ²⁰Ne⁺(2p⁵) ion is determined to be 780.4269(36) cm^-1.

A thermodynamic model is proposed to describe the above-threshold ionisation process. The photoelectron energy spectrum is calculated by means of the canonical ensemble. The ensemble consists of a system of atoms which is in equilibrium with a mean energy E. The result gives a new physical picture of the additional photon absorption process.

It is noted that the usual identification of the rate R^A(t) for termolecular recombination at time t with the downward current past a bound energy level is appropriate to cases which involve energy distributions that satisfy the quasi-steady-state (QSS) condition to the exact master equation. A general expression for R^A(t) is derived which is valid for both QSS and non-QSS distributions, such as those non-QSS distributions obtained from both the diffusional and variational methods or from numerical approximation to the exact QSS condition.

The value of Z_eff, the effective number of electrons per molecule available to the positron for annihilation, is calculated for low-energy positron-hydrogen-molecule scattering using the scattering wavefunctions obtained in recent detailed ab initio calculations. The results are higher than those obtained in previous calculations but much lower than the experimental value.

The electronic excitation of the repulsive b³ Sigma _u⁺ state of H₂ from the ground state is studied using the R-matrix method. Correlation and polarisation effects are included ab initio. Results are compared with other theoretical work. While previous calculations appear to underestimate the cross section, good agreement is obtained with the results of the linear algebraic method and the Schwinger multichannel formulation. The authors also present elastic scattering cross sections in the same approximation and compare the results with other work. They obtain satisfactory agreement with experimental results.

The authors have performed two-state close-coupling calculations on the X ¹ Sigma _g to b ³ Sigma _u transition for e-H₂ scattering within the linear algebraic, effective optical potential approach. The strong orthogonality constraint between bound and continuum orbitals is relaxed by including correlation-type configurations. These correlation terms prove to be very important in the ² Sigma _g and ² Sigma _u symmetries and lead to an increase in the total cross section of a factor of two over earlier approaches, which neglected these terms. The authors obtain good agreement with recent R-matrix and Schwinger variational calculations.

In this and two accompanying letters, the results of calculations of the cross sections for electron impact excitation of the b ³ Sigma _u⁺ state of H₂, for collision energies from near threshold to 30 eV, are presented. The results are obtained using a multichannel extension of the Schwinger variational principle at the two-state level. The quantitative agreement between the integral cross sections of these three studies is very good. Inclusion of correlation terms in the scattering wavefunctions, which relax the orthogonality between bound and continuum orbitals is seen to affect the cross sections substantially. Although a comparison of these calculated cross sections with available experimental data is encouraging, some serious discrepancies exist.

The method presented is based on the recoil measurement of the H₂ molecule in a supersonic beam excited by electron impact and detected in the C ³ Pi _u metastable state. The time of flight spectra permit measurement of the excitation function for inelastic electron scattering at 0 and 180 degrees . The authors found that beyond 25 eV the back scattering is still very important.

The f ³ Pi _g state of molecular fluorine, which is assumed to be the upper level of a VUV F₂ laser at 157 nm, has been directly observed for the first time by high-resolution electron energy loss spectroscopy at low impact energies (less than about 30 eV) and large scattering angles. The new transitions consist of several vibrational components at 11.62, 11.75, 11.86 and 11.99 eV, and show every characteristic feature of spin-forbidden transitions.

Angular correlation measurements have been made of the Lyman- alpha radiation arising from the cascade of the 3²D_j states of atomic hydrogen detected in coincidence with electrons scattered with n=3 energy loss. Data are presented at incident energies of 54.4 and 100 eV and electron scattering angles theta _e=20 and 25 degrees .

The relativistic Thomas-Fermi equation for positive atomic ions in an extremely strong magnetic field is set up. The resulting equation is solved by numerical integration. Comparison is made with earlier non-relativistic results over a range of magnetic fields, atomic numbers and degrees of ionisation. This comparison reveals that relativistic corrections increase in importance as the strength of the magnetic field increases.

The dipole polarisability alpha and the higher order polarisabilities C, B and gamma for the ground state of Ar have been calculated close to the Hartree-Fock limit. The importance of the contribution of the higher order polarisabilities to atom-charge potentials such as Ar-e^-, Ar-H⁺, Ar-He²⁺ and Ar-Li³⁺ is emphasised. The final values for alpha , C, B and gamma are, in atomic units, 10.73, 16.42, -141 and 1190 respectively.

The spectrum of a Rydberg atom in crossed electric and magnetic fields has been calculated by the methods of classical perturbation theory. Much consideration is given to the region of almost degenerate frequencies, involving mutually normal orientations of fields of any relative value, and also the case when one of the fields is very weak. Strong anisotropy of the electric polarisability in a magnetic field is predicted. In particular, longitudinal and transverse polarisabilities have different signs. The magnetic polarisability in an electric field has been found.

Presents a theoretical investigation of some peculiarities of alignment signals from laser levels carried out in simultaneous oscillation of two transitions connected by spontaneous emission. An expression is obtained for a spontaneous emission part caused by the oscillations origination from the laser level. It is shown that the population transfer exceeds the coherence transfer significantly. That is why the spontaneous emission part has opposite signs in two orthogonal polarisations under certain conditions. The more important results are verified experimentally at 1152.3 and 1161.4 nm He-Ne laser oscillation.

The authors measured the angular correlations of two coincident photons resulting from simultaneous excitation of both atoms to the 2¹P state in helium-helium collisions; the beam energy ranged from 0.5 to 3.5 keV. A theoretical expression for the angular correlation function has been derived, which contains the various density matrix elements as parameters. The relative cross sections for excitation to the various possible magnetic substate combinations and the coherence between them is determined by fitting this function to the measured data. The authors found a strong dependence of the parameters alpha and beta and a weaker dependence of Re( chi ) on the collision energy. The combined parameters lambda + mu contains such large statistical errors that it is meaningless to determine its energy dependence. The minimal Sigma - Delta model for the collision process, where the authors take into account two successive rotational sigma _u- pi _u couplings resulting in ¹ Sigma _g and ¹ Delta _g molecular states (with respect to the internuclear axis), predicts the correct behaviour of alpha from the measurements of beta . However, the fact that Re( chi ) is positive cannot be understood within this model.

Absolute cross sections were measured for He II Lyman alpha , beta , gamma and Balmer beta emission resulting from collisions of 1.25-10 keV amu^-1 He²⁺ ions with atomic and molecular hydrogen. Details of the experimental method and the evaluation of the cross section data are presented. From the experimental data cross sections for electron capture into specific (n, l) subshells of He II (2p, 3p, 3s+3d and 4p) were deduced. The cross section for electron capture into the metastable 2s state of He II for atomic hydrogen as target was evaluated using the total capture data of Shah and Gilbody (1974-8) and Nutt et al. (1978). Results of the present measurement are in very good agreement with recent calculations.

By means of optical spectroscopy the authors have determined absolute subshell-selective single electron capture cross sections sigma _nl for collisions of C⁴⁺, N⁵⁺ and O⁶⁺ with atomic and molecular hydrogen and helium at impact velocities between 0.1 and 0.5 au. The absolute sensitivity calibration of the monochromators is discussed briefly. In addition they have measured the total single electron capture cross section sigma _t by means of charge state analysis, for atomic and molecular hydrogen as targets. In all cases it is found that one or two n shells are dominantly populated and that the total cross section is only weakly dependent on the impact velocity, in contrast to the subshell-selective cross sections sigma _nl. The main features of the results obtained are qualitatively explained by means of simple diabatic potential diagrams and the classical overbarrier transition model. For several systems a detailed comparison can be made with recent theoretical calculations, showing satisfactory agreement.

Single-electron-capture cross sections for Kr²⁺-H₂ collisions in the 1-6 keV energy range are determined using the initial growth curve technique. Energy defect measurements reveal that the incident Kr²⁺ beam is purely in the ground electronic state and that the product Kr⁺ ions are predominantly in the ²P and ²S states. Charge transfer occurs via avoided crossings of pseudo-molecular potential energy surfaces as well as by non-crossing, Demkov-type mechanisms (1964).