Table of contents

The accuracy of a calculation by Chambaud et al. (1978) of the potential for O⁺+H₂ to OH⁺+H (linear approach) is re-examined in the light of available experimental data.

Absorption measurements of the 7s13p-7s52p ¹P₁⁰ Rydberg series in Ra I are presented. A value of the first ionisation limit (42573.36+or-0.02 cm^-1) is derived. From these measurements and from a systematic trend in Ca I, Sr I and Ba I a multichannel quantum defect theory (MODT) analysis is used to predict the location of the 7s9p-7s12p and 6d7p ¹P₁⁰ states.

The authors have measured the wavelengths of the 1s2s ³S-1s2p ³P transitions in two-electron Si XIII, S XV, and Cl XVI, produced by foil excitation of 50-90 MeV silicon, sulphur, and chlorine ions. Our wavelength precision provides sensitivity to Lamb shift contributions to the transition energies to +or-3%. The results for S XV represent the first measurements of the ³S-³P transitions for this ion.

Experimental lifetimes are reported for the 1s2p ³P₀ and 1s2p ³P₂ levels in He-like F VIII. Beam-foil measurements yielded the following values: 9.48+or-0.20 ns (³P₀) and 10.44+or-0.15 ns (³P₂). The difference is due to hyperfine quenching of the ³P₀ level.

Depolarising collisions are studied in a three-level system J=0 to or from 1 to or from 0 in Xe I. The authors separately detect the atoms which have undergone a collisional transfer between Zeeman sublevels and those which have not. The disalignment cross section is measured together with the velocity changes which accompany collisional transfers. These velocity changes appear to be very small.

The spin polarisation of the photoelectrons emitted by argon and krypton atoms exposed to unpolarised VUV radiation has been measured at rate-gas resonance wavelengths in the energy range from the threshold up to 41 eV. Comparison of the wavelength dependence of the measured polarisation with theoretical results shows good agreement.

Some questions regarding the applicability of theoretical results (1977, 1978) for the short-time behaviour of the multiphoton ionisation to the experiments are discussed. It is argued that the relevant conditions could not have been satisfied in the experiments concerned.

Total and differential cross sections for charge transfer of Ar^{zeta +} and Kr^{zeta +} (2<or= zeta <or=7) in molecular hydrogen were measured at collision energies below 10 keV. From an energy analysis of the fast projectiles and the slow target ions information on the energy defects of the reaction and the dynamics of the collision process is obtained. For zeta >or=4 cross sections are found to scale with zeta . For zeta <or=2 close collisions influence the shape of the cross section.

Microchannel plates have been applied as position-sensitive particle detectors in photon-scattered-atom coincidence experiments. The time required to obtain a complete coincident angular distribution of the inelastically scattered atoms is reduced considerably as compared with previous experiments. The capability of the new technique is demonstrated in two different collision experiments. (a) The authors report the circular polarisation of the Li 2P to 2s photons emitted in 1000 eV Li-He collisions as a function of the scattering angle. (b) They report the probability for exciting the Li 2p state in (500 to 1000 eV) Li-surface (tungsten) collisions as a function of the projectile's scattering angle.

Matrix elements of delta (r₁) are written in terms of the matrix elements of spatially distributed operators. This is done by making use of identities involving nabla² and delta (r₁) and utilising the Hermiticity of nebla². For the two methods described, considerably improved results are obtained in a variety of examples considered.

For pt.I see ibid., vol.12, no.4, p.567 (1979). A canonical transformation of an infinite system of differential equations describing the motion of the three-body system in the adiabatic basis is suggested. This transformation allows one to reduce the original problem to the solution of a finite set of differential equations. As an example, a system of two differential equations is constructed which represents the infinite (or finite) system of equations within the accuracy (2M)^-2, where M^-1=m_c/M₀ is the ratio of the mass, m_c, of the negative charged particle, c, to the reduced mass, M₀, of two positively charged particles a and b. The physical meaning of the transformation and the solutions obtained is discussed.

For pt.I see ibid., vol.12, no.19, p.3171 (1979). A theory for calculating autoionisation rates and radiative transition probabilities using the Z-expansion technique which is suitable for autoionisation degenerated states of many-electron systems is given. The autoionisation rates are considered as the imaginary part of the energies and are calculated in an analogous manner to the real part. Only the first non-zero term was taken into account. The calculations have been carried out for ions with 4-9 electrons and Z=6-30. The roles of correlation and relativistic effects are discussed. The importance of intercombination transitions is pointed out. The numerical results are given for the Be, N, O and F isoelectronic sequences.

For pt.I see ibid., vol.9, no.14, p.2385 (1976). The authors have calculated 88 doubly-excited energy levels of neon belonging to the 2p⁴3s3p, 2p⁴3s4p, 2p⁴3p4s and 2p⁴3p3d configurations, using a frozen-core superposition of configurations method previously described. The authors' wavefunctions are 'coupling unrestricted' and permit the determination of the best coupling scheme for the subshell angular momenta. For the 2p⁴3s3p configuration, the external electrons are more strongly coupled to each other than to the core. In the 2p⁴3p3d levels a variety of coupling modes are found.

Positions and widths of the autoionising states 2s2p⁶ns, np of Na⁺ are calculated by analysing the resonances obtained in the Na²⁺+e^- scattering problem, which is solved by the close coupling method.

Oscillator strengths and transition probabilities are evaluated for intercombination transitions between the 2s²2p, 2s2p² and 2p³ states of Fe XXII using configuration interaction wavefunctions. The author has also calculated the fine-structure splittings. Some significant differences with previous calculations are obtained. At present there are no experimental results, however, from calculations reported here the intercombination lines 2s2p²⁴P_3/2 -> 2p³²D_3/2^degrees and 2s2p²⁴P_5/2 -> 2p³²D_5/2 degrees could perhaps be found experimentally. For the 2p³²D₃2/^degrees and ²D₅2/^degrees levels lifetimes of 0.474 ns and 0.557 ns respectively are obtained.

The asymptotic behaviour of the resonances in the hydrogen Stark effect at both small and large fields is related to the high-order behaviour of the Rayleigh-Schrodinger perturbation coefficients of the two-dimensional rotationally symmetric anharmonic oscillator. For two resonances at intermediate fields exhaustive numerical results are given, which provide an excellent matching between the two asymptotic behaviours. For the anharmonic oscillator eigenvalues Rayleigh-Ritz-like approximants are used which are uniformly asymptotic in the Watson sense.

A non-hydrogenic single-electron-transition model has been used to obtain partial photoionisation cross sections for values of the principal quantum number n=2,3,4,6 and 10 and the angular momentum quantum number l<n for Mo^{29+,32+,35+,38+} and Mo³⁹⁺. A method for scaling the cross sections to other values of n is described. Radiative recombination rate coefficients calculated from the cross sections are given for kT=0.03, 0.1,0.3, 1.0, 10 keV. Comparisons are made with results of other authors and with Kramers model (1923) predictions. Deviations of more than 10% from Kramers model rate coefficients (summed over n, l) were found for Mo²⁹⁺(kT>0.1 keV), Mo³²⁺(kT>or=1.0), and Mo³⁵⁺ (KT>1.0).

Synchrotron radiation from the 2.5 GeV electron synchrotron at the Physikalisch Institut of the University of Bonn has been used to measure the asymmetry parameter for the photoionisation of CH₄ as a function of photon energy. Good agreement with existing experimental data is obtained at 21.2 eV. However, the observed slow general increase of beta with photon energy is at variance with published calculated curves. Some structure is observed in the experimental curve and similarities with the 2p photoelectron asymmetry parameter for neon are discussed.

Radiation from the 2.5 GeV electron synchrotron at the Bonn Physikalisch Institut has been used to measure the spatial distributions of photoelectrons from the molecules N₂ and O₂. Values of the asymmetry parameter beta of various ionic states have been determined over a photon energy range of 16 to 25 eV using a Seya monochromator to produce plane polarised radiation to a bandwidth of 500 mV. Good agreement with He I and Ne I resonance line measurements by other workers is observed and a comparison with theoretical studies is given. Additional measurements on N₂ and C0 which complement the published date of Marr et al. (1979 , 1980) using the Synchrotron Radiation Facility at the Daresbury Laboratory are provided in tabular form.

A technique, previously used in the authors' laboratory, in which the H(2s) metastable population of a 5-26 keV beam of hydrogen atoms is modulated by a periodic electric quench field, has been adapted to study Balmer alpha emission in the passage of such beams through thin targets of He, Ne, Ar, Kr, H₂, and N₂. Cross section-data for excitation of H(2S) to the n=3 state have been obtained for the first time. When considered in relation to the authors' previous studies of collisional destruction of H(2S) atoms, the present data indicate that a 2s-nl-1s de-excitation mode via intermediate states n>2 only accounts for part of the observed total cross sections for de-excitation. Capture into unstable states of H^- which subsequently decay by autoionisation is therefore believed to be an important collisional destruction process in the present energy range.

The characteristic features of ionisation of inner atomic shells by spin-polarised protons have been studied. In the general case the effects dependent on proton polarisation are considered which can be observed by detection of the subsequent Auger or X-radiation. On the basis of the first Born approximation estimates of these effects are given.

Partial L X-ray production cross sections for proton bombardment of Au, Bi and U have been systematically measured in the energy range 0.3 to 1.8 MeV. This targets have been used in transmission geometry. For Bi and U, detailed low-energy measurements are reported here for the first time. Intensity ratios I_alpha /I_beta ,I_alpha /I_gamma and I_alpha /I_l have been as a function of proton energy. Data are compared with the theoretical predictions of the PWBA and the PWBA with Coulomb deflection and binding corrections. There are indications that inclusion of relativistic corrections in the PWBA substantially improves the agreement with experiment.

A two-potential modified plane-wave approximation is used to calculate differential cross sections for the electron impact excitation from the ground state to the 3¹S and 4¹S states of helium at intermediate energies. Exchange is considered within the framework of the Ochkur-Rudge approximation (Ochkur, 1974; Rudge, 1965). The results are compared with other recent theoretical calculations and the available experimental data.

Measurements of the angular distribution of X-radiation produced by electron bombardment of free atoms (argon, krypton, xenon) at incident electron energies between 3 keV and 15 keV are presented. The observed anisotropy or polarisation of characteristic X-radiation (L alpha _1,2 transition in xenon) is small (a few per cent) and near threshold approaches the expected value for Q₀>>Q₁. The measured anisotropies of bremsstrahlung radiation may be reproduced reasonably well by a modified Sommerfeld formula if the velocity beta is chosen to be half of the incident electron velocity beta ₀. The measured anisotropy may be described by a parameter P, which is compared with the degree of polarisation obtained from theoretical calculations.

The excitation of nitrogen 1s electrons has been observed in N₂, NO, and N₂O by using the electron energy-loss technique at an incident energy of 1.5 keV, and with an energy resolution of 0.075 eV. The energies, widths and vibrational structures of the inner-shell excited states have been obtained. The spectroscopic constants (internuclear separation R_e, vibrational constants omega and omega x and quantum defect delta ) of the inner-shell excited states of N₂ have been derived and have been found to be similar to those of the equivalent core states. The energies and vibrational structures of the inner-shell excited states of NO and N₂O have also been interpreted in terms of the equivalent core model.

Energy-loss spectra of C₂H₄ and C₂D₄ are presented covering the energy-loss range 6-11 eV, using incident electron energies in the range 5-100 eV above threshold, and scattering angles up to 60 degrees . Members of several optically assigned Rydberg series were identified, and their intensities measured at a number of scattering angles. Each transition was then classified as electric dipole or quadrupole allowed, and this information was used to assign the Rydberg orbitals involved. Four new triplet Rydberg states have been observed, and their assignments are also discussed. For two of the triplet states, more than one vibrational level was observed, and where possible the frequencies of the symmetric C=C stretch and CH₂ torsional modes were obtained. For the lowest triplet Rydberg state, the barrier to internal rotation of the CH₂ groups was estimated to be 0.08 eV.