Table of contents

The electric quadrupole moments of hydrogen, deuterium and muonium have been calculated in their 1S_1/2, 2S_1/2, 2P_1/2 and 2P_3/2 states; previous discrepancies for the 2P_1/2 state have been resolved. The energy of interaction between the electric field gradient at the nucleus due to the electronic charge distribution and the nuclear quadrupole moment of the deuteron has been calculated for the deuterium atom.

The Fabry-Perot technique in the far UV range was used to investigate isotope shifts and the hyperfine structure of resonance transitions in singly ionised lead. The experimental ground-configuration data were compared with those computed with Desclaux's (1975) program which uses the relativistic Dirac-Fock method. A good agreement is observed between experimental results and theoretical evaluations, for both the isotope shift and the hyperfine structure.

The conversion of quantum electrodynamics to a configuration-space Hamiltonian formalism introduces three-electron potentials of relativistic origin. For heavy atoms the authors find that the contribution of these potentials to the inner-shell binding energy is no more than 0.21 eV. This is too small to explain the discrepancy between current theory and experiment. They also briefly discuss the uniqueness of the potentials obtained in the configuration-space Hamiltonian.

Taking the real chemical environment explicitly into account, a study of highly excited atom states carrying multiple vacancies revealed a nearly alchemical transmutation of the valence electron structure occurring prior to X-ray emission. While in third-row atoms such as Si an interatomic valence charge flow via covalent bonds was found to prevail in this electron rearrangement, the present study of F emitter atoms discloses the decisive role of adiabatic character-exchanging pseudo-crossings between descending levels of the emitter atom (F,2s,2p,3s,3p,3d) and the upper levels of adjacent atoms. These pseudo crossings, achieved by a temporary covalency, provide a concise and comprehensive picture of the chemical effects observed in the F K X-ray satellite and hypersatellite spectra. These chemical effects furnish evidence of creation and decay of inner-shell vacancies as a single scattering process in which electron relaxation efficiently competes with the sequential ejection of electrons.

The broadening and shift parameters of the sodium D lines perturbed by helium have been measured in the hot gas produced behind the reflected shock in a combustion-driven shock tube. The incident shock waves are propagated at Mach numbers in the range of M=3-6 in pure helium at initial pressures ranging from 20 to 400 Torr. The conditions created in the reflected-shock region are a temperature in the range 3000-8000K and a helium number density of 5*10¹⁸<N_He<10²⁰cm^-3. Sodium seeding of the gas is produced by impurities. The sodium-atom number density is evaluated to be less than 10^-5N_He, by comparing partially reabsorbed D₁ and D₂ profiles. The electron number density which might come from the ionisation of the sodium and a fortiori from helium is negligible.

The authors present exact solutions for N=1 and 2 N-photon off-resonance ionisation in the presence of a fluctuating radiation field with pre-Gaussian statistics. These pre-Gaussian fluctuations are composed of n independent two-state jump processes (telegraphs). Such telegraph-type fluctuations have a second-order intensity coherence function g that is normalised and bounded such that 1<or=g<or=2. In the limit n to infinity the pre-Gaussian noise converges to chaotic light with the well known g=2!. The authors show that an exact discussion of the N-photon ionisation rate can be given for such pre-Gaussian fluctuations.

Experimental results are presented for the spectral shapes for atomic-field bremsstrahlung produced by complete absorption of beta electrons from the decay of ⁹⁰Y in targets of Cu, Cd, Ta and Pb over the photon energy range from 600 keV to 2 MeV. The experimental data are compared with numerical calculations based on the formulation of Pratt et al., the Born approximation theory of Bethe-Heitler and with the classical theory of Kramers. Although small discrepancies exist at some spectral points scattered throughout the energy region studied, the overall agreement between the theory of Pratt et al. and experiment is good. The experimental data agree with the Elwert-corrected Bethe-Heitler theory within experimental uncertainties for Cu, and a systematic positive deviation is found for Cd, Ta and Pb. The overall agreement between the classical theory of Kramers and experiment is very poor. Agreement between Kramers' theory and experiment is only seen within experimental uncertainties in certain restricted energy regions for Cd, Ta and Pb.

Photoionisation partial cross sections, continuum phaseshifts and angular distributions of photoelectrons for the hydrogen molecular ion ground state, H₂⁺1 sigma _g(² Sigma _g⁺) up to an energy of 20 au have been calculated using a two-dimensional numerical treatment, and an exact one-electron treatment in spheroidal coordinates. The calculations presented extend the range and precision of previous work. The cross section, asymmetry parameter and continuum phaseshifts for the hydrogen molecule have been also calculated using fully numerical two-dimensional continuum functions at the Hartree-Fock level with exchange. The cross section and asymmetry parameter are compared with experiment in both the length and velocity dipole forms, and the effect of exchange is assessed.

Photoionisation of H₂ has been studied from first principles by taking into account the rotational motion of both the target and the residual molecular ion. Three difference kinds of wavefunctions for the ground electronic state of H₂ are considered. In order to perform a completely ab initio study, the wavefunction for the final (e^-+H₂⁺) system has been obtained from the polarised-orbital method. Photoionisation matrix elements have been calculated for both length and velocity forms of the electric dipole operator. The effects of various approximations have been studied in detail. It has been found that, unlike the Q branch ( Delta N=0) quantities, the S branch ( Delta N=+2) observables are extremely sensitive to various approximations. They therefore provide a very stringent test of theoretical models. The results also show that it is very necessary to couple partial waves higher than l=1 in the continuum orbital or even the low-energy photoelectron and also to include exchange and polarisation in the final-state wavefunctions. The neglect of the latter effects gives rise to an unphysical behaviour of the photoionisation observables. This calculation also demonstrates that, within the employed level of accuracy, the various photoionisation quantities in the dipole-length approximation are consistently better than those obtained in the dipole-velocity approximation. The best results are obtained in the dipole-length approximation with Huzinaga's wavefunction (1957) for the initial state.

The presence of very low concentrations of phenol has been detected in a 90% argon/10% methane proportional-counter gas mixture by laser-induced resonant two photon ionisation. The resonant two-photon ionisation spectrum has been compared with the single-photon ultraviolet absorption spectrum of phenol in the wavelength range 266-278 nm.

Excitation cross sections into the 1s2p and 1s3p states of 400 MeVFe²⁴⁺ projectiles colliding with He, N₂, Ar and Kr targets have been extracted from the intensities of helium-like 2P to 1S and 3P to 1S transitions de-exciting the ion after one-electron 1s to 2p and 1s to 3p excitation. The exciting partner being neutral, the capture channel is closed, and these experiments thus give an opportunity to study the pure distortion effect even in the case of quasisymmetric systems at intermediate velocity (V_p/V_e approximately 0.6). A striking feature of the results is the experimental evidence of saturation in the excitation cross section as the nuclear charge of the exciting partner increases. This trend is supported by theoretical predictions of a new method based on the Schwinger variational principle that was developed for that purpose.

A method of Bransden and Stelbovic (1984) for the construction of an optical potential in a two-channel system is extended to the case of several channels. Numerical examples are given of an optical potential representing the effect of two open channels on a third and the success of the procedure is assessed.

The authors present optical excitation functions of singly excited helium states, measured by detecting the yield of emitted photons as a function of the incident electron energy from 56 to 66 eV. Many structures are observed, which are caused by negative-ion resonances and by the decay of autoionising states followed by post-collision interaction. Some of the structures are interpreted as being caused by hitherto unknown shape resonances lying very close to the thresholds of a particular class of autoionising states. As these shape resonances almost exclusively decay to their respective parent (autoionising) states, thereby considerably enhancing the threshold excitation cross sections of these states, they can only be observed via the PCI effect on the excitation functions of (higher lying) singly excited states. Using the recently introduced supermultiplet classification for doubly excited states a selection rule for the near-threshold excitation of doubly excited states by electron impact is deduced from the measurements.