Table of contents

The charge state distribution of Xe ions produced by 4.1-8.0 keV synchrotron radiation has been measured by means of a time-of-flight mass spectrometer. It is found that the mean charge of Xe ions resulting from atomic rearrangement following the creation of the inner-shell vacancies in Xe atoms significantly depends on the photon energy and is clearly affected by L-subshell ionisation thresholds. The experimental results are found to be in good agreement with the Monte Carlo calculations.

The authors present calculations of the two-photon one-electron ionisation of the 5p shell of xenon including non-linear response. Screening effects are described within the random-phase approximation (RPA). The authors use both a local-density approximation (LDA) and a Hartree-Fock average basis set. They describe a large energy region from the two-photon ionisation threshold to above the one-photon ionisation threshold ('above-threshold-ionisation' process, ATI). They demonstrate the importance of many-electron screening effects; and in particular point out the role of double excitation mechanisms.

Rotational structure in the A-X system of the indium monobromide molecule has been studied with moderately high resolution. Five bands, namely (2, 0), (1, 0), (0, 0) (0, 1) and (0, 2), were rotationally analysed and rotational constants were determined in the A and the X states for both the isotopic molecules In ⁷⁹Br and In ⁸¹Br. Predissociation has been observed at J=134 in the upsilon =2 level of the A ³ Pi ₀ state. This gives an upper limit to the dissociation energy of the InBr molecule of 3.5 eV.

A simple relationship between the electron spin polarisation function and the electron-photon coherence and correlation parameters associated with the excitation of n³P target states by an unpolarised incident electron beam is derived for LS-coupled targets initially in a ¹S₀ state. This relationship is useful because it facilitates the experimental determination of the electron spin polarisation functions associated with this particular type of excitation process through the measurement of the lambda and chi coherence and correlation parameters, thus avoiding the necessity of measuring the spin of the scattered electrons directly. This relationship is illustrated by using first-order many-body theory (FOMBT) to predict the electron spin polarisation functions Sp(k, k') associated with the excitation of the n³P/sub /J (n=2, . . ., 8; J=0, 1, 2) states of helium by electron impact.

Electron impact excitation cross sections from the 1s²2s²2p³⁴S degrees , ²D degrees and ²P degrees states to the 1s²2s²2p²3s⁴P^e and ²P^e states of singly ionised oxygen are calculated using the R-matrix method. It is found that the collision strengths are dominated close to threshold by previously unreported 1s²2s2p⁴²P^ens³P^e and ¹P^e resonances. These will have important implications in applications of the collision strengths in astrophysical and laboratory observations.

The observations of anomalous emission near the D₁ transition of a dense (>or=10¹⁴ cm^-3) sodium vapour illuminated by a laser detuned to the blue with respect to the D₂ transition is studied and reported. A discussion of existing models is presented. The authors propose a type of parametric down-conversion process to explain this emission. This process involves a laser photon splitting into two photons by the three-level system of sodium, assisted by a magnetic dipole transition.

New variational Schwinger functionals are introduced. These functionals are based on any Green's operator G/sub /k, where the subscript k denotes any kind of decomposition of the total Hamiltonian. It overcomes a difficulty of previous functionals containing only G/sub /i or G/sub /f, where these Coulomb Green's operators include the full final and initial perturbations respectively. Unlike previous functionals, the expressions introduced here could be calculated exactly if suitable G/sub /k were chosen. Second-order Faddeev-type functionals (L.D. Faddeev 1961) and third-order variational functionals are calculated. The functionals are applied to a one-dimensional collision of two particles interacting with square wells.

A method due to Pechukas (1972) and De Leon and Heller (1984) is used to construct a curvilinear coordinate system appropriate to the first-order quadratic Zeeman effect. The method is introduced through the example of the classical Coulomb problem. The curvilinear nodal coordinates in momentum space are shown to be the elliptical cylindrical (or conical) coordinates employed by Herrick (1982) to separate the diamagnetic Hamiltonian exactly.

Many-body perturbation theory within the algebraic approximation has been applied to compute properties of the double open-shell states 3s3p ³P and 3s3d ³D in Mg. The computed results are based on a complete perturbation expansion to third order, and even a few fourth-order core polarisation diagrams have been considered. The properties considered are the specific mass shift and field shift contributions to the ²⁶Mg-²⁴Mg isotope shifts in the transitions 3s²¹S-3s3p ³P and 3s3p ³P-3s3d ³D, the fine structure and isotope shift in the fine structure of the 3s3p ³P state, and finally the magnetic hyperfine structure of the 3s3p ³P state. Good or reasonable agreement with experiments was obtained in all cases, except for the isotope shift in the fine structure where only a qualitative agreement was obtained.

A theory of stepwise pulsed-laser excitation of cascade Zeeman quantum beats has been developed for a four-level system. The laser pulses are treated in the broad-line approximation and the full optical-pumping equations for the atomic-density matrix of each level have been obtained. These can be used to describe saturation and light shift effects. The weak-pumping theory solutions to the optical pumping equations have been used to obtain expressions for the cascade quantum-beat intensity versus time curves. Larmor precession, spontaneous-emission decay and the populating of the final cascade level during the excitation pulse are all allowed for. A Wiener-Levy phase diffusion model with slowly varying amplitudes is used for the laser pulses, and the final theoretical expressions for the quantum beats depend on the time-dependent intensities of the laser pulses as well as on their polarisations. Hyperfine effects have been ignored. The theory applies to both coincident pulses and to pulses with a time delay between them. An illustrative application to the case of two very short but coincident pulses (during which Larmor precession, spontaneous emission etc. can be ignored) is given. In an accompanying paper the theoretical expressions are further evaluated and successfully applied to the interpretation of cascade Zeeman quantum beat experiments in Fe I.

For pt.I see ibid., vol.20, p.251 (1987). Theoretical expressions for cascade light intensity as a function of time have been evaluated for the case of cascade Zeeman beats produced by two-step excitation with broad-line laser pulses, and the results are compared with experimentally observed cascade Zeeman beats in Fe I. The beats were detected in time-resolved fluorescence from intermediate levels populated by cascades from highly excited levels of Fe I, using three different stepwise-excitation-cascade sequences. In the case of coincident laser pulses, good agreement between theory and experiment is obtained over a range of magnetic fields when allowance is made for Larmor precessions during the excitation period, for the width and shape of the exciting laser pulses and for the contribution of impurity processes. The effects of precession during the excitation period are especially important for the excitation sequence a ⁵D₄ to z ⁵F degrees ₅ to e ⁵F₅, for which short-pulse theories predict no quantum beats for the experimental geometry used (parallel linear polarisations in the two excitation steps). For non-coincident pulses a predicted oscillation in the amplitude of the cascade beats with magnetic field has also been observed.

The cooperative spontaneous emission by a fully inverted array of N atoms, interacting with and via the radiation field is studied. The arrays to which this treatment is applicable are symmetric under the exchange of any pair of atoms, including the dipole-dipole interaction in the energy and are confined to a volume with linear dimensions that are small relative to the radiation wavelength. Analytic expressions are obtained for the populations of the levels and the intensity of emission as functions of time, and for the spectrum of the radiation emitted in the cascade to the ground state.

The authors show that, contrary to what is usually supposed, for one-electron heteronuclear diatomic molecules there exists no reference frame with respect to which the radial electronic flux of the molecular wavefunctions vanishes as the internuclear distance tends to zero. The relevance of this fact to the properties of electron translation factors and to the calculation of radial couplings is pointed out.

Dissociative processes in Na⁺-N₂, Na⁺-CO, and Na⁺-CO₂ collisions have been studied by measuring the time-of-flight spectra of the fragment atoms originating from dissociation. The peak positions of the fragment atoms N, C and O are in good agreement with those given by the spectator model and classical trajectory calculations. The atom peaks measured at small scattering angles ( theta approximately=10 degrees ) have an asymmetric form. The experimental results at small laboratory (lab) angles can be related to the rotational rainbow effect at large centre-of-mass angles. At large lab angles ( theta approximately=40 degrees ), on the other hand, both rotational and vibrational dissociation contribute almost equally to the spectra and the atom peaks show a nearly symmetric form.

The symmetric eikonal theory of electron capture by relativistic projectiles is developed. Simple analytical expressions are obtained for K-shell-K-shell charge transfer total cross sections when the spin of the electron flips as well as when it remains unchanged. Comparisons with experimental data and other theoretical relativistic approximations are given. It is also shown that the relativistic symmetric eikonal total cross sections coincide at asymptotically high impact energies with the relativistic second-order Oppenheimer-Brinkman-Kramers and relativistic continuum distorted wave cross sections.

A comparison between the impulse approximation (IA) and the strong-potential Born approximation (SPB) is carried out for radiative capture of a target electron into excited bound states (REC) and into continuum states (radiative ionisation, RI) of the projectile. It is found that in the vicinity of the peak in the photon spectrum, the deviations of the SPB from the IA are small but do not disappear either for excited states or for continuum states with low momentum kappa /sub /f. The contribution of RI to the forward peak is calculated for the systems C⁶⁺ to He and Ne¹⁰⁺ to He, showing a strong skewness towards the high-energy side. For electrons with high kappa /sub /f, large deviations between SPB and IA indicate the breakdown of an atomic description.

A coupled pseudostate approximation for calculated triple differential cross sections for electron ionisation of atomic hydrogen is presented. A partial wave formalism is developed for ionisation from any initial s state. The approximation is best suited to asymmetric geometries where one of the two electrons in the final ionised state has a much smaller energy than the other. Results are presented for ionisation of H(1s) at incident energies of 54.4, 100, 150 and 250 eV and ejected energies of 5, 10 and 14 eV. At 150 and 250 eV comparison is made with the coplanar experimental data of Ehrhardt et al. (1985), Schlemmer and Lohmann et al. (1984) with a generally satisfactory outcome, although there are some discrepancies. The coupled pseudostate approximation substantially confirms the earlier second Born/eikonal-Born series calculations of Byron et al. (1985) for high energy asymmetric coplanar geometries.

The feasibility of determining the Stokes' parameters for the electron impact excited 3³P, 3¹D and 3³D states of helium is shown for several scattering energies and angles. An extensive analysis is made of the optimisation of the experimental variables. The methods used for extraction of the source parameters are given in detail, with emphasis on the use of polarisers and the analysis of the electron-photon coincidence spectrum. The Stokes' parameters for the 3³P state are determined at 39.7 and 23.5 eV for several electron scattering angles. There are no theoretical or experimental values for comparison. The lifetimes of the 3³P, 3¹D and 3³D states are measured to be 102.8+or-2.4, 16.3+or-0.8 and 13.9+or-0.5 ns respectively. The method is free of cascade effects and the values are more accurate than previous determinations.

An interpretation of the low-lying dissociative attachment bands in CS₂ is attempted with the help of the combined application of fragment anion mass spectrometry with ion kinetic energy analysis, electron transmission spectroscopy and electron energy loss spectroscopy. Dissociative attachment bands at 3.35, 3.65, 6.2 and 7.8 eV were found, in agreement with earlier measurements. The ion kinetic energies were found to be nearly zero at all four bands. The onset of the 3.35 eV peak was found to be very gradual, with a weak signal extending down to 2.55 eV, which is in close proximity to the dissociation threshold at 2.41 eV. The transmission spectrum shows evidence of a ² Sigma _g shape resonance at 4.2 eV, in addition to the previously reported ² Pi _u resonance at 1.8 eV. The energy loss spectra reveal three low-lying structured valence bands with maxima at 3.31, 3.63 and 3.90 eV. The energy dependence of the excitation of these bands shows a stepwise onset and two broad bands at 4.8 and 5.6 eV, tentatively assigned as the ( pi )³( pi *)²² Phi and ² Pi _u core-excited shape resonances. Based on these experimental results it is suggested that the 3.35 and 3.65 eV S^-/CS₂ peaks are caused by dissociation or predissociation of negative-ion states in which an electron is loosely bound by the low-lying valence excited states of CS₂, on potential energy curves that nearly coincide with those of the parent states in the Franck-Condon region.

A computer-based analysis has been performed to study the different characteristics of positrons, such as the annihilation constant, the average energy, the diffusion coefficient and the drift velocity in argon gas under the influence of applied external electric and magnetic fields at room temperature. The electric field is varied over the range 0-200 V cm^-1 Amagat^-1 while values of the magnetic field up to 50 kG are considered. Two models of the positron-atom interaction have been employed and compared with experimental results wherever possible.

The authors show that structure produced in the ionisation continuum via multiphoton ionisation by an intense laser field of frequency omega enhances the generation of optical harmonics at frequencies omega /sub /k=(k₀+k) omega , with k=0, 1, 2, . . ., where ionisation requires a minimum of k₀ photons.