Table of contents

The CASSCF and contracted CI methods have been used to generate electronic wavefunctions and potential energy curves for the ³ Sigma ^-, ¹ Delta and ³ Pi states of HCl²⁺. Calculations on the ground state of HCl at its equilibrium geometry and dissociation limit were performed and the vertical double ionisation energies were deduced. These results are compared with experimental results following recent PIPICO experiments. Molecular constants and predissociation rates for the ³ Sigma ^- and ¹ Delta state, which both possess local minima above their dissociation limits, are given.

The authors present a novel instrument for the analysis of highly excited Rydberg states of ions. This set-up is applied to study the ns and nd Rydberg levels of Ba⁺ produced in an atomic beam with values of n varying between 19 and 65. The selective excitation is provided by a two-UV-laser-photon process. The excited Ba⁺ ions are ionised by a microwave field and detected by a double time of flight mass spectrometer. A UV optogalvanic spectrum, resulting from an iron hollow-cathode discharge, is used as a reference for calibration. The accuracy of the energy level wavenumber determination is +or-0.1 cm^-1. These measurements lead the authors to a revised value for the Ba II ionisation limit of 80 686.25+or-0.10 cm^-1.

Doppler-free spectra of gas phase atoms near a glass-vapour interface are obtained by monitoring the reflection of a probe beam-instead of its transmission-in three-level saturation spectroscopy. Experiments performed on the 3S-3P-4D cascade system of Na have allowed the authors to monitor two-photon-induced saturated dispersion of Na atoms.

The authors discuss the contribution of core excited states to multiphoton transition probabilities in alkali atoms. With numerical examples, they show that such contributions are small and cannot be calculated correctly by one-electron-atom treatments.

A general expression for the angular distribution of photoelectrons with defined spin polarisation ejected from oriented molecules is derived. For linear molecules it is shown that in Hund's cases (a) and (b) photoelectrons may be polarised only parallel to the molecular axis, while in Hund's case (c) all three mutually perpendicular components of the polarisation vector are different from zero.

Angular differential cross sections for Ar^q+ ions, where q=8, 9 and 11, colliding with Ar at an energy of 1.8q keV are reported. The final charge state of the projectile together with its scattering angle were measured in coincidence with the charge of the target ion. Processes where the target lost between one and five electrons were observed. The results are compared with two different versions of the classical over-barrier model.

A symmetric eikonal approximation is used to obtain a symmetric approximate formula in closed form for the cross section for 1s-1s electron capture from a target hydrogenic ion with nucleus A having atomic number Z_A by an incident bare positive ion B having atomic number Z_B moving with relativistic velocity referred to A, assuming that alpha Z_A and alpha Z_B<<1 where alpha is the fine structure constant. The formula for the capture cross section yields satisfactory accordance with numerical calculations carried out by Deco and Rivarola (1986) using a similar approximation but produces cross sections which are significantly less than those given by the non-symmetric eikonal approximation formula derived by Eichler (1985) except at extremely high energies.

Using a recently developed VUV circular polarisation analyser, circular polarisation correlations for the electron impact excitation of the C ¹ Pi _u ( nu =2) state of H₂ are measured by the method of electron-photon coincidence. The data are complementary to the previous linear polarisation correlation measurements of Becker et al. Taken together, these show that significant coherence must exist in the excitation process. The absence of hyperfine depolarising effects enables an upper limit of 180 MHz to be placed on the C ¹ Pi _u hyperfine splitting. The circular polarisation correlations indicate that for the electron scattering angles considered (<25 degrees ), the transfer of angular momentum perpendicular to the scattering plane is positive and qualitatively 'helium like'.

The spectra of atoms exposed to black-body radiation suffer temperature-dependent frequency shifts predicted earlier and now becoming measurable, whose thermodynamic status is clarified. To a good approximation, they are calculable as differences between the coupling-induced shifts in the Helmholtz free energies of the coupled system atom plus quantised electromagnetic field, in thermal equilibrium subject to the constraint that the atom is in a given state li). Effectively, the state label i plays the same role as do macroscopically controlled variables in ordinary thermodynamics. With kT well below the electron rest-mass energy, such calculations require only the forward scattering amplitude of light from the atom; some previously overlooked but theoretically interesting relativistic features of the exact expressions are pointed out. For a harmonic oscillator, uniquely, the constrained free-energy shift is the same for all levels (and therefore also the same as the shift in unconstrained equilibrium); consequently there is no thermal frequency shift at all. Recent comments on black-body-induced Stark shifts by Ford et al. (1985-6) are based on their calculation of the free energy of a harmonic oscillator in unconstrained equilibrium with the field. It is shown that as regards frequency shifts their comments are incompatible with, and misrepresent, previous correct work on such shifts.

A paper by Barton (ibid., vol.20, p.879, 1987) comments on some of Ford et al.'s recent work in this general area and takes issue with some of their conclusions. Ford et al. point out that Barton's reinterpretation of his own previous work actually serves to support their conclusions. In particular, he is making extensive use of thermodynamic concepts (free energy, thermodynamic perturbation theory, . . .) which were introduced for the first time into this area in the papers by Ford et al.

The configuration-interaction code CIV3 has been modified to allow the inclusion of model potentials for core electrons. Optimisation of valence orbitals is performed with potentials that model the Hartree-Fock potential of the core, plus modified mass-correction and Darwin terms. After optimisation, the authors add core-polarisation potentials that include an allowance for the penetration of the core by the valence electrons, and a spin-orbit potential that reproduces fine-structure interactions. Using these potentials, configuration interaction in the valence shells has been included for several low-lying states of mercury. For the 6s²¹S₀-6s6p^1.3P^o₁ transitions, the authors obtain oscillator strength of 1.195 and 0.0234 respectively.

Ab initio Al XI and S XIV wavelengths below 400 AA involving transitions between 1s²nl ²L_J levels up to n=7 are reported. The accuracy is better than 1% in all cases where existing data allow for comparison. Corresponding transition probabilities are given. A time-dependent collisional radiative model is used to calculate the populations of Al¹⁰⁺ states in laser-produced plasmas. Population inversions occur between n=5, n=4 and n=3 levels at well defined times of the plasma recombination phase. The conditions of validity of the model are discussed, particularly with regard to the optical depth of the plasma for each resonance line, and conclusions are drawn from an appropriate criterion.

The X-ray satellite structure accompanying the L alpha ₁ emission for the elements iridium (Z=77), gold (Z=79) and uranium (Z=92) has been studied with a curved-crystal spectrometer. The observed satellite contour has been compared with theoretical curves which were computed assuming doubly ionised atoms.

L X-ray fluorescence cross sections ( sigma _{L alpha} , sigma _{L beta} and sigma _{L gamma} ) and relative intensities (I_{L beta} /I_{L alpha} and I_{L gamma} /I_{L alpha} ) measured for Ba, Ce and Nd at excitation energies of 15.2, 17.8, 22.6 and 25.8 keV. Reasonable agreement is obtained between the present experimental results and theoretically calculated values of L X-ray fluorescence (XRF) cross sections and relative intensities. To the best of the authors' knowledge, photon-induced L X-ray fluorescence cross sections for these elements at different energies are reported for the first time.

Reports on measurements of the shift and width of emission profiles of neutral argon lines (3p⁵4p-3p⁵4s transitions) produced in a high-pressure plasma (pressure range 1-2.5 atm) sustained by surface microwave propagation (Surfatron). Broadening and shifts were measured at different gas temperatures (1900-2600 K). The results are compared with theoretical calculations involving the long-range part of the interaction potential (van der Waals and resonant) and with the Lennard-Jones potential. Particular attention is given to the influence of the gas temperature.

A mixed velocity-acceleration formulation of the matter-radiation problem is introduced and used to evaluate cross sections for the absorption of two photons by electrons scattered from atomic targets. This approach avoids errors associated with near-cancellations, either within given radial integrals, as in the velocity gauge, or between the matrix elements of two operators, as in the acceleration gauge. The characteristic infinite summations are evaluated by the inhomogeneous equation or Green function technique. Contrary to a fairly prevalent view, the presence of quasiresonant, non-decaying intermediate states does not limit the usefulness of the Green function method.

For pt.II see ibid., vol.19, p.2109 (1986). This paper is the third in a theoretical study of subradiance, i.e. the cooperative inhibition of spontaneous emission by a destructive interatomic interference, a phenomenon which has recently been observed. The paper is devoted to the problem of the cooperative spontaneous emission of atomic samples with linear dimensions much smaller than the wavelength of the emitted light. The treatment, which is based on a fully quantum mechanical description of the problem, corroborates the results obtained in a semiclassical treatment by Friedberg and Hartmann (1974). Starting from a master equation describing both the cooperative spontaneous emission and the dipole-dipole interaction, a statistical treatment of the latter is introduced and the corresponding symmetry relaxation is studied. The evolution of the atoms is described in three typical cases that correspond to different sample geometries and/or atomic distributions. In one of these cases, in particular, the symmetry is first conserved and the atoms follow Dicke's predictions for a time. A 'retarded' symmetry breaking then occurs suddenly, near the zero value of the collective energy, as in Friedberg and Hartmann's calculations for a small sphere. With regard to the further evolution of the atoms in this case, which cannot be studied in a semiclassical model, it is shown that the decay rate quickly becomes much smaller than the incoherent decay rate. An unquestionable subradiance phenomenon is thus predicted in this case.

Signals with subnatural linewidths observed in a polarisation spectroscopy experiment on sodium have been investigated under various experimental conditions. The authors find that the mechanism responsible for the subnatural structures is based on non-stationary velocity-selective optical pumping. It is discussed in the light of the presented measurements and a theoretical analysis. Possible spectroscopic applications are considered.

The energy spectrum of electrons produced in the multiphoton ionisation of rare gases in the 10¹²-10¹⁵ W cm^-2 range displays interesting features. It generally consists of a series of peaks evenly spaced by an amount equal to the photon energy. The number of peaks depends strongly on the laser wavelength. For example, with Xe only one additional peak is observed at short wavelengths, while about ten and even tens of peaks are observed at longer wavelengths such as 1064 nm. These absorption processes can be described in terms of continuum-continuum transitions or, better still, in terms of the absorption of photons by the electron in the field of the ion to which it was originally bound. Furthermore, as soon as the electron-ion pair is formed, the electron acquires a quiver energy Delta in the presence of the EM field. The absorption of additional photons, corresponding to an energy less than Delta , is made energetically impossible. This leads to the suppression of the first peaks of the electron energy distribution. The disappearance of a certain number of peaks depends strongly on the laser wavelength and intensity. The disappearance of nearly 30 peaks has been observed for He at 1064 nm and 10¹⁵ W cm^-2. Finally, additional effects, such as electron angular distributions and space charge effects, can change the relative amplitude of the first electron peaks. These effects must be taken into account before any comparisons can be made between theory and experiment.

The non-stationary multistep (diffusion-like) ionisation of Rydberg atoms is considered theoretically. The investigation is based on the time-dependent Fokker-Planck equation and is related to collisional and microwave ionisation of Rydberg atoms. Analytical expressions for the mean time and the higher moments of the distribution of the diffusive ionisation times are derived. The conditions for the occurrence of diffusive ionisation are obtained. The relation of the present study to stationary diffusive ionisation and the experimental investigation of the ionisation of Rydberg atoms is discussed.

Quantum mechanical close-coupling calculations have been performed on rotational energy transfer in collisions of para-ammonia with ground-state (j=0) para-hydrogen. Rate coefficients are given for the temperature range 15<or=T<or=300 K. The calculations employed an ab initio potential energy surface derived from separate determinations of the SCF and dispersion energies. The MOLSCAT computer package was used to perform the scattering calculations. Comparison is made with the results of Billing and Diercksen (1986), who adopted a semi-classical approach as well as a different potential. Conclusions on the accuracy of the potential surface are drawn from comparison with experimental microwave double resonance and pressure broadening work. The extent to which He behaves like ground-state para-H₂ in collisions with NH₃ is also investigated.

The alignment of Na* (2p⁵3s²²P_3/2) for H⁺ and He⁺ impact was measured via the non-isotopic emission of autoionisation electrons for the projectile energy range of 10 to 300 keV. The experimental results for both projectiles agree reasonably well with theoretical results of a PWBA calculation; particularly, they confirm the minimum of the alignment parameter A₂ predicted by theory. On the other hand the experimental results disagree with the results of a close-coupling calculation.

Total and differential cross sections for excitation are calculated using three distorted-wave methods: the continuum distorted-wave (CDW) method, the Coulomb peaking impulse approximation (CPIA) and the symmetric eikonal (SE) method. All three methods have the correct Coulomb asymptotic behaviour on both channels. Results for proton-hydrogen excitation to levels n=2, 3 and 4 are presented. Differential cross sections are also calculated and compared with experimental results. The CDW method is found to present a doubtful enhancement that vanishes at high projectile velocities. The dependence of the excitation process on the projectile Coulomb charge is also studied. Cross sections for single excitation of Fe²⁴⁺ by impact of bare and neutral projectiles are calculated and compared with recent experiments.

Relative differential cross sections are presented for elastic scattering of electrons by sodium, potassium and rubidium atoms in the energy range from 150 to 500 eV. The data were obtained at angular intervals of 10 degrees from 30 degrees to 120 degrees . The cross section curves show a minimum around 100 degrees for sodium and potassium and 80 degrees for rubidium, the depth of the minimum decreasing with increasing energy. A comparison with previous experimental data and various theoretical models indicates the dominance of static and absorption potentials in the elastic scattering channel and the weak binding of valence electrons in the alkali atoms.

The authors report experimental elastic differential, integral and momentum transfer cross sections for electrons scattered by Ar and O₂ in the impact energy range 300-1000 eV and in the angular range 5-140 degrees . In their experiments, the intensities of the elastically scattered electrons were obtained in the crossed-electron-beam-molecular-beam geometry and the absolute values of the cross section were derived through the relative-flow technique using N₂ as the secondary standard. For both gases, the cross sections are compared with the available experimental and theoretical results.

The coupled pseudostate model of Curran and Walters (1987) is used to calculate non-coplanar triple differential cross sections for electron impact ionisation of H (1s) in asymmetric geometry. The calculations are for an ejected-electron energy of 5 eV and for incident energies of 250 and 150 eV. The results are displayed in a way which attempts to expose symmetry in the cross section surface. Double and single differential cross sections are also calculated in the pseudostate approximation for ejected energies of 5, 10 and 14 eV and incident energies of 250, 150 and 100 eV. Double differential cross sections are differential with respect to the energy and angles of one of the outgoing electrons. The shape of these cross sections is explained in terms of the structure of the triple differential cross section and the typical form of optically allowed discrete excitation cross sections.

Absolute cross sections sigma _q,_q+1 for electron impact single ionisation of Kr^q+ ions (q=1, 2, 3) have been measured for electron energies up to 700 eV by employing a dynamic crossed-beams technique. The experimental data significantly exceed the Lotz prediction for direct ionisation of Kr²⁺ and Kr³⁺ ions, indicating the importance of indirect processes for multiply charged Kr^q+ ions. The observed ionisation thresholds of cross sections from the present measurements as well as of data from Danjo et al. (1984) and Gregory et al. (1983) indicate the presence of metastable ions in the parent ion beams. The influence of metastable states on the cross sections is discussed on the basis of Lotz calculations (1968) for direct ionisation from different excited states and differences observed in the experiments are rationalised.

A new metastable state in mercury has been observed at an energy of 10.530 eV. It is assigned as 5d⁹6s²(²D₃2/)6p₁2/(3/2¹/₂)₂ and its lifetime is estimated to be approximately 200 mu s. A novel technique has been used to differentiate this new metastable state from the other, lower-lying, metastable states of mercury, and high-resolution electron impact has been used to measure its excitation function, which displays resonance structure. A mechanism is described by which atoms in this metastable state de-excite at the detecting surface.