Table of contents

The anomalously large fine structure of the ns²₍²D) series in Al I, Ga I and In I (n=3, 4 or 5, respectively) is shown to be due to an important magnetic correlation effect in the nsnp²(²D) perturber. Consequently, this fine structure enables a simple derivation of the perturber distribution over the series. The results (the largest component of the perturber is found in the second ²D term) call for a new theoretical treatment of the discrete spectrum.

The authors study the classical electronic motion of a two-electron atom on the Wannier ridge. Stability properties of certain periodic orbits are calculated. The analysis shows that the helium atom is not ergodic. Semiclassical implications of the classical analysis are discussed.

The authors report the observation of the 1S-2S two-photon transition in atomic hydrogen in an atomic beam, excited by light from a narrow-band continuous-wave laser. The excitation takes place in a collision-free environment and detection of the resonance is carried out by Stark-quenching the metastable atoms some distance from the interaction region. This approach removes the limitations of cell excitation employed in previous experiments with continuous-wave lasers, and should allow the full precision of modern tunable laser techniques to be applied to the 1S-2S transition. It also gives a new method for producing a collimated beam of metastable atoms which has particular applications to a variety of future experiments with cold hydrogen.

Using a novel semiclassical approach, the authors analyse the resonance behaviour of spin-1/2 atoms subjected to static and oscillating magnetic fields. Unlike a number of previous calculations, the orientation of the pump beam is taken into account explicitly. Thus it is shown that much previous work is valid only if the pump beam is along the static magnetic field. They also show how one such previous calculation can be generalised, namely that based on diagonalisation of the Floquet Hamiltonian.

The projectile charge state dependence of binary encounter electron (BEe) production was studied at zero degrees with respect to the beam direction for collisions of 19 and 28.5 MeV F^{(3(sup)-or+9)+}= with H₂ and He targets. The measured double differential cross sections for BEe production were found to be increasingly enhanced with decreasing projectile charge state contrary to the usual screening behaviour. The relative enhancement of the BEe F⁸⁺ yields over the F⁹⁺ yields was found to be about 15% increasing to about 50% for F³⁺ projectiles. Plausible mechanisms are discussed; however, no theoretical explanation of this anomalous enhancement is established.

In a first paper, the authors pointed out that valence-only calculations based on the effective potential of Barthelat et al. (1977) yield satisfactory low-energy electron-molecule scattering results at a reduced computational cost. In the present letter, they explain why this potential seemed superior to more recent ones when the calculation was performed in standard molecular bases. They clarify why bases adapted to the pseudopotential philosophy must be used and they show that, by optimising them adequately, valence-only scattering results remain as accurate as all-electron ones up to large scattering energies (100 eV).

The He II n=2 satellite in the He ionisation spectrum is investigated by measuring the angular dependence of the (e, 2e) triple differential cross section (TDCS) in asymmetric geometry for the first time, at moderate momentum transfer (0.8 a^-1₀) and low ejected electron energy (20 eV). The generalised oscillator strengths for the transitions to the n=1 and n=2 He II states are derived from the TDCS and their ratio is in fair agreement with a recent close-coupling calculation. The experimental angular distribution of the n=2 TDCS shows peculiarities, typical of continuum final-state correlations, which appear to be much more relevant when the final ionic state is the excited one.

The authors report differential cross sections for the X ¹ Sigma ⁺_g to b ³ Sigma ⁺_u, a ³ Sigma ⁺_g and c ³ Pi _u excitations of H₂ by electron impact at 20 eV. They are particularly interested in studying correlation effects of the target on these cross sections. Comparisons are made with available theoretical and experimental results.

Using the language of scattering theory a Floquet theory for the interaction of atoms with a strong laser or microwave field is developed incorporating bound and continuum states at the same time. The various boundary conditions to be used are discussed and the T-matrix and K-matrix approaches are outlined separating the effects of the bound and continuum states with the help of projection operators. The theory is then exemplified by a solvable model, which neglects the continuum-continuum interactions. The properties of the solutions in this model are discussed and it is shown that the photoionisation probability exhibits a general above threshold ionisation (ATI) resonance structure, where the positions of the resonances are given by a self-consistent eigenvalue equation.

For pt.I, see ibid., vol. 22, no.18 p.L503-9 (1989). A detailed analysis of the Sturmian expansion of the off-shell Coulomb wavefunction is presented. Some analytic properties of the representation are clarified and expressions for the off- and half-shell T matrix and for the near-shell approximation are derived. Finally, an algorithm ensuring rapid and stable calculations of the expansion series is given and extensions to non-Coulombic systems are briefly mentioned.

The tables of f-electron matrix elements calculated by Nielson and Koster (1963) for single-electron tensors of the types V^(k) and W⁽¹¹⁾ are examined for proportionalities that go beyond what would be expected on the basis of a straightforward application of the Wigner-Eckart theorem. The tensors V^(k) (for k even) transform according to the irreducible representation (irrep) (20) of th exceptional Lie group G₂, and the -triple Kronecker product U*(20)*U' that occurs when a matrix element is being evaluated frequently contains the G₂ scalar (00) twice in its reduction. In spite of this, blocks of matrix elements are often proportional to each other, especially when U or U' is the irrep (20) or (21). Various methods are considered to explain such proportionalities (as well as a few unexpected orthogonalities), the aim being to avoid a detailed calculation of the type undertaken by Nielson and Koster. Commutation relations provide the most striking approach, though separating the spin-up and spin-down electrons and using UU coupling to bring the two parts together can also be effective. In the case of W⁽¹¹⁾, the analysis is extended to embrace operators that possess the same G₂ label (namely the irrep (11)) and that come from two-electron interactions of the spin-other-orbit type. Proportionalities are much less common here. Although the authors have been unable to account in a direct way for a few relations, their special nature suggests that they arise from some particular set of circumstances and not as a result of some underlying group-theoretical structure that has eluded their detection.

Accurate ab initio calculations are presented of the electronic factor F for the field isotope shift in the resonance line in K and for the hyperfine structure of the states involved. The relation between the authors' calculated F value and that obtained from experimental hyperfine structure data is discussed. The isotope shift data by Touchard et al for the chain ^38(sup)-or+4 ⁷K are reanalysed and revised values for delta (r²) are presented and compared with corresponding results for neighbouring Ca isotones.

A configuration-average method is developed to investigate the properties of multiply-excited configurations of many-electron ions produced during multiple-electron capture collisions. For doubly-excited configurations of Ar⁷⁺ lying below the 2p⁵5s ionisation limit, the average energy is calculated and a classification into core-excited Rydberg series is presented. The energy range spanned by Wannier-type configurations 2p⁵N'n with N approximately=n also includes high members of the Rydberg series with N<<n. The radiative and non-radiative decay rates are calculated and the cascade processes from excited configurations are analysed explicitly. The stabilisation ratio R(N', n), defined as the probability of reaching non-autoionising configurations of Ar⁷⁺ during the cascades from the configuration 2p⁵N'n, is calculated for all configurations with N<or=5, n<or=9. Its dependence on the quantum numbers N, n, and on the energy of the configuration is investigated. Highly-excited configurations with very asymmetrical excitation of the two electrons (N<</n, '<<) correspond to high R ratios.

With the purpose of refining diagnostic methods for laser plasmas and of improving technological applications such as X radiation sources and X-ray lasers, the authors have examined the combined effects of radiative transfer and temperature transverse gradient on spectral line intensities. To this end they have used composite targets with a suitable geometry and a varying transverse dimension. The measurements concern mainly Al H gamma , Al He_gamma and Al H_beta , Al He_beta spectral lines. The latter are sensitive to both the inhomogeneity and radiative transfer effects while the former depend essentially on the temperature gradient. The comparison between theoretical results and experimental ones has shown that plasmas with a large transverse dimension (D>or approximately=40 mu m) are simultaneously inhomogenous and self-absorbent.

Some analytical methods to compute the Coulomb free-free and bound-free matrix elements are described and applied to the calculation of the autoionisation probabilities of high angular momentum doubly excited states 6pnl in barium. Emphasis is put on the use of analytical formulae deriving from methods originally developed in photoionisation studies of high Rydberg states. The semiclassical formulae for collisional excitation-extensively used in line broadening theory-are derived as special limits of the Gordon quantum expression. It is shown that the domain of validity of the semiclassical approach extends to low-energy collisions, and even to near-threshold excitation, when proper average electron kinetic energy and angular momentum are used.

The R-matrix method is used to obtain accurate values of energies and autoionisation widths for intermediate resonance states formed during dielectronic recombination processes. Rate coefficients are calculated for the Al¹⁰⁺ ground state, both in the L-S and intermediate coupling schemes. The effect of the fine structure splitting of the core level is studied using quantum defect theory. The relative importance of dielectronic and direct radiative recombination processes in Al¹⁰⁺ plasmas is discussed.

The photoabsorption and fluorescence cross sections of Cl₂ were measured in the 50-105 nm region. Excitation of Cl₂ in the 90-109 nm region emits two UV bands with peaks at 258 and 307 nm. The emission bands are attributed to the Cl₂ (D' to A') transition that is produced by the energy transfer from Cl*(⁴P₅2/) to Cl₂. The Cl⁺₂(A to X) emission band is produced at excitation wavelengths shorter than 89 nm.

This work reports the five- and nine-photon ionisation of the N₂ molecule in the nanosecond range with laser intensity up to 10¹⁴ W cm^-2. Both ion mass spectra and photoelectron energy distributions are recorded. At 694.3 nm the authors demonstrate the absorption of six photons above the ionisation threshold (ATI) populating a unique vibrational level (X ² Sigma ⁺_g, nu ₊=0) which is subsequently dissociated into N(²D^O) atomic fragments by absorption of seven photons. At 347.1 nm the N⁺₂ vibrational population is distributed over X ² Sigma ⁺_g, nu ₊=0-3 and A ² Pi _u, nu ₊=0-5 levels which then dissociate into N(²D^O) and N(⁴S degrees ) by absorption of three or four photons. The effects of the high laser intensity are discussed.

The low-lying electronic excited states of H₂Se and H₂Te, accessed by multiphoton absorption, have been investigated at ab initio level according to quantum electrodynamical formalisms, by utilising RPA vertical transition energies and amplitudes. The spectral observables associated with absorption of two, three and four photons with identical frequency and polarisation have been evaluated. The molecular response to concerted absorption of two and three photons from two tunable laser beams under various polarisation situations has been investigated as a function of the photon frequencies.

The spin polarisation parameters S, T, U and differential cross sections are calculated for low-energy electrons elastically scattered from mercury atoms. Additionally the moduli and the relative phases of the scattering amplitudes are calculated. The applied theoretical method is based on the scattering Dirac-Fock equation, where the exchange is calculated exactly. The scattering potential is a combination of two terms: the frozen-core Dirac-Fock potential and the polarisation model potential with one adjustable cut-off parameter. The results obtained are in very good agreement with the recent experimental and theoretical data of the spin polarisation parameters. Some discrepancies are noted in the case of the absolute differential cross sections.

Electron impact optical cross sections of thirteen infrared transitions from the 5d levels of xenon have been measured over an electron energy range of 10 to 150 eV. The measurements were made using a commercially built Fourier transform spectrophotometer and an electron gun constructed in-house. Absolute calibration of the data was accomplished by comparison with the helium 728.1 nm cross section. Pressure in the experiment was kept at 4 mTorr or less, but at these pressures the excitation functions still show some dependence on operating pressure. No attempt was made to determine the degree of polarisation of the radiation in the experiment.

Cross sections for the production of N⁺₂, N⁺+N²⁺₂ and N²⁺ by electron impact on N₂ have been measured for an energy range from threshold to 1000 eV. These cross sections have been fitted to an empirical formula for future use in modelling various plasmas. Considerable differences from some previous data have been found.

Differential cross sections for electron impact vibrational excitation of ethane (C₂H₆) have been determined for incident energies from 3 to 20 eV over a scattering angular range of 20' to 130'. In addition, vibrational excitation functions were determined for impact energies of 2 to 15 eV at scattering angles of 30', 60', 90' and 120'. These measurements confirm the broad shape resonance at 7.5 eV but provide no evidence for a (second) resonance near 2.3 eV. A decomposition of the vibrational energy-loss spectrum shows strong resonance enhancement of selected vibrational modes, which may be explained by decay of a temporary negative ion (TNI) of symmetry species e_u. A continuum multiple scattering calculation (CMS) reproduces these experimental results fairly well and indicates the simultaneous involvement of a TNI of species a_2u.

Fine structure transitions in the first excited state of positronium (Ps) have been measured using 'Backscatter Ps' production on a Mo surface by observation of a change in the emitted Lyman- alpha intensity under resonant microwave irradiation. Production, fine structure transitions and Lyman- alpha decay of the Ps atoms took place inside a waveguide designed to transmit the microwave frequencies of 8.6, 13.0 and 18.5 GHz for the transitions from the 2³S₁ state to the 2³PJ, J=2, 1, 0, states, respectively. In the presence of a magnetic field, all transitions observed show a shift to higher frequencies, compared with earlier calculations and measurements in zero magnetic field. The deviations exceed the expected Zeeman shift significantly but may be explained by assuming a motional Stark effect as predicted by Lewis and Hughes for Ps with kinetic energies of several eV.

N identical atoms interacting with photons are considered. Different kinds of collective variables were used to study collective effects in the system (e.g. superradiance) taking as an example Dicke collective spin. A new kind of collective operator is suggested, namely the Green para-Fermi operators a_n labelled by an index n that enumerates atomic levels. The system Hamiltonian, conservation laws and superradiant states are written in terms of the para-Fermi operators. Their use is discussed.

The interaction of the pump wave, the scattered wave and the space-charge wave in a free-electron laser with an electromagnetic wave wiggler and an axial guide field is analysed, where both the scattered wave and pump wave are circularly polarised. The linear dispersion and non-linear growth of these three waves are derived in the laboratory frame. The possibility of explosive instabilities for a fast electromagnetic wave pump propagating counter to the electron beam is discussed and its critical condition is ( omega _p+k_p nu _z0)- omega _b/ gamma _zo< omega _c<(k_p nu _z0), where omega _p and k_p are the pump wave frequency and wave number, omega _b, nu _zO and gamma _z0 are the plasma frequency, initial axial velocity and Lorentz factor of the relativistic electron beam, respectively, and omega _c is the electron cyclotron frequency in the axial guide field. Group II orbits ( omega _c> omega _p+k_p nu _z0) may no longer give positive gain in the process of three-wave interaction.