Table of contents

The Thomas-Reiche-Kuhn sum rule is determined for an atom consisting of an arbitrary number of discrete, equally spaced eigenenergy levels. This generalization is consistent with a two-level atom and with a usually seen form of the sum rule involving the atomic ground state, and resolves an apparent paradox reported elsewhere.

The EUV spectra of tin, antimony and tellurium have been observed in laser produced plasmas. Lines of Ag I-like Sn IV, Sb V and Te VI were recorded in the 250-450 AA region and most of those arising from the 4d¹⁰5s-4d⁹5s5p transition array were identified by isoelectronic comparison and MCDF calculations.

The R-matrix method has been employed to study the electron excitation of atomic helium. Twenty-nine target states (n=1, 2, 3, 4 and 5) have been included in the expansion of the total wavefunction. The results of the calculation are used to obtain the electron-photon coincidence parameters lambda and chi for the excitation 1¹S-2¹P at 500 energies ranging from the 2¹P threshold to 24 eV. The calculated parameters lambda and chi are compared with values obtained from experiments and with earlier five-, 11- and 19-state R-matrix calculations.

The authors have performed 11-state and 37-state R-matrix close-coupling calculations of the contributions of double excitation followed by autoionization to the electron impact ionization cross section of Li⁺. Through the use of pseudo-orbitals in the configuration-interaction expansions for the bound states, the authors obtained energies in good agreement with observed energies for both the singly excited and doubly excited terms. The 37-state calculation, which employed a basis set with a total of 58 terms in all of the configuration-interaction expansions, is an improvement over our earlier calculation of this process and predicts a resonance structure which is in general agreement with that observed in an earlier high-resolution measurement of this process.

Energy spectra of electrons ejected from an argon target by positron impact have been measured and compared with theoretical expectations. No major cusp structure was observed in any of these spectra. Instead a small ridge is present in some cases at energies compatible with electron capture to continuum (ECC) states of positronium. Its magnitude suggests that ECC makes a minor contribution to ionization by a light positively charged projectile.

The method of minimization of the variance of Froelich and Brandas (1975, 1976) is generalized by considering other mean values than the energy. More generally, quantum resonances are also considered as incompletely specified mechanical systems. They are described by density operators which provide the least biased statistical description available from relevant mean values. Two model examples illustrate the relevance of this approach. This way of considering resonances is a new application of the basic ideas advocated by Brillouin (1988) and Jaynes (1957) in information theory and statistical physics.

A method for calculating the perturbation of hydrogenic manifolds, the emerging bound states and resonances, for arbitrary combinations of external fields, is presented. It requires the combined use of complex dilation, an orthonormal Laguerre basis e^{- lambda r}L^2l+2_k( lambda r) rather than the non-orthogonal Sturmians e^{- lambda r}L^2l+1_k( lambda r), and the recursive residue generation method (RRGM) version of the Lanczos algorithm. Generalized eigenvalue problems are avoided. Furthermore, direct computation of the residues of resolvents, transition amplitudes and sum rules is achieved. Comparison with other methods and with previous calculations, suitable for one perturbation at a time, indicates that high accuracy is achieved separately both for the 1s Stark resonance and for the 1s Zeeman effect. Accurate results for the 1s Stark-Zeeman resonance, for various combinations of fields, are given.

Three different theoretical methods (MCDF, HFR and SST) taking configuration interaction into account are compared with respect to their ability to predict energy levels and radiative transition rates for forbidden transitions within the 3d² configuration and for transitions of the type 3d²-3d4s or 3d²-3d4d. A set of new M1 and E2 transition probabilities is presented for the ions Ti III to Ag XXVIII. It is shown that the three methods considered in this paper are basically equivalent in their predictive power but the method most suited for the evaluation of oscillator strengths at a particular degree of ionization is identified.

Photoionization spectra of atomic hydrogen of a homogeneous electric field are calculated by exact integration of the Schrodinger equation for both negative and positive energies and compared with various experimental data. The number method allows the spectra to be decomposed into contributions from the parabolic substates labelled by the quantum number n₁. On this basis the field-induced continuum oscillations are discussed.

The authors have recorded the time-resolved photoabsorption spectra of aluminium plasmas in the 50-200 AA wavelength range using the dual laser produced plasma technique (DLPP). Absorbing plasmas were generated by focusing the output from a pulsed dye or a Q-switched ruby laser onto an aluminium slab target. The continuum backlight was provided by irradiating a tantalum rod with the output from a time-synchronized Q-switched Nd-YAG laser. Some of the spectra were recorded with the aid of a toroidal mirror. By combining time and space resolution the authors have been able to isolate almost exclusively Al⁺ and record its photoabsorption spectrum. They provide an analysis of 2p to nd, (n+1)s Rydbergs in Al⁺ based on multiconfiguration ab initio atomic structure calculations and quantum defects for the higher members of each series. They have also observed and tentatively classified a number of members of the 2p⁶3s² to 2p⁵3s3p² doubly excited transition array.

Photoexcitation of the Kr 3d to np resonances and photoionization of the 3d inner shell have been studied by measuring the emitted electrons with energies from 0 to 3 eV and from 0 to 12 eV, respectively. Intense discrete structure due to the Kr⁺ to Kr²⁺ transitions has been observed in photoelectron spectra. Assignments are proposed based on the spectator shake model and by analysis of effective quantum numbers of proposed Kr⁺ states. The observed energy shift and asymmetric line broadening, related to the post-collision interaction, of the Kr⁺ 3d^-1(²D_512.3) ionic states have been compared with previous experimental results and with theoretical predictions.

The diagrammatic solution to a (sequentially coupled) one-electron three-level system in an intense laser field is derived. The solution is in the form of a continued-fraction expansion and is equal to the Floquet state that is adiabatically connected to the ground state. The formalism used is a recently presented diagrammatic many-body theory by Jonsson that can be used for non-perturbative calculations in strong fields. The purpose of the calculation is to demonstrate how a non-trivial one-electron problem can be solved exactly within the formalism.

The vibrationally resolved spectra of the ground state A ² Sigma ⁺_u of Xe⁺₂ were observed at wavelengths from 1023 AA to 1116 AA by using the method of single-photon ionization with synchrotron radiation as a light source. A long progression beginning at v=0 was clearly observed in the threshold electron spectra for the first time. The spectroscopic parameters for the ground state of Xe⁺₂ (average mass=131.3, 131.3) calculated from the experimental data are: omega _e=122.9 (+or-0.1) cm^-1 omega _e chi _e=0.481 (+or-0.001) cm^-1, the dissociation energy D₀=7849 (+or-9) cm^-1, the zero point energy 61.3 (+or-0.1) cm^-1, T_e=90 119 (+or-8) cm^-1, and the adiabatic ionization potential 90 170 (+or-8) cm^-1. Autoionizing Rydberg series were found and assigned to converge to the C ² Pi ₃2u/ and B² Pi ₁2g/ states of Xe⁺₂.

A merged beam apparatus is described that has been developed to study mutual neutralization. Initially, it has been used to measure cross sections for H⁺/H^- neutralization for centre-of-mass energies between 3 and 500 eV. The results are in excellent agreement with those of Szucs et al. (1984), which covered the range from 5 to 2000 eV, and with inclined beam measurements of Peart et al in the overlapping range of energies from 30 to 500 eV. There is also good accord with a variety of recent theoretical results, notably those by Sidis et al. (1983) Fussen and Kubach (1986) and Shingal and Bransden.

The authors show that for the study of recombination of ions and electrons in weakly ionized low-temperature hydrogen plasmas, the processes H+H⁺+e to H+H*(n) and H⁺₂+e to H+H*(n) must both be considered since their contributions are comparable. A simple method for the calculation of the corresponding rate coefficients is presented.

The ionization cross sections for thermal energy collisions Ne(3s ^1.3P₁)+X are calculated where X is an Ar, Kr or Xe atom or an H₂ molecule. The results are compared with experimental data. A theoretical analysis of the Ne(3s) rare gas term structure and autoionization width is performed. The interaction potentials of the Ne(3s)+H₂ system are calculated using the pseudopotential method.

At wavelengths lower than 550 nm, addition of argon or neon to xenon gives rise to the emission of continua produced by heteronuclear processes (ArXe, Ne⁺Xe and NeXe⁺) indicating large energy transfers from the preponderant and lighter gas. Above 550 nm the transitions observed were all produced by atomic processes. Addition of argon to xenon causes an enhancement of certain xenon emissions ( lambda =770 nm in particular): this does not occur in Ne-Xe mixtures. These processes involve, above all, high energy states of xenon (configuration 5d). The kinetic study of certain emissions is dealt with in paper II (Galy et al., 1993).

A combined experimental and theoretical investigation has been carried out on CS⁺ and CS²⁺ radical ions. Ion translational energy spectrometry has been applied to study charge stripping of CS⁺ ions to yield metastable CS²⁺ ions and to study the energetics of spontaneous and collision-induced dissociation of these metastable doubly charged ions of CS. All-electron ab initio molecular orbital calculations, inclusive of fairly large-scale configuration interaction procedures, have been carried out to compute the potential energy functions of the different ionic systems involved in the collision processes studied experimentally.

The problem of electron impact ionization of the hydrogen atom at incident electron energies close to the breakup threshold is considered within the framework of three-particle Faddeev-type equations. To solve these equations an approximated modified strong channel coupling method is applied: each Faddeev component is expanded in the eigenfunctions of the discrete spectrum of the target Hamiltonian and final state wavefunctions. The expression for the triply differential cross sections (TDCS) is given for the coplanar symmetric case of equal energy partition between the final state electrons outgoing in opposite directions.

The polarization fraction of the 3¹D-2¹P (667.8 nm) line in helium following electron impact excitation of the 3¹D state has been measured over the incident electron energy range from threshold to 300 eV. Serious discrepancies are observed with the only previous experimental measurement over an extended energy range, particularly at energies under 200 eV. All calculations underestimate the polarization fraction at incident electron energies above 30 eV, but a theoretical prediction of lack of dependency on the principal quantum number n is confirmed.

Relative differential electron impact cross sections ( sigma ( theta )) for elastic and inelastic (3p, 4s, 3d, 4p) scattering at 10 eV, 20 eV (3p and 4s excitation only) and 54.4 eV by sodium atom have been measured. The sigma ( theta ) for the 3s-3p transition are normalized to the optical oscillator strength. The angular range of measurements is from 2' for inelastic and from 6' for elastic scattering, up to 150' for both. Results are compared with the available experimental and theoretical data. Present results of sigma ( theta ) are lower at large scattering angles than coupled-channel theory predicts. Comparison with the recent distorted wave calculations by Madison et al. (1992) is given in the following paper.

Second-order distorted wave calculations are reported for elastic scattering and excitation of the 3²P⁰, 4²S, and 4²P⁰ states of sodium by 10-150 eV electrons. The second-order results are summed over bound intermediate states and integrated over continuum intermediate states to numerical convergence. The effects of atomic charge cloud polarization and exchange distortion are isolated and examined. Experimental and theoretical results are compared for differential cross sections, spin asymmetry parameters, transferred angular momentum, and triplet/singlet ratios. It is found that the second-order results are typically in reasonably good agreement with experiment for energies above about 10 eV.

A coplanar electron coincidence spectrometer has been used to measure Auger lineshapes in coincidence with the scattered electron produced after electron impact ionization of the 2p shell in argon. The coincident lineshapes have been measured at an excess energy which is close to the nominal Auger energy. The L₃-M₂₃M₂₃(¹D₂) lineshape was measured for two different emission angles, -95' and 132', of the Auger electron. The angles chosen correspond to the recoil and binary regions, respectively, of the triple differential cross section for 2p ionization of argon. Additional structure was observed in the coincidence Auger lineshape at one angle (-95'), consistent with an angular dependence of the post collision interaction.

Differential electron scattering cross sections for excitation and deexcitation of Ba (. . . 6s6p ¹P₁) excited atoms were calculated at 30 eV impact energy in the unitarized distorted wave approximations using multiconfiguration wavefunctions. The results are compared to available experimental data. It is found that theory and experiment agree quite well as far as the shapes of the differential cross section curves are concerned. For some transitions, even the cross section magnitudes are in agreement, but for others, only order of magnitude agreement is found. Further experimental studies are needed to get a more accurate assessment of the situation.

Electron-impact, energy-loss spectra of anthracene were obtained in the 12 to 100 eV impact energy range at angles ranging from 5' to 40'. From these spectra energy level information, relative electron-impact excitation probabilities and relative optical f values have been derived. Cross sections for 250-525 nm band emission have been measured over an electron-impact energy range from threshold to 60 eV. Many processes, both direct and indirect, are observed to contribute to the excitation. The results are compared with those obtained using naphthalene targets.