Table of contents

Several rigorous upper bounds to the atomic ionization potential in are obtained from the Schrodinger equation for atomic systems. They are expressed in terms of radial expectation values (r^alpha ) and/or the electron density and its second derivative at the nucleus, rho (0) and rho "(0) respectively. A numerical study in a nonrelativistic Hartree-Fock framework is carried out for some atoms. It is shown that the accuracy of the relationship among in , rho (0) and rho "(0) is higher than 80% for all atoms studied. An interesting property of the bounds in terms of various (r^alpha ) is that they reach the exact value of the ionization potential in a limiting case.

The authors propose to prepare oriented circular Rydberg states in atoms (i.e. states where one valence electron has spherical quantum numbers: m=l=n-1>>1) by combining the specific advantages of the adiabatic crossed-field method, recently tested in alkali and alkaline earth atoms, and of the adiabatic RF field method whose principle was described in 1986. The formal equivalence of the two methods is pointed out. Their association is made possible by considering the dynamical symmetry of the Coulomb potential. In order to illustrate the new process, calculations have been performed for the manifold n=5 of hydrogen.

Cross sections for double photoionization of the Ne L shell into the 2s2p⁵³P⁰ and ¹P⁰ and the 2s⁰2p⁶¹S^e states were measured for energies from threshold up to 150 eV, using photon induced fluorescence spectroscopy. Both 2s2p⁵ channels were observed with comparable magnitude in contradiction to a propensity rule based on the Wannier-Peterkop-Rau theory. A comparison of the summed ³P⁰ and ¹P⁰ cross sections with MBPT calculations results in a deviation of 50%.

The non-linear absorption coefficients of C₆₀ thin films at 633 nm were measured with the absolute measurement and the Z-scan method, respectively. Both measurements yield a large non-linear absorption coefficient beta =4.4 cm W^-1 and 8.0 cm W^-1. A thermal self-focusing effect was also observed. The possible mechanisms of the larger non-linear absorption were discussed. The authors predict that C₆₀ thin film may be a good optical limiter for a continuous wave (CW) laser.

The authors study the dependence of the production of binary encounter electrons on the charge state of recoil ions for 2.4 MeV u^-1 Xe²¹⁺ on He and Ar. Doubly differential cross sections of electron emission are calculated with the nCTMC method. They find that the contributions to the binary electron cross section from various recoil ion charge states reach a maximum near Ar⁵⁺ for the Ar target. In the case of the He target, double ionization dominates over single ionization for all ejected electron energies above 100 eV. However, an unexpected local drop of double ionization in the binary peak region has been observed. This decrease is found to be related to a two-step, sequential removal of the two electrons.

The ion mobilities of N₂⁺ and CO⁺ in He have been measured using a liquid-helium-cooled, selected ion drift tube at 4.35 K. Mobility reaches a maximum at an effective temperature of between 400 K and 600 K, where it is in reasonable agreement with the room temperature measurements of other experimenters. Below 100 K there is a region in which the mobilities are constant, N₂⁺ close to the polarization limit and CO⁺ somewhat lower, possibly due to the occurrence of collisionally induced, rotational excitation. At very low effective temperature though, below 10 K, the mobilities decrease sharply, below the polarization limit. This may be further evidence for the existence of orbiting resonance such as that suggested previously by Kojima et al. for He⁺ in He.

Total and partial cross sections for electron capture from excited H(2p) and Li(2p) by protons and alpha -particles at intermediate energies are calculated within the time-dependent close-coupling method. The total capture cross sections are found to be relatively insensitive to the target system, but strongly dependent on the projectile charge. Special emphasis is put on the system sensitivity of electron capture cross sections to the initial p-state charge cloud alignment. It turns out that the cross sections at intermediate energies are largest when the p-state is aligned parallel with the ion beam, independent of projectile and target system.

The distorted-wave approximation combined with the Born-Oppenheimer approximation is applied for the first time to calculate the rovibrationally resolved final-state-specific integral cross sections for the X¹ Sigma _g⁺( upsilon ₀=0,N₀) to c³ Pi _u^-( upsilon '=0,1,2,3,N'=2) transitions of H₂. The comparison with the available recent LIF experimental data in the 12-22 eV range is very encouraging. In addition, the calculated relative intensities for these final vibrational levels are essentially proportional to the Franck-Condon factors for incident energies above 18 eV.

Ionization cross sections for positrons impacting on atomic hydrogen have been measured for kinetic energies in the range 15-700 eV. This has been done in a crossed-beam geometry where a magnetically guided positron beam intersects a hydrogen gas jet emanating from a radio frequency discharge tube. Electron impact ionization cross sections were also measured with the same apparatus thus facilitating comparison with, and normalization to, published results. The positron-atomic hydrogen results are found to be significantly lower than those obtained by Spicher et al. (1990).

The electron angular distribution near the double ionization threshold is considered in the framework of the Wannier mechanism. The problem is reformulated in the physically appealing terms of the harmonic oscillator with time-dependent frequency. The characteristic features of the process are described in terms of the time propagation in the adiabatic regime and the deviations from it. The boundary conditions at the border of the reaction zone are crucial for the final relative angular distribution. The omission of this point is shown to invalidate previous calculations. The simplest possibility to formulate a reasonable boundary condition comes from the assumption of the oscillator zero-mode population at the border of the reaction zone. The deficiencies of this formulation necessitate consideration of arbitrary boundary conditions; for this case the general solution of the propagation problem is presented.

The electric static dipole polarizability alpha , quadrupole polarizability C, dipole-quadrupole polarizability B, and the second dipole hyperpolarizability gamma have been calculated for the alkali metal atoms in the ground state. The results are based on a pseudopotential which is able to incorporate the very important core-valence correlation effect through a core polarization potential, and, in an empirical way, the main relativistic effects. The calculated properties compare very well with more elaborated calculations for the Li atom, excepting the second hyperpolarizability gamma . For the other atoms, there are neither theoretical nor experimental information about most of the higher polarizabilities. Hence, the results of this paper should be seen as a first attempt to give a complete account of the series expansion of the interaction energy of an alkali metal atom and a static electric field.

A polynomial version finite element method has been applied, within the Moller-Plesset theory, to calculate second-order pair and total correlation energies for several closed-shell systems with d electrons: Zn, Zn²⁺ and Cu⁺. The authors have employed basis sets including functions up to l_max=12, which are so close to completeness that radial extrapolation is not necessary, and produce extremely accurate results. The final values are compared with the most accurate values found in the literature, which were calculated using Slater basis functions. The agreement is relatively good in the case of the total correlation energies but not in some details of the electron correlation.

The author examines the relativistic, QED and nuclear finite-size corrections of the hyperfine splitting for ground-state muonic ³He up to alpha ². The author finds that the nuclear finite-size correction and the magnetic correction to the electron wavefunction of the hyperfine splitting, Delta v^e, which accounts for the magnetic interaction between the electron and the ³He nucleus, are quite different from the previous results of Huang and Hughes (1979,1980,1982). The author obtains the total hyperfine splitting, Delta v=4166.540 MHz, with the uncertainty, +or-0.001 MHz, which only originates from the calculation of the Schrodinger wavefunction, and the uncertainty, +or-0.004 MHz, which originates from the neglect of order alpha ³ and above in calculating the relativistic, QED and nuclear finite-size correction, and the uncertainties of the parameters of the electric and magnetic form factors of the ³He nucleus. This result is more consistent with the experimental result, 4166.3+or-0.2 MHz, than the previous theoretical results.

The 5p-subshell spectrum of Yb I has been photographed at high resolution in the wavelength range 250-550 AA. Twenty seven new lines have been revealed including three probable Rydberg series converging on three new limits. The spectrum has been re-analyzed through comparison with ab initio calculations.

After excitation of the 7¹S₀ mercury state by two photon absorption of a laser radiation at 313 nm the time integrated intensities of the 407.8 (I₀) and 404.7 nm (I₁) lines emitted from the 7¹S₀ and 7³S₁ states were determined as functions of the pump laser intensity, I, and mercury pressure. At high pump laser intensities I₀ and I₁ do not follow anymore a quadratic law in I, I₀ weakens to I^n<2 while, in contrast, I₁ increases to I^n>2. The time evolutions of the populations of these 7¹S₀, 7³S₁ states and of the 6¹P₁, 6³P_0,1,2 states were determined. The set of experimental results is well reproduced by introducing collisions of two Hg(6P) atoms as well as two Hg(7¹S₀) collisions. Effects of these collisions are discussed.

Excitation energies and oscillator strengths from the ¹S₀ ground state to the first ³P₁⁰ and ¹P₁⁰ excited states of Hg-like ions are calculated by using the multiconfiguration relativistic random-phase approximation including excitation channels from core electrons. The discrepancies between the theoretical and experimental results are much reduced for the oscillator strengths but more enhanced for the excitation energies when these core-polarization effects are taken into account.

In two-component Xe/HCl mixtures irradiated with alpha -particles, emissions centred at 308, 345 and 470 nm are observed; they correspond to the (B to X) and (C to A) transitions of XeCl* and to the transition (4² Gamma to 1² Gamma ) of Xe₂Cl*, respectively. The Xe₂Cl* emission was only seen when HCl was present as traces and Xe was at a pressure of over 200 Torr. The experiments showed the simultaneous presence of the emissions of the third continuum of xenon (290 and 390 mm) which compete with those of XeCl*. This implies that the Xe₂⁺* ions and their precursors, which participate in the reactions of XeCl (B, C) formation are also at the origin of the third continuum. By means of intensity measurements, the energy gap between states B and C of XeCl was estimated at 118+or-40 cm^-1.

The authors study multiphoton ionization of molecular iodine up to I₂⁶⁺. These highly charged unstable ions are inertially confined during the duration of the laser pulse (80 fs) and can be identified by their characteristic fragmentation energy. This enables them to measure the ion yields for a large number of molecular ions. They generalize an atomic tunnel ionization model by taking molecular Stark shifts and induced dipole moments into account. This model predicts the observed appearance intensities.

The quantitative investigation of the energy pooling collision (EPC) process between resonantly laser excited In atoms is reported. The process is studied in a laser induced fluorescence (LIF) experiment where the nP populations, with n=11, 10 and 9, are monitored. The population mechanism for these levels is verified via a temporal analysis of the fluorescence signals. This is possible in In due to the strongly reduced self trapping of the resonant radiation. The cross section for energy pooling collision to these nP levels has been derived by considering the Rydberg character of these levels.

The maximum entropy method is implemented in order to describe equilibrium distributions arising in beam-foil spectroscopy. Since there are very few charge states involved, the usual moment conditions, based on simple powers xⁱ, give rise to severe numerical difficulties already for three moments and cannot be applied in these systems. Earlier devised methods, based on Lagrange interpolation polynomials Q_i(x) with abscissae chosen as zeros of Chebyshev polynomials in the interval being studied, are adapted and implemented for the present problems of charge state distributions. A reduced variable x=(q-(q))/(q), where q is the charge and (q) the mean charge, is chosen. Using the method described above calculations of equilibrium charge state distributions for copper ions (exit energy range 0.559-2.304 MeV u^-1) colliding with carbon foils are carried out in order to exhibit the usability of the method. The new moment conditions associated to the Q_i(x) provide a framework for a systematic analysis of equilibrium distributions.

Charge-inversion spectra were recorded for C⁺(²P, ⁴P), Si⁺(²P, ⁴P), CH⁺(¹Sigma⁺, ³Pi), SiH⁺(¹Sigma⁺, ³Pi) and C₂⁺(²Pi) cations in order to establish whether any of the resulting undissociated anions were formed in electronically excited states. For ground state C⁺ and Si⁺ cations sequential single-electron-capture reactions with CS₂ were found to produce anions which were mainly in the ²D excited states, but capture by metastable C⁺ and Si⁺ cations gave, almost exclusively, ground state ⁴S anions. This is probably due mainly to preferential capture in the first stage, i.e. by ⁴P state cations into ⁵S states of the neutrals and by ²P state cations into singlet states as supported, in the case of C⁺, by evidence drawn from a target-gas variation study. Spectra recorded for CH⁺ and SiH⁺ showed that, in these cases also, ground-state (¹Sigma⁺) cations were converted in part into excited (singlet) state anions and to a greater extent than that found for the metastable-state cations. The conversion of C₂⁺(²Pi) cations into excited C₂^-(B ²Sigma_u⁺) as well as C₂^-(X ²Sigma_g⁺) and possible C₂^-(A ²Pi_u) anions was also observed but, in contrast with the case of C⁺, no large variation with target gas was found, presumably because of the lesser importance of first-stage state specificity.

The results of Ali and Fraser have been extended numerically to obtain still higher-order terms in the Born iteration expressions for elastic scattering phaseshifts arising from long-range inverse power forces. The expressions, in terms of the energy (to the sixth power) and the asymptotic strengths of the long-range forces (up to the coefficient of the inverse twelfth power), directly give accurate values of the phaseshifts for high enough orbital angular momentum and low enough energy. The present work is useful for extending in an easy way phaseshift results from orbital angular momentum/energy regimes for which longer and more detailed calculations must be carried out.

The authors have used a pulsed crossed beam technique incorporating time-of-flight spectroscopy of the collision products to study the electron impact ionization of ground state Fe atoms. Relative cross sections sigma _n for the formation of 1 to 4 times ionized iron have been measured within the energy range 8-1250 eV. Individual cross sections have been obtained by normalization to lower energy values of sigma ₂ previously measured by Freund et al. (1990) using a fast crossed beam experiment where analysis was complicated by the presence of metastable atoms in Fe beams. Measured cross sections exhibit evidence of contributions from inner shell electrons. The authors' high energy values of sigma ₁ are in excellent agreement with theoretical predictions based on the first Born approximation.

Radiation emitted from excited atomic states of the heavier rare gases is depolarized due to the influence of non-zero nuclear spin. In previous analyses of electron-photon angular correlation data for krypton and xenon this depolarization has not been considered. It is shown that parameters describing the nascent charge cloud excited in the collision can be obtained from existing relationships after the effect of hyperfine depolarization on the amplitudes of the measured angular correlations is taken into account. Corrected data for electron impact excitation of the 4p⁵(²P₁2/)5s'¹P₁ and 4p⁵(²P₃2/)5s³P₁ states of krypton at 60 eV are presented. The observation of significant spin effects for scattering angles >or=35 degrees which are inconsistent with theoretical predictions is discussed.

Excitation of quintet states of the hafnium atom has been investigated by a method of extended crossing beams by recording the optical signal from the intersection region. About 110 effective excitation cross sections of Hf I spectral lines within the region 248 to 800 nm have been measured; optical excitation functions changing the electron energy from the excitation threshold value to 200 eV have been recorded. The measured excitation cross sections range from 3*10^-19 to 9.6*10^-17 cm² (at an electron energy of 30 eV).

The R-matrix method has been used to calculate electron impact excitation cross sections for the lowest seven electronically excited states of CO in the energy range 6-18 eV. These states are represented using configuration interaction (CI) expansions and an algorithm for treating long CI expansions in scattering calculations is presented. The calculations are carried out for a range of internuclear distances 1.8<or=R<or=3.0 a₀. Several new resonance structures are identified. Good agreement is obtained with the experimental results of Furlong and Newell (1993) for the excitation of the a ³ Pi state.

Using a two-potential localized exchange approach the authors present results for the partial depolarization due to exchange in the inelastic collision of the intermediate energy polarized incident electrons with unpolarized sodium target atoms. Further, in the situation when the target fine structure is resolved the authors study the left-right scattering asymmetry S_A (1/2) for the superelastic scattering. Comparison of their results is made with the other available experimental and theoretical results.

The structure of the triply-differential cross section for fast electron or positron impact ionization of atomic hydrogen is studied as a function of the energy sharing between the projectile and electron detected in a plane containing the beam direction. Pronounced peaks or minima at certain angles are traced to the occurrence of double-binary collisions or to a final-state interaction between the projectile and ionized electron.

Results from a high resolution study of vibrational excitation of the 4 eV shape resonance of carbon dioxide are presented. The role of the Fermi resonance (i.e. the near-degeneracy of and interaction between the vibrational modes (1, 0, 0) and (0, 2, 0)) is discussed. Evidence for differing structure in the excitation functions of members of Fermi polyads of the types (n, 0, 0) and (n, 1, 0) is presented. This difference is structure is attributed to bending deformation. Local complex potential calculations are presented which support this finding.

The light absorption coefficient is evaluated in strongly coupled plasmas. Analysis is based on the calculation of the dynamic collision frequency of electron-ion scattering. Under plasma conditions when electron scattering has quantum character, the Born approximation for screened electron-ion interaction is used. In the case of classical scattering non Born corrections become essential at high frequencies. Interpolation formulas for the collision frequency and Gaunt factor are presented at intermediate frequencies. Theoretical calculations are compared with measurements of bremsstrahlung for dense Ar and Al plasmas and computer simulation data.