Table of contents

A crossed-beams apparatus has been used to measure, for the first time, the angular distributions of electrons photodetached from B⁻ ions. The angular dependent yields of the ejected photoelectrons were determined using energy-and angle-resolved electron spectroscopy. The asymmetry parameter was found to be essentially constant at β ≃ 0.18 over the spectral range 1.78–2.09 eV

For the first time, charge-changing reactions in collisions of two negative ions were investigated. Absolute cross sections for mutual ionization were determined for H⁻ colliding with H⁻, using crossed beams and coincident detection of the reaction products. The centre-of-mass energy range covered in the experiment extended from 1.5 keV to 90 keV for the reaction channel H⁻ + H⁻ to H⁰ + H⁰ + 2e⁻ and from 4 keV to 40 keV for the channel H⁻ + H⁻ to H⁰ + H⁺ + 3e⁻. The measured cross sections are compared with results of CTMC calculations obtained with different model potentials for the interaction between the outer electron and the H⁰ core

Total cross sections for the production of He⁺(2p) in collisions of H⁺ and e^- with He at energies from 75 to 800 keV and from 0.8 to 7 keV respectively were measured. The ratio of the cross sections for He⁺(2p) production and for single ionization is discussed to investigate the contribution of electron correlations in the two-electron transition leading to He⁺(2p). For the first time the impact of H₂⁺ and H₃⁺ ions in comparison with H⁺ impact has also been studied for the energy range mentioned above.

The global effects of configuration mixing on the total intensity of a transition array have been interpreted. The changes of the subarray intensities by the mixing of pure jj subconfigurations (breakdown of jj coupling) in heavy atomic ions can be calculated in the same way. The perturbed relative intensities are deduced from the eigenvectors of a 2 × 2 energy matrix. This matrix is not changed if the subarrays contain spectator electrons. The calculated values give the interpretation of some conspicuous features appearing, for instance, in highly ionized Ta, Pt and Hg x-ray spectra.

The linear, approximate and full coupled-pair (CP) methods are applied to calculate the ground state energy of the C²⁺ion using one- and two-dimensional model spaces. The one-dimensional model space consists of the 1s² 2s²configuration whereas the two-dimensional one consists of the 1s² 2s²and 1s² 2p²configurations. The procedure is based on the numerical solution of coupled radial pair equations using a finite difference method. By iterating the system of pair equations, the most important correlation effects are included to all orders. The pair equations of the full CP method are solved for the first time. The excitation energies for the 2p²levels are calculated. The accuracy of the excitation energy values changes from 2.5% to 0.3%. Explicit formulae for the contributions from nonlinear terms to the right-hand side of the equations are given. The results of the work are compared with those of earlier many-body calculations on this system.

Oscillator strengths are presented together with excitation energies for the np² -np(n+1)s transitions in Ge I, As II, Sn 1, Sb II, Pb I and Bi II spectra computed using the relativistic configuration interaction (CI) approach. A slight modification in the standard CI method is proposed and investigated. The effect of the CI on calculated oscillator strengths computed in the Babushkin and Coulomb gauges is discussed. The results obtained are compared with other theoretical calculations as well as with available experimental data.

The optical isotope shifts in the transition 4f⁴ 5d ⁶ K_9/2 -(23 537)_9/2⁰ (5702 Å) of Nd II by collinear fast-ion-beam laser spectroscopy have been used to deduce for the first time that the mixing probabilities of the configurations 4f³ 5d² , 4f⁴ 6p and 4f³ 5d6s of the level (23 537)_9/2⁰are 0.84, 0.15 and 0.01 respectively.

Some new energy levels belonging to the 4f¹⁴ 6s np ³ P_0.2series of neutral ytterbium have been measured by a three-step laser excitation technique. The spectra of the 6snp ³ P₂series were analysed with MQDT. It is found that 4f¹³ 5d² 6s ³ P₂and ¹ D₂are two perturbers of the 6snp ³ P₂series and the admixture coefficients of these perturbers in the bound 6s np ³ P₂series were calculated.

A method is presented for an asymptotic expansion of the hydrogenic radial dipole integral for discrete-discrete transitions from initial state n'l' to final state nl. The advantage of this simple method is that it allows all the necessary algebraic manipulations to be done by the computer programming package REDUCE-2. Expansion coefficients up to the coefficient of the 1/n¹⁷term are obtained for all initial states with n'<or=10. However, only the coefficients which are not calculated by Klarsfeld (1978) are tabulated in the paper.

An exact quantum adiabatic evaluation of the shift of an isolated spectral line is applied to the potassium 7s-4p transition perturbed by argon. The long-range interaction of excited alkali states with noble gases is computed with a large basis set of unperturbed atomic states, a multipole interaction and a pseudopotential to compare with a measurement of the dependence of the shift on temperature. The parameters of the theoretical potassium-argon excited-state potential are determined by adjusting the theoretical temperature dependence to fit the observations. Previous pseudopotential models, fitted to ground-state interactions derived from scattering experiments, show weaker long-range interactions than the authors find. The conclusions are consistent with independent line wing experiments on this and other alkali-noble-gas systems that reveal significant interactions at long range.

The authors present in graphical form (Z-diagrams) an extension, to nuclear charges Z(2, of previous study on doubly excited ¹ S^estates of helium-like systems. The structure of the diagrams and closed-channel wavefunctions are discussed from the viewpoint of the quantum defect expression, which is coherent with an independent electron picture. The analysis also contributes to a better understanding of the H^-resonances. To apply to the complete Z- correlation diagram, the (N, n alpha ) and (K, T) notation may be taken as equivalent, except in pseudocrossing regions where both labelling schemes break down. Energy ordering together with Z-diagrams provide the only general classification of doubly excited states.

The authors present a detailed comparison between results of a R-matrix calculation and laser-excited 6pn s (J=1, odd parity) autoionizing Rydberg states in Ba. After combination with standard MQDT methods, the R-matrix results are compared with new and previous experimental results on resonance profiles, electron branching ratios and angular distributions of 6pns (J=1) autoionizing Rydberg states. The overall agreement between theory and experiment is very satisfactory in all the observables and is in line with similar investigations for Ca and Sr.

Pseudorelativistic Hartree-Fock radial wavefunctions have been used to provide ab initio estimates of discrete-continuum electrostatic interaction widths for all levels of the 4d⁹ 5s² 5p configuration of Cd decaying into the 4d¹⁰ 5s in p and 4d¹⁰ 5 in f continua.

A sensitive electron spectrometer has been used to study photoionization processes in the region of the double-ionization threshold of helium. Analysis of the cusp observed at the He²⁺potential in a threshold photoelectron spectrum indicates that this structure would support the predictions of Wannier theory with a range of validity of 2.0+or-0.5 eV. Asymmetry parameters ( beta ) have been obtained for near-threshold excitation of He⁺ *(n) satellite states up to and above n=10 at photoelectron energies of 0.14 and 0.25 eV. For n)6, beta was found to have a roughly constant value of about -0.5 and did not approach the predicted value of -1. Similarly, for the double-ionization process, beta is approximately constant at -0.4 in the energy range from 0.25 to 2.0 eV above the He²⁺potential and does not display the steep rise from beta =-1 to positive values which is predicted to occur in this energy range.

Angular distributions of photoelectrons ejected through 2+1 REMPI via nf (3/2)₂^eand nf (5/2)₂^e(n=4, 5 and 6), J_c =1/2 and 3/2 states of Xe have been measured. Ionization ratios between the two channels leading to the two fine-structure levels of the ground state of the Xe⁺ion have been extracted. The experimental results have been compared with the results of the QDT calculations of Tang and Lambropoulos (Ref. unobtainable).

An analytic solution of large amplitude bending vibration for linear triatomic molecules is given. The matrix elements of the total vibrational Hamiltonian are then evaluated in the basis of these bending eigenfunctions and the Morse oscillator eigenfunctions. The vibrational energy expression, similar to the usual one, can be obtained by perturbation treatment and used to fit the experimental data.

The authors report on measurements of the differential change in kinetic cross section, Delta sigma / sigma , upon excitation of Rb immersed in various atomic and molecular buffer gases. For the first time negative values for Delta sigma / sigma are found, Large positive values are found for saturated hydrocarbons as a buffer gas.

A general theory of spontaneous electromagnetic emission in symmetrical ion-atom collisions has been developed using a semiclassical adiabatic model of collision. The spontaneous emission probability has been obtained numerically for the p+H case, in an intermediate range of velocities. It has been shown that an earlier suggested quasistatic model of the process works well in a wide range of slow collisions. However, in a range of intermediate collision energies approximately 1 keV the quasistatic cross sections for spontaneous emission may be in error by a factor of 3. The significance of the reported results for plasma applications is emphasized.

A crossed-beam technique has been used to determine cross sections for Balmer alpha emission from hydrogen atoms excited by H⁺ , He⁺and He²⁺impact within the energy range 2.5-100 keV amu^-1 . Results have been compared where possible with Balmer alpha cross sections derived from theoretical predictions of cross sections for excitation of the n=3 sublevels of H.

The study of electron spectra in H^- -rare-gas collisions (0.5-6 keV) at low observation angles ((5') reveals structure due to decay of the H^- * (2s2p ¹ P⁰ ) shape resonance to H (n=2). Projectile kinematics allow the transformed cross section in the H^-frame to be obtained with a resolution that can be as high as 1 meV which then yields the resonance parameters. At very low angles ((1') a cusp is observed due to zero-energy electrons in the projectile frame, implying a nonzero cross section at threshold. This behaviour originates from a long range potential interaction which can be the dipole interaction between the excited hydrogen atom and the detached electron. Furthermore the authors have observed a target effect on the angular distribution of these near-zero energy electrons which could be related to the target polarizability. Very weak structure in the electron spectra corresponding to energies below 4 meV could be the first observation of the predicted dipole oscillations.

Symmetric resonant charge transfer in proton-hydrogen collisions is examined theoretically using phase-integral halfway-house variational continuum distorted wave theory. The formulation involves the factorization of the scattering matrix into two Moller matrices, one describing the motion for - infinity <or=t<or=O_-and the other the motion for + infinity >or=t>or=O₊ . The authors use outgoing continuum distorted waves for t(O and incoming continuum distorted waves for t)O with a matching at t=O, the associated heavy particle motion remaining incoming for t(O and outgoing for t)O. Results for total cross sections are presented and compared to previous theoretical models and experimental results. Differential cross sections in the centre-of-mass range 0-3.2 mrad are presented at 25, 60 and 125 keV and are compared with the same theoretical models and previous experimental results.

The paper reports on measurements of the cross section of transfer ionization in He²⁺ -H^-collisions over the barycentric energy range 0.16-1300 eV. The experimental method involves merged beams and coincident detection of the reaction products. The dependence of the cross section on the energy is rather similar to that of the one-electron exchange, suggesting that a two-step mechanism dominates the process.

The authors report on experimental data for electron capture by H(3l) hydrogen atoms, in collisions with neon or argon targets. The energy range is 0.6-3 keV. The fine structure of the colliding atoms is experimentally resolved and its impact on the electron capture process established.

The authors describe a method for producing a fast atomic hydrogen beam with a large amount of population selected in 3l_{j, mod mj mod} levels. This is realized by a one-step tunable laser excitation of the metastable component of the atomic beam inside a nonhomogeneous static electric field. The linear laser polarization is taken either parallel or perpendicular with respect to the external field. It allows the selective excitation of all the states of the n=3 Stark manifold (with the exception of the mod mj mod =5/2 state). The electric field strength vanishes immediately after the laser excitation. At this point the population of the excited states are better referred to the atomic basis. The shapes of the resonant lines show that two transient effects are leading the excitation process. The first is of an optical kind (i.e. Rabi oscillation and rapid adiabatic passage) and the second is the intra Stark manifold state mixing due to the nonadiabatic time evolution of the external field strength. The internal state of the atoms crossing these fields is completely described by the density matrix evolution calculated independently on two bases (the atomic basis and the Stark instantaneous basis).

One- and two-electron collision processes between fully stripped ions (Z=2-9) and helium atoms have been investigated in the intermediate energy range within the independent electron approximation. The transition probabilities for single excitation, single ionization and electron capture for an effective one-electron helium atom colliding with bare ions calculated in the coupled channel semiclassical impact parameter model employing a travelling atomic orbital expansion were combined to obtain cross sections for two-electron processes, such as transfer ionization, double capture and double ionization. It is found that all one-electron processes are well described by an independent electron approximation where the effective potential of the target atom gives the correct single ionization energy, while accurate cross sections for all two-electron transitions can be obtained only if one adopts a different effective potential where the single ionization energy of each electron is half the double ionization energy of helium.

The spin polarization of electrons elastically scattered by Xe is found in a Dirac-Fock calculation for an impact energy range of 40 to 100 eV. All electrons are treated as indistinguishable and experience the same interactions, namely those arising from the relativistic frozen-core and model polarization potentials of the target. Exchange, including polarization exchange, is calculated exactly for the large and small components of the Dirac wavefunctions. On the whole, the results are in good agreement with experiments and with theoretical calculations using a different treatment of the core polarization.

A quasirelativistic approach is applied to the calculation of the atomic wavefunction and the low-energy e-Xe scattering process. For the scattering calculation the exchange and static interactions are treated exactly; the correlation, distortion and polarization effects are accounted for approximately by a parameter-free correlation-polarization potential. The relativistic, correlation, distortion and polarization effects are considered effectively and very good agreement between theory and experiment is obtained for the elastic total, differential and momentum transfer cross sections in the energy range 0.1-20.0 eV.

A correlated three-body continuum wavefunction, already successfully employed to describe hydrogen atom impact ionization, is used to calculate the triply-differential cross section for electron impact ionization of helium. A good description is obtained of all the major structure in the differential cross sections in both symmetric and asymmetric geometries. It is demonstrated how interference between the various projectile-target interactions is necessary to reproduce the experimentally observed structure.

Collision strengths have been calculated for electron impact excitation of neon-like Ar⁸⁺from its ground state. The first 15 eigenstates of the Ar⁸⁺target have been included in an R-matrix expansion of the total wavefunction. Configuration-interaction wavefunctions have been used to represent the target states. Effective collision strengths for the 14 excitation transitions from the ¹ S^eground state are tabulated for electron temperature in the range (5*10⁴ -10⁶ ) K.

The detailed competition mechanism between two Stokes waves in stimulated multipole Raman scattering in the presence of a coupled four wave parameteric process has been studied in K. The amplification of the coupled nonlinear processes is discussed. Based on the experimental data an expression is given to account for the interdependence.

The method of induced phase modulation has been extended to the UV. Pump and signal waves are involved, the sum of their frequencies lying near a two-photon resonance in gas media. The variational method for solving the Schrodinger equation for two waves has been applied.