Table of contents

Total Auger rates have been calculated using the statistical model for the isoelectronic series of molecules: CH₄, NH₃, H₂O and HF based upon bound-state Hartree-Fock ab initio wavefunctions. Minimal, double zeta, and triple zeta basis sets were used. A projection operator of Beebe and Lunnell (1972) is introduced to convert the double zeta and triple zeta basis sets into an atomic orbital basis set. Application of the statistical model to determine the total Auger rates, based upon atomic orbital electron populations, is then possible with a higher quality basis set. Both core-hole and equivalent-core wavefunctions are used in the determination of the Auger rates. These results are compared to the statistical and explicit rates calculated using INDO equivalent-core wavefunctions. While the INDO approach provides a satisfactory first estimate when the statistical method is used, whenever possible ab initio core-hole studies are to be preferred.

We report here on new spectroscopic measurements of transitions in highly ionized gallium. Employing a Nd:YAG laser and a grazing incidence monochromator with toroidal grating, the soft X-ray spectrum of a laser-produced gallium plasma was observed in the wavelength region between 2 and 13.5 nm. Using in addition the results of Hartree-Fock calculations, the dominant transition arrays of the ionization stages Ga VII to Ga XIV have been classified. The main lines are ascribed to the 3p⁶3d^n-14f-3p⁶3dⁿ transitions (n=1-7); furthermore, 5f-3d, 6f-3d, 3d-3p and 4p-3d emission lines were identified.

A theoretical study of hydrogen production in collisions between protons and hydrogen-like beryllium ions Be³⁺ is reported, for a proton impact energy range between 100 keV and 3 MeV. Cross sections are obtained in close-coupled atomic-orbital calculations within the semiclassical impact parameter method. The presented results are relevant to design and interpretation of current experiments at JET where a substantial flux of neutral hydrogen is observed to be generated at the centre of the tokamak plasma.

The set of analytic formulae with overlapping areas of applicability, enabling approximate calculation of the dipole matrix elements between any quasi-classical states, is presented. In the Coulomb field the energies of the quasi-classical states border on the continuous spectrum limit on two sides and, consequently, these are the matrix elements of the bound-bound, bound-free and free-free transitions that are considered in the review. The analytic formulae are obtained both in the context of and without taking into account the Heisenberg correspondence principle. In the purely Coulomb field a comparison of approximate and exact results is performed. The non-Coulomb character of the atomic potential at small distances is taken into account by introducing the quantum defects and non-Coulomb phases of scattering. Approximate formulae for transitions between the states of the parabolic basis are considered. Quasi-classical results find application in quantitative calculations and qualitative estimations when investigating one-photon radiative transitions, multiphoton excitation and ionization of the Rydberg states, including the problems elated to the above-threshold ionization, stabilization and coherent atomic wavepackets.

It is shown that under atomic or molecular interaction with a light wave there appear P and T non-invariant effects of spin rotation and spin dichroism. Effects of P and T non-invariant rotation and birefringence appear depending on the polarization of the atomic or molecular ensemble.

We study the single-particle and the electron-pair densities for the ground state of two-electron atomic systems by means of wavefunctions which provide very good values for the energy. Special attention is paid to the convergence of some properties of the densities when we increase the dimension of the used basis. The best results obtained for the densities and for Kato's cusp conditions indicate that we have really got a quite good description for the ground-state wavefunction.

A simple configuration interaction method to calculate accurate electronic energies of two-electron molecules at intermediate internuclear distances is presented. The correlated basis functions are constructed from H₂⁺ molecular orbitals (exact solution of the two-centre Coulombic problem), and include an explicit dependence on the interelectronic distance in order to describe the major part of the electronic correlation. The first Green transformation is used to give an important simplification of the expression for the matrix elements of the Hamiltonian. The method is shown to give accurate energies for the H₂ molecule in the ground ¹ Sigma _g state at intermediate distances, and is applied to determine new potential curves of highly excited states. The simplicity of the wavefunction will allow it to be used relatively easily in subsequent calculations of dynamical properties.

The relative motion of two ground state hydrogen atoms is considered in the framework of the resonating group method and using the Heitler-London electronic basis. It is shown that, with correct simultaneous account for boundary conditions and indistinguishability of electrons, the effective reduced mass involved in the relative motion of atoms may be written as 1/2 (M+1+ delta M) where M is the proton mass in atomic units. The correction delta M, which is obtained as a function of the internuclear distance R, is different in sign and amplitude for the singlet and triplet states, it is of the order of unity at R=3 au and goes exponentially to zero as R increases.

In the presence of weak external fields, we have investigated doubly-excited Ba atoms with one valence electron in a Rydberg circular state (n₁=21, l₁=n₁-1), oriented with respect to the fields ( mu ₁=l₁), and the other electron in one of the low-lying states n₂l₂j₂=6p_1/2,3/2, 6d_3/2,5/2 or 7s_1/2. A theoretical model, including the e-e Coulomb interaction, predicts weak correlation effects leading to an angular-momentum mixing of the circular state when the inner electron is excited to anisotropic core states j₂> 1/2 . Taking advantage of the field-ionization detection technique sensitive to the structure of the outer orbital, we have found evidence of such mixing. The implications and the limits of the model have been checked.

Ab initio MRD-CI calculations have been carried out on electronic states of ArH at intermediate and large internuclear distances, where ion-pair, Ar⁺H^-, states as well as excited Ar(³P, ¹P) states interact with the states involving excited H limits. The results show that charge transfer character contributes to the lowest three ² Sigma ⁺ states and two ² Pi states for internuclear distances from 4.0 to 10.0 a₀. The ion-pair states are found to have minima at 4.5 a₀ and at 6.5 a₀ where they correspond to the lowest ² Pi and the first excited ² Sigma ⁺ states of ArH. In addition to the above, potential energy curves have been produced for a large number of Rydberg states of ArH, correlating with dissociation limits up to H n=4.

We calculate photoabsorption spectra of atoms in crossed electric and magnetic fields using a truncated basis of Coulomb eigenfunctions. The method yields spectra in the regime where inter-n-mixing is not dominant and allows for the treatment of non-hydrogenic atoms via a simple recourse to quantum defects. We compare results for hydrogen to those obtained in second order perturbation theory where the residual degeneracy left in first order perturbation theory is completely lifted and we show that only a very small basis size is needed to achieve convergence to within the accuracy of second order perturbation theory. In the case of lithium the coupling of an incomplete hydrogen-like manifold to states with non-negligible quantum defects substantially modifies the spectra obtained in comparison to the purely hydrogenic spectra. In the inter-n-mixing regime we also compare our convoluted results directly with an experimental spectrum for hydrogen and find good agreement below the saddle point.

Close-coupling methods for electron impact broadening have been extended to the Stark broadening of neutral helium lines. The comparison with semiclassical impact theories reveals significant differences, particularly at low electron temperatures. Ion broadening is included by the use of the model microfield method. For the 2³P^o-4³D^e and 2¹P^o-4¹D^e transitions, comparison has been made with experimental measurements.

The mesh method of solving the Hartree-Fock equations for atoms in magnetic fields of arbitrary strength is presented. The method is applied for the ground and a series of excited states of the He atom. The energy levels of the ion He⁺ in the states with n=1 and 2 are also calculated for the same strengths of the magnetic field as for the He atom. The use of the mesh method results in reaching the Hartree-Fock energy limit for fields from weak to strong, including the intermediate field region.

Measurements of the resonance between the Ly- alpha line of sodium and the 2s to 4p transition in neon-like cobalt are presented, which show a good resonance between cobalt and sodium. The cobalt line is measured at 10.0222 AA+or-1.1 mAA, just 1 mAA below the short wavelength component of the Ly- alpha sodium line. Results from model calculations are presented that show that this resonance can be used in experiments to study enhanced fluorescence due to photo-pumping and as part of a resonantly photo-pumped X-ray laser scheme which would lase on several 2p to 2s transitions near 83 AA and 92 AA in neon-like cobalt.

Many-body perturbation theory is applied to the calculation of photoionization cross sections with excitations (2p, 3s) to (4s, kl) and (3p, kl) of sodium. The double-electron excitations (2p, 3s) to (4s, nl) and (3p, nl) are included and lead to the resonance structure between 38.5 and 46.5 eV. The cross section for the transition (2p to kd+ks) is obtained taking into account the random-phase approximation (RPA) and an important part of the Brueckner-orbital (BO) and structure-radiation (SR) corrections. A comparison of the contributions from RPA, BO and SR for the transition (2p to kl) is presented. The contributions of substantial correlations shake-off, virtual-Auger, knock-out and ground-state correlation (GSC) have been calculated and discussed for the transitions (2p, 3s) to (4s, kd) and (3p, kp), respectively. The ratios of the contributions from the cross section of photoionization with excitations to the cross section of the transition (2p to kl) are in good agreement with experiment.

The theory of the photodetachment of negative ions in an external static electric field developed earlier is applied to photodetachment of the sulphur and chlorine negative ions. The photodetachment cross section in the presence of the electric field is modified by a modulation factor which is calculated including the rescattering effect, i.e. the scattering of the electron wave reflected by the electric field's potential barrier. The modulation factor is expressed in terms of the scattering lengths for electron-neutral-atom scattering. The scattering lengths are calculated by the method of extrapolation of potential parameters along isoelectronic series of positive ions and corresponding neutral atoms. The low-energy electron scattering cross sections for the S and Cl atoms are obtained, and the Ramsauer effect for these atoms is studied. The rescattering effect in the photodetachment cross sections is small, and cannot explain the observed reduction of the oscillations in the modulation factor.

In order to determine the effect of molecular rotation on gas kinetic collisions, light-induced drift measurements were performed on CH₃F in the nu ₄ rovibrational band. Data were taken for a large set of transitions of the type v=0 to 1, Delta J=1, Delta K=1 over a range of values of J or K. Systematic trends in the J- and K-dependence of the kinetic collision rate have been observed. For Kr and CH₃Cl as collision partners the K-dependence is found to be large and nearly linear in K, whereas the J-dependence is found to be small. We present a theoretical model to describe the dependence on the rotational quantum numbers of the relative change in collision rate upon excitation; this model is in satisfying agreement with the experimental results.

An approximate, analytic expression is derived for the distribution of saddle-point electrons emitted in ionizing one-electron atomic collision systems. The electron distributions of two other relevant ionization mechanisms, the direct S and the radial decoupling mechanisms, are also taken into account. The electron velocity distributions of 1-6 keV amu^-1 collisions of H⁺ and He²⁺ with atomic hydrogen are presented.

An experimental study is presented, using coincident energy gain spectroscopy, of transfer excitation processes in the low keV energy range, for lithium-like F⁶⁺ and Ne⁷⁺ ions colliding with He and Ne atoms. A theoretical model is developed in a quasimolecular framework, implying the same basic interactions as invoked in non-resonant transfer excitation (NTE), a mechanism well established at large collision velocity. This model describes transfer excitation as a two-step process essentially governed by the electron-nucleus interactions.

The cross sections for the excitation energy transfer in the collisional processes Na*(3P₁2/)+M(nS₁2/) to or from Na*(3p₃2/)+m(Ns₃2/), m being ground-state Rb (n=5) or Cs (n=6) atom, have been measured by laser fluorescence method. With either of the two sodium 3P fine-structure states optically excited, the intensity ratio of the sensitized to direct fluorescence was measured. The polarization effects due to partially polarized and anisotropic direct fluorescence were investigated. The measured cross sections (T=530 K) for the sodium 3P fine-structure excitation transfer induced by collisions with Rb are: sigma (¹/₂ to ³/₂)=170 AA², with an overall accuracy of 20%. The corresponding values in the Cs case are: sigma (¹/₂ to ³/₂)=140 AA², sigma (³/₂ to ¹/₂)=83 AA². The theoretical calculations reveal the decisive role of the dipole-quadrupole interaction. The obtained values are in satisfactory agreement with the experiment only if both the dipole-dipole and dipole-quadrupole interactions are taken into account, while the calculations involving the dipole-dipole term alone yield the cross section values which are as much as one order of magnitude lower than the experimentally observed ones.

Angular and energy distributions of ejected electrons in the direct and resonance He(e, 2e)He⁺ reactions are studied. It is shown that the first Born approximation (FBA) can be used to described ejected electron characteristics at incident energies E₀>or=500 eV and ejection energies E_b<or=40 eV. It is also shown that the anisotropy of ejected electrons and the predominance of higher-order multipole terms are due to the peculiarities of the angular distributions of double-differential cross sections under different kinematical conditions. Under the conditions of FBA validity the profiles of the overlapping ¹P and ¹D autoionization resonances (AIR) in the ejection spectra of the He(e, 2e)He⁺ reaction are investigated by using the resonance structure of the angular coefficients. The resonance structure of the angular coefficients might provide more detailed information on the profiles and interaction mechanism of the ¹P and ¹D AIR at different multipoles L. Calculations carried out in the concrete structure model are compared with the available experimental data and theoretical estimates.

We report an investigation of collisional processes in Ca vapour in the presence of noble gases under ¹S₀ to ¹P₁ laser excitation. We observe the process of energy transfer to the triplet ³P_J(J=1, 3) states and the process of energy pooling to many higher energy states. The combination of these processes makes it possible, by changing pressure and kind of noble gas, to 'drive' the population distribution of the upper states.

The three-body eikonal approach to ion-atom collisions based on the Alt-Grassberger-Sandhas equations is developed. The amplitudes for scattering and charge exchange, written in the impact parameter representation and using the eikonal approximation, are represented as a sum of two terms. One of them describes the scattering via the Coulomb potential acting between the colliding heavy particles. The second one could be factored into a product of two terms. One is an explicitly known factor which contains all the information about the interaction between the heavy particles. The remaining part does not contain any reference to this interaction any longer. For these residual amplitudes effective-two body equations are written down. Explicit expressions for the effective potentials occurring therein, for both the scattering and the transfer channels, are derived in the lowest order approximation. This approach is then used to calculate total and partial cross sections for the electron capture processes in collisions of the ions H⁺, He²⁺ and Li³⁺ with atomic hydrogen. The latter is taken to be in its ground state, except for the case of incident H⁺ when the target hydrogen atom is considered also in some low-lying excited states.

The orientation of the (1s²2p)²P excited state of the doubly charged B²⁺ ions, resulting from electron capture in collisions of triply charged B³⁺ ions with He, is investigated theoretically. The collision dynamics are described using a quasi-molecular adiabatic representation of the system and calculations are carried out in the range of ion energies from 0.3 to 1.5 keV. Our results, which are in excellent agreement with experiment, confirm the strong propensity for orientation of the ²P state at small scattering angles. The main contributing factors responsible for the orientation are carefully analysed.

The alignment and orientation effects for charge exchange in 1-1.5 keV Li⁺-Na(3s, 3p) collisions are studied by semiclassical close-coupling calculations employing adiabatic molecular data and a common electron translation factor. The differential cross sections from Na(3s) to Li(2s) and Li(2p) are compared to experiment, a large orientation effect is predicted for charge exchange to Li(2p_+or-1). From Na(3p) the charge exchange is important only to the Li(2p) channel, the corresponding alignment and orientation effects are calculated. The orientation effect follows the propensity rules. The formulae for total and differential cross sections for a transition from aligned p to p state or from oriented p to p state are derived. Two differential total cross sections for oriented p_+or-1 to p_+or-1 states can be defined, the same for aligned p+or-45 degrees to p+or-45 degrees states. The important role of the translation factor is discussed and a four state model is able to describe charge exchange from Na(3p) to Li(2p) at large impact parameters.

The time evolving electronic density and current of the active outer electron in 1-2 keV Li⁺-Na(3p) collisions have been calculated and graphed. The corresponding time-dependent electronic wavefunction is provided by the 28 coupled molecular state calculations of the preceding paper with a common translation factor. The mechanism of the alignment effect for charge exchange from initially aligned Na(3p0 degrees /90 degrees ) or Na(3p45 degrees /-45 degrees ) is clearly seen in the graphs of the current.

Single differential and total cross sections of the electron-impact ionization of the hydrogen atom are calculated numerically in the two-potential distorted-wave approximation for excess energies from 0 to 1 eV. Partial-wave contributions to the cross sections are also investigated. The near-threshold cross section is parametrized by the power gamma and the proportionality constant a₀ for models with various asymptotic charges, and the dependence of a₀ on the asymptotic charge is studied. The validity range of the threshold power law is also discussed.

State-to-state rotational transitions of planar structures forming regular polygons are studied within a simple spectator model, which for electron-diatomic molecule scattering has been found to be in excellent agreement with experiments. Selection rules in the state-to-state transition probabilities for the excitation from the ground state are discussed. Transition probabilities for the excitation of initially rotating planar highly symmetric targets are derived using a new factorization relation which relates the transition probabilities for initially excited states to those from the rotational ground state.

Electronic-state analyses of 6 keV beams of the title ions have been carried out through translational-energy spectroscopy of their charge-inversion products formed in Xe or CS₂ target gas. All beams were found to contain CN⁺ ions in the a ³ Pi state, of term energy 0.12+or-0.20 eV, while beams formed using electrons of energy greater than about 23 eV also contained CN⁺ ions in a triplet state (³ Sigma ⁺) of 1.32+or-0.10 eV term energy. The proportions of ¹ Sigma ⁺, ³ Pi and ³ Sigma ⁺ ions in the beam formed from 100 eV electroionized HCN, were estimated to be 0.80, 0.11 and 0.09, and the ³ Pi :¹ Sigma ⁺ population ratio was found to increase with decreasing electron energy, to about 30:70 at the CN⁺ appearance threshold of 20(+or-1) eV. In the beam formed from C₂N₂ (or NH₂CN) the ³ Pi and ³ Sigma ⁺ proportions were 0.35 and 0.16 (or 0.35 and 0.11) at 100 eV, and no significant variation of the ³ Pi :¹ Sigma ⁺ population ratio with decreasing electron energy was detected. Under certain circumstances, electron capture by CN⁺(³ Pi ) ions was found to be less efficient than capture by CN⁺(¹ Sigma ⁺) ions and thus beam composition may be an important factor in reactivity studies involving this cation.

The process of bremsstrahlung, formed in the fast e^+or-+H collisions with a simultaneous excitation or ionization of the target, is considered in the low-frequency limit. It is demonstrated that in the case of the positron scattering the intensity of the 'inelastic' bremsstrahlung greatly exceeds the intensity of the radiation formed during the collision without the change of the target's state. The role of 'inelastic' channels in the formation of the total bremsstrahlung spectrum of the electron is relatively small.

We present a theory of harmonic generation by a strongly driven atom within the framework of non-relativistic QED. The atom is assumed to possess a finite number N of levels and ionization is neglected. This atom is dressed by the driving field using a unitary time-dependent transformation. Within the quasistatic approximation this transformation is shown to lead from the bare to the Floquet atom. The interaction of the Floquet atom with the vacuum modes is derived and it is shown to give rise both to the high-order harmonic and to the hyper-Raman spectrum. In the case N=2 the spectrum of the light scattered into the vacuum modes can be evaluated analytically and it is shown to consist of two parts. Neglecting interference between these two parts, the first of them is the high-order harmonic spectrum displaying peaks at the odd harmonics of the driving field. The second part is the hyper-Raman spectrum which has finite bandwidth and which displays peaks at the even harmonics of the laser frequency, centred at the Stark-shifted natural atomic frequency. These two spectra partially overlap and they are discussed in various ranges of intensity of the driving field, both in the minimal coupling and in the multipolar scheme. The results support recent suggestions that the presence of the plateau is related to interference between the two spectra. The dependence of the high-order harmonic spectrum from the driving field intensity is found to be related to that of the hyper-Raman spectrum in such a way that when the former increases the latter decreases and vice versa. It is argued that this might help to explain the non-visibility of the hyper-Raman lines when the high-order harmonics are visible.