Table of contents

Optical oscillator strengths have been calculated for the electron impact excitation of the X ¹ Sigma ⁺_g state of the N₂ molecule to the b' ¹ Sigma ⁺_u, c' ¹ Sigma ⁺_u, b ¹ Pi _u, c ¹ Pi _u and o ¹ Pi _u electronic excited states using highly correlated configuration interaction wavefunctions, including up to quadruple excitations from the respective reference configurations. The results are compared with other theoretical calculations and with available experimental values.

The authors present an investigation of the modification caused by an intense laser field to the rates of m_J changing collisions for Hg perturbed by argon. They have studied experimentally the competition between radiative and collisional coupling between m_J states with a strong laser field tuned to the Hg J=1 to J=1 transition at lambda 435.8 nm. Population is pumped into the strongly dressed manifold from the ground state with a weak UV 'probe' laser detuned slightly from the lambda 253.7 nm intercombination line. The fluorescence observed at lambda 404.7 nm depends critically on whether the excitation proceeds via collisional redistribution or resonance through Stark shifted levels. The authors present data that clearly exhibit the effects of this competition. The data show in particular how one can produce excitation to transient Stark shifted levels in the presence of collisions. This will be critical in future experiments on collisions in strong fields.

The authors present cross sections for double ionization of helium by alpha-particle impact within the independent-event model. Within this model the probability of ionizing the first electron (P₁) from neutral helium is calculated in the continuum distorted-wave (CDW) approximation using an explicitly correlated wavefunction of Pluvinage (1950), whereas the probability of ionizing the second electron (P₂) from the He⁺ ion is calculated in the CDW approximation with an eikonal initial state (CDW-EIS). The total probability for double ionization is then P₁P₂ rather than the P² which is usually assumed in the independent-electron model. The calculated cross sections in the present model show excellent agreement with the measurement of Shah and Gilbody (1985), at all energies above 200 keV amu^-1.

A recently developed method for dealing with the second Born approximation to the laser-assisted potential scattering is applied to the case of superelastic scattering (where no photons are exchanged). The explicit expression of the transition amplitude is analysed in the case of long-range scattering potentials in order to obtain information about the importance of second-order corrections. The analytic properties of the T-matrix in the complex-momentum plane are also studied. It is shown that the field considerably weakens the steep enhancement occurring in the field-free phase of the transition amplitude for long ranges although it does not remove the Coulomb divergence: the field also introduces a new logarithmic singularity, and yields a real part to the existing one, implying an imaginary part to the complex energy. The overall results show the limitations of widely used approximations, and provide evidence of inherent interplay between the radiation field and the static potential.

The exact wavefunction of stimulated one-photon direct and inverse bremsstrahlung in the Coulomb potential is obtained, which describes the coherent states of the electron at asymptotically large distances. It is shown that as a result of a superposition of such states, quantum modulation of the current density of the beam takes place at the frequency of the stimulating wave. With the help of the exact wavefunction the one-photon absorption coefficient of an electron beam and of an isotropic Maxwellian plasma are calculated treating electron-ion interaction exactly. Formulae for summing of series containing hypergeometric functions and Legendre polynomials are obtained.

The accuracy of phaseshifts calculated by means of the Pais variational procedure is investigated and the method is employed to calculate partial derivatives of the phase shifts with respect to a number of potential parameters. One of these, the partial derivative of the phase shift delta _L with respect to the orbital angular momentum L, plays an essential role in the semiclassical approach to scattering. Another is the partial derivative delta delta _L(k)/ delta E, where E is the energy, which is related to physical quantities like the time delay, virial coefficients and resonance widths. The relative simplicity of the numerical calculations involved in the Pais approach and the fact that it gives accurate results for the range of values of L that are important in semiclassical calculations makes the use of Pais phaseshifts an attractive possibility in such calculations. The formalism that results has possible application to both electron scattering from atoms and molecules and to heavy ion scattering in nuclear physics.

A number of works have reported on the calculation of the parity non-conservation E1 matrix for Cs and Tl atoms. However little has been done on other more complex atoms. The authors give a MQDT wavefunction for an alkaline-earth atom at the origin, and upon this base a semi-empirical calculation of Im E1_pnc of atomic barium. The results show that the Im E1_pnc of the 0 to 32480 cm^-1 transition is 8.9 *10^-11 ea₀, which implies that the barium atom might be a possible candidate for PNC investigation in atomic spectroscopy.

Autoionizing and Auger transitions in atomic manganese and samarium have been experimentally investigated by observation of the ejected electrons in the energy region 0 to 40 eV following electron impact excitation with incident beams in the energy range is 15-500 eV. Seventy-four spectral features are tabulated for manganese and a number of new assignments have been made based on pseudo-relativistic Hartree-Fock calculations and quantum defect analysis. A similar study of samarium reveals only a number of broad features in the ejected-electron energy range 8-10 eV. Three features have been observed consistently in the ejected-electron spectrum of samarium and assigned by comparison with previous work.

The authors have implemented a recently proposed L²-integrable basis method to study photoionization of He-like systems below the n=2 threshold in the framework of a Fano-Feshbach theory. Non-resonant background cross sections and Fano parameters for the (2, n)¹P degrees doubly excited states of He and Li⁺ up to N=5 are calculated. For n>or=3 an anomalous behaviour of the profile parameter q is found in He, which is the result of the existence of interfering, but nevertheless non-overlapping, resonances in the (2, nb) and (2, nc) series. The behaviour of photoionization parameters as a function of the nuclear charge is also discussed.

The S-matrix version of the Kohn variational procedure is examined. It is shown that the method is a particular case of other Kohn variational procedures which have the limitation that they give results that are dependent on the asymptotic form chosen for the wavefunction. In particular it is shown that when using S-matrix boundary conditions the calculated S-matrix need not be unitary. However, if the trial functions are adapted according to the prescription previously given by Rudge (1973-80), then the calculated S-matrix is unitary and coincides with the S-matrix calculated by any of the other Kohn methods using these same basis functions. A numerical illustration is given.

The laser-induced collisional energy transfer process: Rb(5²P₃2/)+Na(3²S₁2/)+hv implies Rb(5 ²S₁2/)+Na(3²D₃2.5/2/), is experimentally investigated. The relative transfer cross section or transfer profile is measured versus the frequency of the photon. The far wing of the transfer profile is well interpreted using the quasi-static approximation. However, it is shown that the central part of the profile is reproduced only by taking into account non-adiabatic mixing between molecular levels during the collision.

Multiple electron capture by highly charged ions in collision with rare gas targets is experimentally investigated. Differential cross sections are measured for specific processes in which both the number of electrons lost by the target and captured by the projectile are determined. This set of data is compared with the predictions of the molecular Coulombic barrier model. The good predictive quality of this model is confirmed but deficiencies are pinpointed which are attributed to the neglect of 'simultaneous' electron transfers as well as to the intrinsic static character of the model.

The energy distribution of protons and pairs of fragment protons produced in 15 keV He⁺-H₂ collisions has been investigated experimentally. The results are compared with earlier measurements involving H⁺ projectiles and show some surprising differences. A possible explanation is proposed which assumes that two-electron excited states can play an important role in both processes, especially in the double-ionization process where they are formed in the first step of a two-step collision mechanism.

A new low-energy 29-state R-matrix calculation including the n5 states of neutral helium is reported. The 1¹S-2^3.1S transitions are re-examined and the authors show that the close-coupling expansion has converged for these transitions below the ionization threshold. In addition, collision strengths for transitions from the ground state to n=4 states are presented for the first time.

The authors report calculated differential and integral cross sections for e-C₂H₂ collisions in the 10-200 eV energy range. These cross sections were derived from fixed-nuclei scattering amplitudes in such a way that the low angular momentum components are obtained using the Schwinger iterative method and the large ones are treated by a Born closure. Using this combined theory, it can be shown that the elastic differential cross sections obtained at the static-exchange level are nearly exact, even at higher energies. The inclusion of a semi-empirical polarization potential is discussed as well.

The R-matrix method has been employed to study the electron excitation of atomic helium. Nineteen states (n=1, 2, 3, 4) have been included in the expansion of the total wavefunction. The results of this 19-state calculation are used to obtain the electron-photon coincidence parameters lambda and mod chi mod for the excitation of 1¹S to 3^3.1P at 29.6 eV. The calculated parameters lambda and mod chi mod OE are compared with the only available experiments.

The authors have studied in a crossed-beam experiment under optically thin conditions the extreme ultraviolet (EUV) spectrum of argon produced by electron impact excitation. The most prominent features of the EUV spectrum between 40 and 110 nm are the resonance lines of Ar I at 104.8 nm and 106.7 nm and of Ar II at 91.96 nm and 93.21 nm. Absolute cross sections of these lines at 200 eV are measured by the relative-flow technique and compared with previous estimates. The measured emission cross section values at 200 eV for the Ar I lines at 104.8 nm and 106.7 nm are 23.1*10^-18 cm² and 9.32*10^-18 cm², respectively, with an uncertainty of 14%. When compared with electron energy loss estimates of the direct excitation cross section these values establish that cascading is larger for the Ar I resonance lines than previous emission experiments have indicated. The far ultraviolet (FUV) spectrum of Ar is also surveyed for the first time and is found to consist of Ar II multiplets from simultaneous ionization-excitation.

The semiclassical approach in the framework of the non-local theory of resonance electron-molecule collisions is developed. The basic integro-differential equation of this theory is reduced to the recursion set of linear equations, the number of the equations is equal to the total number of the vibrational levels of the target molecule. The continuum of the molecule dissociative states is taken into account by the boundary condition. The semiclassical approach makes it possible to calculate the whole set of cross sections for all the channels which are accessible at the value of the total energy under consideration. These cross sections are calculated for the model systems, F₂ and HCl molecules. The relation between local and non-local theories is considered. It is shown that the local theory is well founded if the molecule and resonance energy curves cross far from the equilibrium geometry and if the threshold dependence of the width of the resonance state on the electron energy is smooth enough.

The authors present a general formalism for describing the photon statistics of the atomic fluorescence of driven collectively interacting N-atom systems. The formalism is applied in several special cases to obtain explicit results for the k-fold intensity correlation functions. The authors find that collective interactions diminish the non-classical properties of the emitted radiation.

The cooperative spontaneous emission from three identical, initially excited two-level atoms separated by distances much smaller than the resonance wavelength is studied taking into account the dipole-dipole near-field interaction. Approximate expressions are obtained for the time evolution of the atomic system, the radiated intensity and the one-time intensity correlation function. As a new result the authors find that the near-field interaction may lead to the occurrence of beats in the intensity. Moreover, the destructive effect of the near-field interaction on the coherence properties of the emitted radiation is discussed.