Table of contents

The quality of the expressions used in line-broadening calculations based in the semiempirical Gaunt approximation is examined. Comparison with more than one hundred experiments carried out in the singly ionized noble gases shows that the Gaunt factor for ns-np transitions is fairly different from that for n p-nd lines. Simple empirical expressions for the Gaunt factor were developed that permit an agreement with experimental values better than 20%. Thus, a strong discrepancy between experimental and theoretical results for Xe II is, in principle, removed.

The authors consider the effect of a high-frequency field with random phase (random telegraph) noise on an excited one-dimensional hydrogen atom and show by numerical simulation that if the phase changes are sufficiently large then the classical and quantal systems behave similarly.

The L₃-M₂₃M₂₃(¹S₀) and L₃-M₂₃M₂₃(¹D₂) Auger lineshapes in argon have been measured in coincidence with the scattered electron at two different excess energies, 2 and 5 eV. The measured lineshapes exhibit strong post-collision interaction and interference effects.

The authors propose a computationally optimized scheme in which the iterative procedure can be efficiently employed to treat exchange-correlation exactly in electron-polyatomic molecule collisions. The scheme is more suitable for polar species. First, it adequately solves integro-differential coupled equations for only a few low partial waves for the exact-exchange treatment. Second, the continuum function determined iteratively at any one energy is a good starting point to start iteration at other energies. Some text results are presented for the e-H₂O system in its A₁ symmetry at the static-exchange level.

The angular anisotropy coefficients and spin polarization parameters of Auger electrons for the transitions L₃M_2.3M_2.3 in Ar, M_4.5N_2.3N_2.3 in Kr and N_4.5O_2.3O_2.3 in Xe atoms are calculated. The relativistic Dirac-Fock model with intermediate coupling is used for the description of the initial and final state of the Auger transition. The results of calculation are compared with earlier theoretical predictions and recent experimental data in the case of angular anisotropy. With a few exceptions good agreement with the experimental anisotropy is found.

The authors propose a new model for the Fermi correlation function for a pair of electrons of the same spin in a system with a finite number of electrons. Using this correlation function, they have derived new expressions for the exchange and kinetic energy density functionals, depending explicitly on the number of electrons. The numerical results obtained for several atomic systems show good agreement, thus demonstrating the accuracy of prediction of the proposed model.

For the calculation of radial integrals of dipole transitions between nu l and nu 'l' Rydberg states of a non-hydrogenic atom or positive ion a general formula for any Delta nu (0( Delta nu ( infinity ) is developed using the semiclassical and Coulomb approximations. In the limiting case Delta nu << nu the formula is equivalent to the known one. The main point of this paper is the choice of the mean value of the effective principal quantum number as nu _c=(2 nu ²v'²/( nu + nu '))¹3/. This choice ensures the correct behaviour in the region near the continuum threshold. The conditions of validity are examined. The criterion of applicability of the Coulomb approximation is obtained. Comparison with the results of the previous papers is made.

² Sigma ⁺ Rydberg states of the NO molecule have been calculated by ab initio methods up to the 8s sigma state. In addition to the SCF procedure the coupled electron pair approximation (CEPA) has been applied in order to account for electron correlation effects. Quantum defects and partial wave expansion coefficients are presented for states of the Rydberg series ns sigma , np sigma , nd sigma and nf sigma . The energy dependence of the quantum defects and the angular momentum composition of the Rydberg states is discussed. Finally quantum defects and partial wave expansion coefficients obtained by the CEPA method are used for a calculation of electronic transition moments in the Coulomb approximation.

The authors report high resolution absorption spectra of palladium in the 1500-1300 AA region. The observations were made in the first order of a 3 m vacuum spectrograph equipped with a 6000 lines/mm holographic grating using synchrotron radiation as the background source of continuum. More than 45 new transitions are reported, all of which can be classified into Rydberg series due to excitation of one electron from the 4d subshell: 4d¹⁰¹S₀ to 4d⁹(²D_5/2) n p (9(n<or=30)nf(6(n<or=16) to 4d⁹(²D_3/2) n p (8(n<or=17). The ionization energies obtained are 67241.3+or-0.8 cm^-1 and 70779.8+or-0.8 cm^-1 for the ²D_5/2 and ²D_3/2 states of Pd II respectively. To study the interchannel interactions, a graphical multichannel quantum defect analysis of the observed energy levels is presented.

The behaviour of an atom placed in an external adiabatically cone-rotating constant electric field is considered. Taking into account the Berry phase in this problem is shown to be necessary. The radiation spectrum for this atom in such a field is obtained and photon polarizations are considered. The Berry phases are shown to be easily observable during optical transitions in atoms placed in an external varying electric field.

A study is made of the spectra of a highly excited hydrogen atom in crossed external l-mixing electric and magnetic fields. A semiclassical model is suggested for the radiative transitions between the perturbed Rydberg levels and the low-lying levels of the atom. The emitted light is shown to be a superposition of radiation of two effective linear vibrators symmetrically placed with respect to the external fields' plane. The interference between the vibrators is the source of rapid oscillations of the observed line intensities and polarization accompanying even small variation of the external fields. The averaged spectral intensity is shown to be unchanged by the presence of the magnetic field, being the same as in Stark splitting.

A description is given of a method for calculating forces on atoms from periodic laser fields in an arbitrary number of dimensions. A crucial approximation is the introduction of an average force which depends on the magnitude and direction of the particle velocity but not on position. A simple two-dimensional example is studied and a strong force component perpendicular to the two-dimensional atomic velocity is found. It may become of interest for the construction of well collimated atomic beams of high intensity. Fluctuations are not included but results from simulations at high laser intensity indicate that average forces alone serve as useful guides in the prediction of modifications of velocity distributions.

The authors have performed quantal calculations of the ³ Sigma ^- and ³ Pi potential energy curves for interactions of C and He atoms in their ground states. The corresponding cross sections for transitions between the fine structure levels of C have also been determined. Rate coefficients are tabulated for these transitions in the temperature range 10<or=T

Differential cross sections for collisions between protons and helium atoms have been calculated in the range of impact energies from 25-100 keV. The eikonal formula for the differential cross section was used, in conjunction with semiclassical transition amplitudes. The latter were calculated in the impact parameter approximation, with straight line trajectories. Comparison is made with experimental results for elastic scattering, excitation and electron capture. Overall, good agreement with the measurements is obtained. An outstanding discrepancy between the experimental and theoretical elastic cross sections is resolved, at least for the lower energies in the range considered.

Double ionization of He by fully stripped ions are studied within the independent-event model. Ionization of each of the two electrons in He are considered to be independent events and probabilities for these events are calculated individually by using the continuum-distorted-wave approximation. Electron-electron correlation has been accounted for through the explicitly correlated two-electron wavefunction of Pluvinage (1950). Good agreement between our results and measurements are attributed to this static correlation as well as to the dynamical correlation of events as against particles.

The energy imparted to the ions in the transfer ionization reaction He²⁺+H^- to He⁺+H⁺+e has been measured in a merged beam experiment using two different techniques (magnetic analysis and time-of-flight). The result obtained in both cases is of the order of 6 eV and confirms that the transfer ionization proceeds in two steps: a single electron transfer at large internuclear separation followed by a resonant Penning ionization at small distances. With this view, a simple calculation of the cross section is presented, with results that are in good agreement with the available experimental data.

A fast intersecting beam technique previously developed in this laboratory has been used to study collisions of protons with the alkali-like ions Ba⁺ and Sr⁺ at CM energies within the range 50-500 keV. Absolute cross sections for Ba²⁺ and Sr²⁺ production from the combined processes of charge transfer and ionization have been measured. Separate measurements of the cross sections for charge transfer also allowed cross sections for ionization to be determined. Measured cross sections for charge transfer are shown to be in poor agreement with available theoretical predictions. Cross sections for ionization (believed to be dominated by excitation-autoionization) are compared with those for equivelocity electron impact and Born predictions. The applicability of a simple classical scaling relation is also considered.

The collision of low-energy (below positronium formation threshold) positrons with methane (CH₄) and silane (SiH₄) molecules is investigated by employing two types of parameter-free model polarization potentials within the density functional description of the target correlation. The first potential is based on the correlation energy, in _corr, of one positron in a homogeneous electron gas of the target, while the second one is a modified version of the electron correlation potential in which the exchange interaction is absent. In the asymptotic region, both types of polarization potentials have the correct analytic form (- alpha ₀/2r⁴, where alpha ₀ is the polarizability of the target). Results for the total and differential cross sections are compared with recent measurements. The authors found that the first polarization potential mentioned above, is better than the second one.