G Peach

Atomic data for opacity calculations. IX. The lithium isoelectronic sequence

Bound-state energies, oscillator strengths and photoionisation cross sections have been calculated for members of the lithium isoelectronic sequence with nuclear charge Z in the range 3<or=Z<or=10. Two independent approaches to the problem give virtually the same results. Detailed comparisons with experiment and other theoretical results indicate that the present data are of high accuracy.

Ionization of atoms and positive ions by electron and proton impact

Earlier work on cross sections for the removal by electron and proton impact of a 2p or 3p electron from all neutral atomic systems in the sequences boron to neon and aluminium to argon is corrected. The new results are in much better agreement with experiment and the work has now been extended to obtain cross sections for certain isoelectronic positive ions.

Collisional broadening of alkali doublets by helium perturbers

We report results for the Lorentzian profiles of the Li I, Na I and K I doublets and the Na I subordinate doublet broadened by helium perturbers for temperatures up to 3000 K. They have been obtained from a fully quantum-mechanical close-coupling description of the colliding atoms, the Baranger theory of line shapes and new ab initio potentials for the alkali–helium interaction. For all lines except the 769.9 nm K I line, the temperature dependence of the widths over the range 70 ≤ T ≤ 3000 K is accurately represented by the power law form w = aTbb with 0.38 < b < 0.43. The 769.9 nm K I line has this form for 500 ≤ T ≤ 3000 K with b having the higher value of 0.49. Although the shifts have a more complex temperature dependence, they all have the general feature of increasing with temperature above T ~ 500 K apart from the 769.9 K I line whose shift decreases with temperature.

The shape and shift of the resonance lines of Ca+ perturbed by electron collisions

The impact theory of Baranger (1958), in which the perturbing particles are treated quantum-mechanically, is clarified and extended to give a general relatively simple expression for the width and shift of an isolated line using an LS coupling scheme to describe the radiating atomic system. For the case of a radiating positive ion this result is shown to be free of explicit dependence on the phase shifts for Coulomb scattering. The width and shift of the resonance lines in Ca+ perturbed by electron collisions are then calculated using the scattering data of Burke and Moores, and the results are in reasonable agreement with existing semi-classical estimates. The discrepancy between theory and experiment is partially resolved.

Optical transitions in excited alkali + rare gas collision molecules and related interatomic potentials:

A comparison is made of experimental far-wing profiles for the system with quantum calculations of thermally averaged free - free continua. Using input adiabatic potentials and transition moment functions from both ab initio and semi-empirical approaches, the comparison shows: (i) new ab initio potentials for reproduce the spectral positions of the observed rainbow satellites well; the height of the barrier predicted agrees with experiment to within , whereas its position is too large by ; (ii) potentials obtained with various semi-empirical methods reproduce the satellite structure qualitatively, but are generally too repulsive in the state at intermediate and large internuclear separations; (iii) transition moment functions for the asymptotically forbidden transitions reproduce, with different degrees of accuracy, the intensity of the red-wing satellite related to the potential extremum around . The vibrational energies in the and states calculated with the ab initio potentials reproduce to within a few those obtained from rotationally resolved band spectra reported in the literature.

Ionization of neutral atoms with outer 2p and 3p electrons by electron and proton impact

The Born approximation is used to calculate cross sections for the ionization by electron and proton impact of all neutral atomic systems in the sequences boron to neon and aluminium to argon. Possible exchange between the incident and atomic electrons is approximately taken into account using the Ochkur approximation to the exchange amplitude. In the cases where experimental data exist for comparison there is agreement at high energies to within about a factor of 2, a discrepancy which may be partly explained by the simple approximation made to the true wave function of the ejected electron.

The classical theory of charge transfer and ionisation processes in collisions between complex atomic ions

The classical theory of charge transfer and ionisation of hydrogen atoms by protons, first developed by Abrines and Percival (1966), is extended to the case in which the interactions between the three particles are not of pure Coulomb type. The model adopted is one in which only the active electron is treated explicitly; the two residual atomic ions are considered to be simply polarisable cores. The two-body interactions are represented by semiempirical central potentials and additional three-body terms are included in order to allow fully for polarisation.

Ultracold atomic collisions in tight harmonic traps: Quantum-defect model and application to metastable helium atoms

We analyze a system of two colliding ultracold atoms under strong harmonic confinement from the viewpoint of quantum defect theory and formulate a generalized self-consistent method for determining the allowed energies. We also present two highly efficient computational methods for determining the bound state energies and eigenfunctions of such systems. The perturbed harmonic oscillator problem is characterized by a long asymptotic region beyond the effective range of the interatomic potential. The first method, which is based on quantum defect theory and is an adaptation of a technique developed by one of the authors (G.P.) for highly excited states in a modified Coulomb potential, is very efficient for integrating through this outer region. The second method is a direct numerical solution of the radial Schrodinger equation using a discrete variable representation of the kinetic energy operator and a scaled radial coordinate grid. The methods are applied to the case of trapped spin-polarized metastable helium atoms. The calculated eigenvalues agree very closely for the two methods, and with the eigenvalues computed using the generalized self-consistent method.

Charge transfer and ionisation processes in collisions involving atoms and ions of hydrogen and helium

The classical theory which the authors previously presented is used to study collisions between hydrogen and helium atoms and ions. The total classical cross sections for charge transfer and ionisation are obtained by using standard Monte Carlo methods.

Effects of non-adiabatic and Coriolis couplings on the bound states of He(2 3S)+He(2 3P)

The effects of non-adiabatic and Coriolis couplings on the bound states of the He(2 ^3S1) + He(2 ^3Pj) system, where j = 0, 1, 2, are investigated using the recently available ab initio short-range 1, 3, 5Σ+g, u and 1, 3, 5Πg, u potentials computed by Deguilhem et al (2009 J. Phys. B: At. Mol. Opt. Phys. 42 015102). Three sets of calculations have been undertaken: single-channel, multichannel without Coriolis couplings and full multichannel with Coriolis couplings. We find that non-adiabatic effects are negligible for 0−u, 0±g, 1u, 2g, 2u, 3g Hund case (c) sets of levels in the j = 2 asymptote but can be up to 15% for some of the 0+u and 1g sets of levels where near degeneracies are present in the single-channel diagonalized potentials. Coriolis couplings are most significant for weakly bound levels, ranging from 1% to 5% for total angular momenta J = 1, 2 and up to 10% for J = 3. Levels near the j = 1 and j = 0 asymptotes agree closely with previous multichannel calculations based upon long-range potentials constructed from retarded resonance dipole and dispersion interactions. Assignment of theoretical levels to experimental observations using criteria based upon the short-range character of each level and their coupling to metastable ground states produces well-matched assignments for the majority of observations. After a 1% increase in the slope of the 5Σ+g, u and 5Πg, u input potentials near the classical turning point is applied, improved matching of previous assignments is obtained and further assignments can be made, reproducing very closely the number of experimental observations.