K T Taylor

Atomic data for opacity calculations. II. Computational methods

For pt.I see ibid., vol.20, p.6363-78 (1987). A general description of the data requirements for opacity calculations has been given in paper I. The present paper gives a detailed description of the methods being used in a collaborative effort which is referred to as the Opacity Project. The close-coupling approximation of electron-atom collision theory is used to calculate energies and wavefunctions for bound states, oscillator strengths, photoionisation cross sections and parameters for line broadening by electron impact. The computations are made using the R-matrix method together with new codes for calculating outer-region solutions and dipole integrals. Use of these techniques provides an efficient means of calculating large amounts of accurate atomic data.

R-matrix theory of photoionization. Application to neon and argon

The R-matrix method, which has been used recently to calculate electron-atom and ion collision cross sections and atomic polarizabilities is extended to enable atomic photoionization processes to be studied. Both the initial atomic bound state and the final atomic continuum state are expanded in terms of R-matrix bases. The method is programmed for a general atomic system and then used to calculate the photoionization cross sections of ground state neon and argon atoms leaving the residual ions in their ground or first excited states. Good agreement is obtained with recent experiments using synchrotron radiation both in resonant and non-resonant regions, showing that the method has a wide range of applicability.

Numerical integration of the time-dependent Schrödinger equation for laser-driven helium

The full time-dependent Schrodinger equation for 2-electron atoms in intense laser fields is solved using a mixed finite-difference/basis set approach. We discuss Krylov subspace techniques for the propagation of the equation in time, and numerical methods for optimizing the description of the system on a finite-difference lattice. We describe the implementation of a parallelized code based on these numerical methods, and review performance and scaling results of the code on the Cray T3D/E.

Double-electron above threshold ionization of helium

We present calculations of intense-field multiphoton ionization processes in helium at XUV wavelengths. The calculations are obtained from a full-dimensional integration of the two-electron time-dependent Schrodinger equation. A momentum-space analysis of the ionizing two-electron wavepacket reveals the existence of double-electron above threshold ionization (DATI). In momentum-space two distinct forms of DATI are resolved, namely non-sequential and sequential. In non-sequential DATI correlated electrons resonantly absorb and share energy in integer units of ωlaser.

Rydberg states of the hydrogen atom near a metal surface

We consider n = 20 Rydberg states of the hydrogen atom interacting with a nearby metal surface over an atom - surface distance range of 105 nm - 530 nm. We show that the traditional quadratic approximation to the potential is insufficient to follow the true potential and one needs to include higher order terms to obtain accurate energies for these Rydberg states. We include terms up to eighth order in the electron-proton distance in the potential and demonstrate by means of basis set calculations that the resulting spectra differ significantly from those obtained in the quadratic approximation. We observe an energy spread of the n = 20 manifold which is roughly twice that given by the quadratic approximation. Observed level repulsions are attributed to z-parity symmetry breaking. We compare our results with those from degenerate perturbation theory.

Double-ionization dynamics of laser-driven helium

We report a new method which allows sequential and non-sequential double-ionization rates in laser-driven helium to be distinguished and calculated separately. The method is applied to calculate such rates for two laser pulses, one of 0.236 au frequency and 8.0 × 1015 W cm-2 peak intensity, the other of 1.0 au frequency and also of 8.0 × 1015 W cm-2 peak intensity.

Intense-field multiphoton ionization of a two-electron atom

Multiphoton ionization of helium at high laser intensities has been investigated by direct numerical integration of the full time-dependent two-electron Schrodinger equation on a Cray T3D. At field intensities above , two-photon ionization occurs in a few field periods, and double ionization and high harmonic generation are prominent. We present calculations of double ionization yields, investigate the sensitivity of double ionization to the electron - electron electrostatic repulsion, and present evidence of unexpected non-exponential behaviour in ionization decay rates.

Photoionization of ground-state carbon and oxygen atoms

The R-matrix theory of photoionization described by Burke and Taylor (1975) is used to obtain cross sections for the photoionization of ground-state carbon and oxygen atoms. The calculations are the first to couple systematically all channels corresponding to the 2s22p1, 2s2pq+1 and 2p1+2 configurations of the residual ion. The results are in good agreement with experiment where comparison is possible.

Ionization dynamics of laser-driven H2+

We set out aspects of a numerical algorithm used in solving the full-dimensionality time-dependent Schrodinger equation describing the electronic motion of the hydrogen molecular ion driven by an intense, linearly polarized laser pulse aligned along the molecular axis. This algorithm has been implemented within the fixed inter-nuclear separation approximation in a parallel computer code, a brief summary of which is given. Ionization rates are calculated and compared with results from other methods, notably the time-independent Floquet method. Our results compare very favourably with the precise predictions of the Floquet method, although there is some disagreement with other wavepacket calculations. Visualizations of the electron dynamics are also presented in which electron rescattering is observed.

Intense-field multiphoton ionization of helium

The dynamics of multiphoton ionization of helium are investigated through numerical integration of the two-electron time-dependent Schrodinger equation. Using this work as a benchmark, a new single-active-electron model is introduced that gives agreement with He ionization rates to within a few per cent on average, and gives good agreement with He harmonic generation spectra over a laser intensity range of to , and frequencies corresponding to four- and five-photon ionization.