Laura R. Moore

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.

One- and two-electron numerical models of multiphoton ionization of helium

We discuss the calculation of ionization rates of helium using time-dependent solutions of the full-dimensional Schrodinger equation in conjunction with time-dependent solutions of a single active electron (SAE) model of helium. The SAE model is a one-electron atom with a non-Coulombic effective potential that can be tuned in certain limits to give near quantitative agreement with the results of the full-dimensional integration. We show how the tuned SAE model can be used to improve the accuracy and reliability of the calculations of ionization cross sections. In addition we consider a case in which failure of the SAE model can be attributed to strong correlation effects in laser-driven helium, specifically interaction with an intermediate doubly excited (autoionizing) state of helium.

Time-dependent and time-independent methods applied to multiphoton ionization of helium

A comparison is made between results from the R -matrix Floquet approach and the direct numerical integration of the full-dimensional time-dependent Schrodinger equation for the multiphoton ionization of helium. Good agreement is obtained away from resonances in the intensity range 5 × 1013 to 4 × 1014 W cm-2 at a laser wavelength of 248.6 nm. This agreement confirms error estimates made independently of the comparison. We review and contrast the two approaches, and discuss details of the convergence tests by which error estimates are obtained.

Accurate computational methods for two-electron atom-laser interactions

We discuss the application of quantitatively accurate computational methods to the study of laser-driven two-electron atoms in short intense laser pulses. The fundamental importance of such calculations to the subject area is emphasized. Calculations of single- and double-electron ionization rates at 390 nm are presented.

Direct versus delayed pathways in strong-field non-sequential double ionization

We report full-dimensionality quantum and classical calculations of the double ionization (DI) of laser-driven helium at 390 nm. Good agreement between the quantum and classical results is observed. We identify the relative importance of the two main non-sequential DI pathways: the direct—with an almost simultaneous ejection of both electrons—and the delayed. We find that the delayed pathway prevails at small intensities independently of total electron energy, but at high intensities the direct pathway predominates up to a certain upper limit in total energy, which increases with intensity. An explanation of this increase with intensity is provided.

Laser-driven helium, H+ 2 and H2

Abstract We review work carried out in recent years at Belfast devoted to handling the dynamics of laser-driven few-electron atomic and molecular systems in full dimensionality with the goal of bringing quantitative discipline to the field. The design and application of quantitatively accurate computational methods are discussed. Electron-electron correlations induced by intense external fields are observed by calculating the position and momentum space distributions of the doubly ionizing two-electron wave packets and the application of the techniques of scientific visualization analysis to such studies is emphasized. Agreement of results obtained with those from recent laboratory experiments is demonstrated. Work in hand and plans for future calculations are outlined.

Two electron response to an intense x-ray free electron laser pulse

New x-ray free electron lasers (FELs) promise an ultra-fast ultra-intense regime in which new physical phenomena, such as double core hole formation in at atom, should become directly observable. Ahead of x-ray FEL experiments, an initial key task is to theoretically explore such fundamental laser-atom interactions and processes. To study the response of a two-electron positive ion to an intense x-ray FEL pulse, our theoretical approach is a direct numerical integration, incorporating non-dipole Hamiltonian terms, of the full six-dimensional time-dependent Schroedinger equation. We present probabilities of double K-shell ionization in the two-electron positive ions Ne8+ and Ar16+ exposed to x-ray FEL pulses with frequencies in the range 50 au to 300 au and intensities in the range 1017 to 1022 W/cm2.

Numerical Grid Methods

Over recent years in Belfast we have developed numerical grid methods for solving the two-electron time-dependent Schrodinger equation (TDSE) in its full-dimensionality 1–4]. We have specifically had in mind during these developments the TDSE for laser-driven helium and, more recently, that for the laser-driven hydrogen molecule. Our developments were initially prompted, and continue to be stimulated, by the experimental interest [5–8] in these few-electron strongly time-dependent systems. As reported elsewhere in this volume, advances in laser technology and in detection techniques [9] steadily increase the possibilities and refinement of experimental measurement. For instance, angular information regarding the ionization of both electrons can now be gained and the upcoming free-electron lasers will soon make available unprecedented high radiation intensities in the UV to soft X-ray regimes. If theory is to play a meaningful role, and especially a predictive one in such circumstances, sophisticated calculational methods, typified by those we set out to summarize below, are required.

Accurate full-dimensionality calculations of laser-driven helium

Recent progress on the development and exploitation of a computer code that accurately solves the Time-Dependent Schrodinger Equation for the laser-driven helium atom in full-dimensionality is reported. Attention is focussed on the design of Single Active Electron models, on comparison with results from the R-matrix Floquet approach and finally, on comparison with results from laboratory experiment at a laser wavelength of 390 nm.