Bernard Piraux

Two-electron atoms in short intense laser pulses

We discuss a method of solving the time-dependent Schrodinger equation for atoms with two active electrons in a strong laser field, which we used in a previous paper [A. Scrinzi and B. Piraux, Phys. Rev. A 56, R13 (1997)] to calculate ionization, double excitation, and harmonic generation in helium by shea laser pulses. The method employs complex scaling and an expansion in an explicitly correlated basis. Convergence of the calculations is documented and error estimates are provided. The results for He at peak intensities up to 10(15) W/cm(2) and wavelength 248 nm are accurate to at least 10%. Similarly accurate calculations are presented for electron detachment and double excitation of the negative hydrogen ion. [S1050-2947(98)00208-X].

Evidence for highly correlated electron dynamics in two-photon double ionization of helium

Experiments using new sources of XUV pulses now tackle the difficult problem of few-photon direct double ionization of atoms. Despite its apparent simplicity, the fundamental process of two-photon direct double ionization of helium is far from being understood. Here, we use a time-dependent approach to study the process. Our results for the electron angular and energy distributions demonstrate that the dominant mechanism for double-electron escape involves a highly correlated electron motion. Angular correlations strongly favour back-to-back electron emission along the polarization axis, while dynamical screening leads to an equipartition of the electron energy for a broad range of field frequencies. These features are reflected in the recoil-ion-momentum distributions that are presently accessible to experiments.

Triple differential cross sections for the ionization of atomic hydrogen by fast electrons

Abstract The triple differential cross section for the ionization of atomic hydrogen by fast electrons is analyzed in the case of a coplanar, asymmetric geometry by using the eikonal Born series theory. Our calculations are in good agreement with recent measurements performed at an incident electron energy of 250 eV.

Attosecond timescale analysis of the dynamics of two-photon double ionization of helium

We consider the two-photon double ionization (DI) of helium and analyze electron dynamics on the attosecond timescale. We first re-examine the interaction of helium with an ultrashort XUV pulse and study how the electronic correlations affect the electron angular and energy distributions in the direct, sequential and transient regimes of frequency and time duration. We then consider pump-probe processes with the aim of extracting indirect information on the pump pulse. In addition, our calculations show clear evidence for the existence under certain conditions of direct two-color DI processes.

Absolute triple- and double-differential cross sections for ionisation of helium by electron impact

The authors have measured absolute (e,2e) cross sections for electron impact ionisation of helium with a much higher accuracy than obtained before. In a coplanar symmetric geometry ( theta =45 degrees ) both outgoing electrons-with equal velocities-were detected in coincidence. The resulting experimental triple differential cross sections for incoming electrons of 200-2800 eV contain an experimental error smaller than 20%. At the same time the authors have determined the absolute double differential cross sections at 45 degrees to an accuracy of 10%. The data are compared with various theoretical results.

Ionization of H- by a strong ultrashort laser pulse

We compare the outcome of two different numerical methods aimed at solving the time-dependent Schrodinger equation associated with the interaction of H- with an ultrashort laser pulse. These methods of spectral and configuration interaction type are based on an expansion of the total wavefunction on eigenstates of H- built as products of either B-spline or complex Sturmian functions. A careful analysis of our results together with a comparison with other existing theoretical data sheds some light on subtle aspects of the theoretical treatments of H- in a strong laser field. A particular emphasis is put on the crucial role played by the density of states in the continua.

Ionization of hydrogen atoms by intense vacuum ultraviolet radiation

We study the ionization of hydrogen atoms by an intense pulsed beam of photons with energies of 17 or 50 eV. The work is motivated by the demonstration of the free-electron laser (FEL) action at the DESY Laboratory. The parameters chosen for the incident field are in the regime accessible by the FEL. Ionization yields are obtained within three different approaches, namely the strong-field approximation, the Floquet method and a numerical solution of the timedependent Schr¨ odinger equation. A marked stabilization effect for 50 eV photons is shown.

Electron-impact double ionization of He by applying the Jacobi matrix approach to the Faddeev-Merkuriev equations

We apply the Jacobi matrix method to the Faddeev-Merkuriev differential equations in order to calculate the three-body wave function that describes the double continuum of an atomic two-electron system. This function is used to evaluate within the first-order Born approximation, the fully differential cross sections for (e, 3e) processes in helium. The calculations are performed in the case of a coplanar geometry in which the incident electron is fast and both ejected electrons are slow. Quite unexpectedly, the results obtained by reducing our double-continuum wave function to its asymptotic expression are in satisfactory agreement with all the experimental data of Lahmam-Bennani et al. [A. Lahaman-Bennani et al., Phys. Rev. A 59, 3548 (1999); A. Kheifets et al., J. Phys. B 32, 5047 (1999).] without any need for renormalizing the data. When the full double-continuum wave function is used, the agreement of the results with the experimental data improves significantly. However, a detailed analysis of the calculations shows that full convergence in terms of the basis size is not reached. This point is discussed in detail.

Adiabatic stabilization of a circular state: Theory compared to experiment

The probability that an-atom of hydrogen, initially in the 5g (m = 4) state, survives a 90-fs pulse of 620-nm wavelength is calculated both by direct; integration of the time-dependent Schrodinger equation and by a Floquet calculation. The two methods give virtually identical results. The survival probability calculated for a one-electron model of neon, for the same initial state, pulse duration, and wavelength, is in fair quantitative agreement with the experimental data of van Druten er al.

On the Theory of (e,2e) Reactions in Atomic-hydrogen and Helium

Abstract We compare the results of eikonal-Born series calculations which we have performed for the (e, 2e) reaction in atomic hydrogen with recent absolute measurements of triple differential cross sections for that process, carried out in the asymmetric coplanar geometry. We find that second-order effects play a crucial role in understanding both the angular positions and the magnitudes of the binary and recoil peaks. The implications of our analysis for the case of (e, 2e) reactions in helium are also discussed.