A Huetz

Photodouble ionization and the dynamics of electron pairs in the continuum

The study of the emission of two electrons from an atom by absorption of a single energetic photon has become of much current interest because it provides the most detailed information on the interaction of the electrons between themselves. Its investigation in the simplest two-electron system, the He atom, in the last ten years has challenged experimentalists and theorists alike. By using selected examples from electron–electron and electron–recoil ion coincidence experiments the main achievements in the field are reviewed. It is shown that the dynamics of the electron pair is strongly constrained by its own symmetry and the Coulomb repulsion. The further aspects brought in by the experiments in diatomic molecules (H2, HD and D2) as well as in the heavier rare gases are also illustrated. Future perspectives which involve other processes which result in an electron pair in the continuum are considered.

Double photoionization: I. A new parametrization of the triple differential cross section from first principles

We have combined a laboratory frame independent particle approach with a body-fixed frame collective coordinates approach to derive new exact parametrizations of the triple differential cross section for double photoionization. The latter disentangle the kinematical from the dynamical effects to the largest possible extent. In addition, they provide an expansion of the so-called correlation factor with respect to the configurations of the electronic pair. The present approach is particularly suitable for the physical analysis of experimental data, as illustrated by the applications reported in the companion paper.

Evolution of angular distributions in two-colour, few-photon ionization of helium

Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.

Optimization of momentum imaging systems using electric and magnetic fields

Time-of-flight (TOF) momentum imaging systems utilize the x, y, t information from charged particles striking a position-sensitive detector to infer the x, y, and z components of the particles’ initial momenta. This measurement capability can lead to the complete experimental determination of multi-ionization/fragmentation dynamics. In the case of electron detection, the addition of a magnetic field leads to a significantly increased operational energy range. This study shows that the TOF system has to be carefully designed in order to optimize the magnetic confinement effect. Expressions for the optimal dimensions of a single electric field TOF system are derived and factors contributing to the resolution are discussed, along with their application to an existing imaging system.

Multicolour above-threshold ionization of helium: quantum interference effects in angular distributions

Energy- and angle-resolved photoionization spectra of He irradiated by linearly polarized intense 810 nm laser radiation and several of its XUV odd harmonics are investigated. The angular distribution of the odd-order peaks, produced by single-photon ionization by one harmonic, is, surprisingly, broadened by the IR field. The even-order ones, due to two-colour, two-photon ionization, show at 90° lobes which depend on the relative IR-XUV phase. Application to the characterization of attosecond pulses is suggested.

Double photoionization: II. Analysis of experimental triple differential cross sections in helium and neon

We use the general formalism established in the companion paper I to analyse recent measurements of the triple differential cross sections for double photoionization of He and Ne, for equal energy sharing and for symmetry of the residual ion. A dynamical factor, which depends on the energy and on the mutual angle between the two electrons, is extracted from the experiments without relying on any dynamical approximation. This factor is expanded with respect to the one-electron angular momentum , up to a maximum value , which measures the degree of angular correlation attained by the electron pair. We discuss the physical meaning of , and the dependence of the dynamical factor on the target, which is observed when comparing helium and neon results.

Analysis of e-e angular correlations in near-threshold electron impact ionisation of helium

Using a coincidence technique in a coplanar geometry, triple differential cross sections (TDCS) for electron impact ionisation of helium are measured in the 0.5-2 eV energy range above threshold. As a few (LS Pi ) states (0<or=L<or=2) of the two outgoing electrons are obviously involved in the process, their respective intensities appear as unknown parameters in the theoretical TDCS as deduced in the frame of the Wannier theory. We show that almost all these parameters can be determined through normalisation to the measured TDCS in two specific geometries: in the first one the two electrons are kept in opposite directions while in the second one they remain symmetrical with respect to the incident beam. A comparison with the complete set of data is then performed. The measured TDCS are in agreement with the Wannier theory for the lowest energies (0.5 and 1 eV). The angular correlation predicted by this theory is confirmed but a precise determination of the angular correlation width cannot be achieved. At 2 eV the overall agreement becomes poorer, although some predictions of the Wannier theory still apply: in particular the measured TDCS are almost independent of the energy sharing between the two electrons. Finally specific measurements at 8 eV clearly show from considerations of symmetry that the Wannier theory no longer applies at this energy.

Helium (γ, 2e) triple differential cross sections at an excess energy of 60 eV

γ, 2e) triple differential cross sections (TDCS) are presented for helium at 60 eV above the photodouble ionisation threshold with energy sharing ratios ( R = E2/E1) for the two ejected electrons of R = 1, 5 and 11. The measurements were taken using a toroidal spectrometer and linearly polarised light from an undulator beamline (SU6) at the Super-ACO synchrotron. Good agreement is found with TDCSs obtained by the CCC method and by the length gauge of 3C theory.

Molecular frame photoemission in dissociative ionization of H2 and D2 induced by high harmonic generation femtosecond XUV pulses

We report the first results of molecular frame photoelectron emission for dissociative photoionization (DPI) of H2 and D2 molecules induced by a spectrally filtered single high harmonic of a few femtosecond duration, using coincident electron-ion velocity vector correlation techniques. For the studied photon energies around 32 eV, where the resonant excitation of the Q1 and Q2 doubly excited states occurs, autoionization and nuclear dynamics are coupled on a few femtosecond timescale, giving rise to quantum interferences. Molecular frame photoelectron angular distributions (MFPADs), traced as a function of the kinetic energy release of the atomic fragments, provide the most sensitive observables for such complex dynamics. These results compare well with recent spectrally resolved experiments using synchrotron radiation which are also reported. As a novel XUV light source running at multi-kHz repetition rate and synchronized with laser pulses, high-order harmonic generation (HHG) opens new possibilities for extending these investigations to time-resolved studies at the femtosecond scale.

Flexible attosecond beamline for high harmonic spectroscopy and XUV/near-IR pump probe experiments requiring long acquisition times

We describe the versatile features of the attosecond beamline recently installed at CEA-Saclay on the PLFA kHz laser. It combines a fine and very complete set of diagnostics enabling high harmonic spectroscopy (HHS) through the advanced characterization of the amplitude, phase, and polarization of the harmonic emission. It also allows a variety of photo-ionization experiments using magnetic bottle and COLTRIMS (COLd Target Recoil Ion Momentum Microscopy) electron spectrometers that may be used simultaneously, thanks to a two-foci configuration. Using both passive and active stabilization, special care was paid to the long term stability of the system to allow, using both experimental approaches, time resolved studies with attosecond precision, typically over several hours of acquisition times. As an illustration, applications to multi-orbital HHS and electron-ion coincidence time resolved spectroscopy are presented.