Problème coulombien à trois corps en champ haute fréquence : application à l'étude de l'ionisation double à deux photons de l'hélium

Ce travail porte sur l'étude théorique de la double ionisation à deux photons de l'atome d’hélium avec comme objectif de comprendre le rôle des corrélations électroniques dans le mécanisme de double éjection. En analysant les distributions en énergie et les distributions angulaires des électrons émis, nous montrons que lors du processus direct, le système initialement dans son état fondamental évolue vers un état hautement corrélé. Les corrélations angulaires forcent les deux électrons à être éjectés dans des directions opposées, le long de l'axe de polarisation. Sous l'effet de "l'écrantage dynamique" c'est-à-dire des corrélations radiales, les deux électrons ont tendance à partager équitablement l'énergie disponible au dessus du seuil de double ionisation. Pour valider ou invalider ce mécanisme, nous proposons de mesurer la distribution des impulsions des ions doublement chargés He++. Tous ces résultats s'obtiennent en résolvant l'équation de Schrödinger dépendante du temps à l’aide d'une méthode spectrale combinée à celle de la matrice de Jacobi. En parallèle, et toujours dans le cas de l'ionisation double à deux photons de l'hélium, nous analysons les effets des corrélations électroniques à l'échelle attoseconde.(PHYS 3) -- UCL, 200

(2 gamma,2e) total and differential cross-section calculations for helium with (h)over-bar omega=40-50 eV

We consider two-photon double ionization of helium and analyse the electron dynamics in the region where the process is direct (39.49 eV omega omega = 46 eV and (h) over bar omega = 50 eV, angular distributions are also analysed. The theoretical approach is based on the resolution of the time-dependent Schrodinger equation (TDSE), using a spectral approach. At the end of the pulse the TPDI probability is extracted from the total wavefunction using two different approaches. The first one neglects the electron interaction in the double continuum while the second one includes electron correlation effects. At (h) over bar omega approximate to 45 eV the electrons are preferably emitted back-to-back with equal energy. At (h) over bar omega = 50 eV the excess energy is likely to be transfered to one of the electron, while the electrons are emitted in opposite or same directions

Two-photon double ionization of helium: An experimental lower bound of the total cross section

We report on the experimental estimation of a lower bound of helium. Our experiment differs from a previous one described by Hasegawa et al. [Phys. Rev. A 71, 023407 (2005)]. The photons that correspond to the 27th harmonic of a Ti:sapphire laser have an energy of 41.8 eV. The extraction of the cross section rests on a theoretical model, based on the rate equations, that fully includes the contribution from the sequential channels and the macroscopic effects. Some of the experimental parameters are very difficult to measure accurately. For those parameters., we deliberately consider values compatible with their experimental determinations and that lead systematically to an underestimation of the two-photon double ionization cross section. Our estimation of this lower bound is higher than a bunch of various theoretical results. It stays, however, too close to these latter ones to draw any definite conclusion about their validity, even though it clearly indicates that the actual value of the double ionization cross section is higher than expected

Dynamics of the direct double ionization of helium by two XUV photons

As a highly correlated process, direct two-photon double onization of helium represents a very challenging theoretical problem. A wide rande of computational methods have been used to evaluate generalized cross sections. However, despite considerable efforts, a quantitative agreement between all calculations has not been reached. In this contribution, we address several issues regarding the origin of the discrepancies in the total cross section and the behaviour of this cross section close to the sequential regime. Our ab initio calculations are based on the numerical solution of the time-dependent Schrödinger equation by means of a spectral method combined with Jacobi-matrix calculations. This method which, in principle, takes fully into account electron correlations, provides also a consistent physical picture of the double escape mechanism

Dynamics of two-photon double-ionization of helium at the attosecond scale

We study two-photon double ionization of helium. We demonstrate that attosecond pulses can be used to probe in the time-domain the electron dynamics within the atom. The interaction of helium with attosecond pulses whose duration is close to the correlation time provides information about the relaxation of the residual ion after the first electron ejection. Here, we show that such interaction of attosecond pulses with helium also modifies the double ionization probability in a non-trivial way. We examine the situations where sequential double ionization is permitted (omega >= 2 au) and where only the direct two-photon double ionization is allowed (omega < 2 au). All the calculations rely on both a model and the numerical solution of the time-dependent Schrodinger equation in its full dimensionality

Theory of multiphoton single and double ionization of two-electron atomic systems driven by short-wavelength electric fields: An ab initio treatment

We give a detailed account of an ab initio computational treatment of multiphoton single ionization (with or without excitation) as well as double ionization of two-electron atoms exposed to short-wavelength electric fields. This treatment is time dependent and based on a spectral method of configuration interaction type combined with Jacobi or J-matrix calculations. It involves a complete treatment of electron-electron correlation in the initial and final states as well as during the time propagation. The atom eigenvalue problem is first solved by means of the spectral method. It consists of expanding the atom wave function in a basis of products of complex Coulomb-Sturmian functions of the electron radial coordinates and bipolar harmonics of the angular coordinates. This method allows a high-resolution study of many atomic states, in particular high-lying singly excited states as well as many doubly excited states. Results for He are presented and discussed in detail. The time-dependent Schrodinger equation is then solved by means of an explicit scheme of Runge-Kutta type. An accurate calculation of the probability of single and double ionization is carried out by projecting the ionizing wave packet on fully correlated multichannel scattering wave functions generated by means of the J-matrix method. After a detailed analysis of the accuracy of this method, we show that our results for the total cross section of one-photon single and double ionization of He and H- are in very good agreement with those obtained by the most sophisticated approaches. Two-photon double ionization of He is then considered, and results are presented in a frequency regime where substantial discrepancies subsist between all existing calculations. Our results demonstrate that electron correlations in the final state play a significant role