62 research outputs found
Spectral data for doubly excited states of helium with non-zero total angular momentum
A spectral approach is used to evaluate energies and widths for a wide range
of singlet and triplet resonance states of helium. Data for total angular
momentum is presented for resonances up to below the 5th single
ionization threshold. In addition the expectation value of
is given for the calculated resonances.Comment: 35 pages, 16 tables, to be published in Atomic Data and Nuclear Data
Table
Improved convergence of scattering calculations in the oscillator representation
The Schr\"odinger equation for two and tree-body problems is solved for
scattering states in a hybrid representation where solutions are expanded in
the eigenstates of the harmonic oscillator in the interaction region and on a
finite difference grid in the near-- and far--field. The two representations
are coupled through a high--order asymptotic formula that takes into account
the function values and the third derivative in the classical turning points.
For various examples the convergence is analyzed for various physics problems
that use an expansion in a large number of oscillator states. The results show
significant improvement over the JM-ECS method [Bidasyuk et al, Phys. Rev. C
82, 064603 (2010)]
Electron correlation in two-photon double ionization of helium from attosecond to FEL pulses
We investigate the role of electron correlation in the two-photon double
ionization of helium for ultrashort XUV pulses with durations ranging from a
hundred attoseconds to a few femtoseconds. We perform time-dependent ab initio
calculations for pulses with mean frequencies in the so-called "sequential"
regime (photon energy above 54.4 eV). Electron correlation induced by the time
correlation between emission events manifests itself in the angular
distribution of the ejected electrons, which strongly depends on the energy
sharing between them. We show that for ultrashort pulses two-photon double
ionization probabilities scale non-uniformly with pulse duration depending on
the energy sharing between the electrons. Most interestingly we find evidence
for an interference between direct ("nonsequential") and indirect
("sequential") double photo-ionization with intermediate shake-up states, the
strength of which is controlled by the pulse duration. This observation may
provide a route toward measuring the pulse duration of FEL pulses.Comment: 9 pages, 6 figure
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
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
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