439 research outputs found
Two-photon Double Ionization of H in Intense Femtosecond Laser Pulses
Triple-differential cross sections for two-photon double ionization of
molecular hydrogen are presented for a central photon energy of 30 eV. The
calculations are based on a fully {\it ab initio}, nonperturbative, approach to
the time-dependent Schroedinger equation in prolate spheroidal coordinates,
discretized by a finite-element discrete-variable-representation. The wave
function is propagated in time for a few femtoseconds using the short,
iterative Lanczos method to study the correlated response of the two
photoelectrons to short, intense laser radiation. The current results often lie
in between those of Colgan {\it et al} [J. Phys. B {\bf 41} (2008) 121002] and
Morales {\it et al} [J. Phys. B {\bf 41} (2009) 134013]. However, we argue that
these individual predictions should not be compared directly to each other, but
preferably to experimental data generated under well-defined conditions.Comment: 4 pages, 4 figure
Application of the Complex Kohn Variational Method to Attosecond Spectroscopy
The complex Kohn variational method is extended to compute light-driven
electronic transitions between continuum wavefunctions in atomic and molecular
systems. This development enables the study of multiphoton processes in the
perturbative regime for arbitrary light polarization. As a proof of principle,
we apply the method to compute the photoelectron spectrum arising from the
pump-probe two-photon ionization of helium induced by a sequence of extreme
ultraviolet and infrared-light pulses. We compare several two-photon ionization
pump-probe spectra, resonant with the (2s2p)1P1o Feshbach resonance, with
independent simulations based on the atomic B-spline close- coupling STOCK
code, and find good agreement between the two approaches. This new finite-pulse
perturbative approach is a step towards the ab initio study of weak-field
attosecond processes in poly-electronic molecules
Breakup of the aligned H molecule by xuv laser pulses: A time-dependent treatment in prolate spheroidal coordinates
We have carried out calculations of the triple-differential cross section for
one-photon double ionization of molecular hydrogen for a central photon energy
of ~eV, using a fully {\it ab initio}, nonperturbative approach to solve
the time-dependent \Schro equation in prolate spheroidal coordinates. The
spatial coordinates and are discretized in a finite-element
discrete-variable representation. The wave packet of the laser-driven
two-electron system is propagated in time through an effective short iterative
Lanczos method to simulate the double ionization of the hydrogen molecule. For
both symmetric and asymmetric energy sharing, the present results agree to a
satisfactory level with most earlier predictions for the absolute magnitude and
the shape of the angular distributions. A notable exception, however, concerns
the predictions of the recent time-independent calculations based on the
exterior complex scaling method in prolate spheroidal coordinates
[Phys.~Rev.~A~{\bf 82}, 023423 (2010)]. Extensive tests of the numerical
implementation were performed, including the effect of truncating the Neumann
expansion for the dielectronic interaction on the description of the initial
bound state and the predicted cross sections. We observe that the dominant
escape mode of the two photoelectrons dramatically depends upon the energy
sharing. In the parallel geometry, when the ejected electrons are collected
along the direction of the laser polarization axis, back-to-back escape is the
dominant channel for strongly asymmetric energy sharing, while it is completely
forbidden if the two electrons share the excess energy equally.Comment: 17 pages, 9 figure
A Multi-Center Quadrature Scheme for the Molecular Continuum
A common way to evaluate electronic integrals for polyatomic molecules is to
use Becke's partitioning scheme [J. Chem. Phys.88, 2547 (1988)] in conjunction
with overlapping grids centered at each atomic site. The Becke scheme was
designed for integrands that fall off rapidly at large distances, such as those
approximating bound electronic states. When applied to states in the electronic
continuum, however, Becke scheme exhibits slow convergence and it is highly
redundant. Here, we present a modified version of Becke scheme that is
applicable to functions of the electronic continuum, such as those involved in
molecular photoionization and electron-molecule scattering, and which ensures
convergence and efficiency comparable to those realized in the calculation of
bound states. In this modified scheme, the atomic weights already present in
Becke's partition are smoothly switched off within a range of few bond lengths
from their respective nuclei, and complemented by an asymptotically unitary
weight. The atomic integrals are evaluated on small spherical grids, centered
on each atom, with size commensurate to the support of the corresponding atomic
weight. The residual integral of the interstitial and long-range region is
evaluated with a central master grid. The accuracy of the method is
demonstrated by evaluating integrals involving integrands containing Gaussian
Type Orbitals and Yukawa potentials, on the atomic sites, as well as spherical
Bessel functions centered on the master grid. These functions are
representative of those encountered in realistic electron-scattering and
photoionization calculations in polyatomic molecules
Vortex stability of interacting Bose-Einstein condensates confined in anisotropic harmonic traps
Vortex states of weakly-interacting Bose-Einstein condensates confined in
three-dimensional rotating harmonic traps are investigated numerically at zero
temperature. The ground state in the rotating frame is obtained by propagating
the Gross-Pitaevskii equation for the condensate in imaginary time. The total
energies between states with and without a vortex are compared, yielding
critical rotation frequencies that depend on the anisotropy of the trap and the
number of atoms. Vortices displaced from the center of nonrotating traps are
found to have long lifetimes for sufficiently large numbers of atoms. The
relationship between vortex stability and bound core states is explored.Comment: 5 pages, 2 embedded figures, revtex. To appear in Phys. Rev. Let
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