328 research outputs found
Two photon excitation of atomic oxygen
A standard perturbation expansion in the atom-radiation field interaction is used to calculate the two photon excitation cross section for 1s(2) 2s(2) 2p(4) p3 to 1s(2) 2s(2) 2p(3) (s4) 3p p3 transition in atomic oxygen. The summation over bound and continuum intermediate states is handled by solving the equivalent inhomogeneous differential equation. Exact summation results differ by a factor of 2 from a rough estimate obtained by limiting the intermediate state summation to one bound state. Higher order electron correlation effects are also examined
Dynamics of a two-species Bose-Einstein condensate in a double well
We study the dynamics of a two-species Bose-Einstein condensate in a double
well. Such a system is characterized by the intraspecies and interspecies
s-wave scattering as well as the Josephson tunneling between the two wells and
the population transfer between the two species. We investigate the dynamics
for some interesting regimes and present numerical results to support our
conclusions. In the case of vanishing intraspecies scattering lengths and a
weak interspecies scattering length, we find collapses and revivals in the
population dynamics. A possible experimental implementation of our proposal is
briefly discussed.Comment: 7 pages, 5 figure
Observing collapse in two colliding dipolar Bose-Einstein condensates
We study the collision of two Bose-Einstein condensates with pure dipolar
interaction. A stationary pure dipolar condensate is known to be stable when
the atom number is below a critical value. However, collapse can occur during
the collision between two condensates due to local density fluctuations even if
the total atom number is only a fraction of the critical value. Using full
three-dimensional numerical simulations, we observe the collapse induced by
local density fluctuations. For the purpose of future experiments, we present
the time dependence of the density distribution, energy per particle and the
maximal density of the condensate. We also discuss the collapse time as a
function of the relative phase between the two condensates.Comment: 6 pages, 7 figure
Neutron Ionization of Helium near the Neutron-Alpha Particle Collision Resonance
Neutron-impact single and double ionization cross sections of the He atom are calculated near the neutron-alpha particle collision resonance. Calculations using the time-dependent close-coupling method for total and differential cross sections are made at 8 incident neutron energies ranging from 250 to 2000 keV. At the resonance energy peak the double ionization cross sections unexpectedly become larger than the single ionization cross sections. This finding appears to be related to the high velocity of the recoiling alpha particle, which makes it unlikely that the atomic electrons can recombine with the alpha particle nucleus, enhancing the double ionization cross section.Peer ReviewedPostprint (author's final draft
Relativistic many-body calculation of low-energy dielectronic resonances in Be-like carbon
We apply relativistic configuration-interaction method coupled with many-body
perturbation theory (CI+MBPT) to describe low-energy dielectronic
recombination. We combine the CI+MBPT approach with the complex rotation method
(CRM) and compute the dielectronic recombination spectrum for Li-like carbon
recombining into Be-like carbon. We demonstrate the utility and evaluate the
accuracy of this newly-developed CI+MBPT+CRM approach by comparing our results
with the results of the previous high-precision study of the CIII system
[Mannervik et al., Phys. Rev. Lett. 81, 313 (1998)].Comment: 6 pages, 1 figure; v2,v3: fixed reference
A two-dimensional, two-electron model atom in a laser pulse: exact treatment, single active electron-analysis, time-dependent density functional theory, classical calculations, and non-sequential ionization
Owing to its numerical simplicity, a two-dimensional two-electron model atom,
with each electron moving in one direction, is an ideal system to study
non-perturbatively a fully correlated atom exposed to a laser field. Frequently
made assumptions, such as the ``single active electron''- approach and
calculational approximations, e.g. time dependent density functional theory or
(semi-) classical techniques, can be tested. In this paper we examine the
multiphoton short pulse-regime. We observe ``non-sequential'' ionization, i.e.\
double ionization at lower field strengths as expected from a sequential,
single active electron-point of view. Since we find non-sequential ionization
also in purely classical simulations, we are able to clarify the mechanism
behind this effect in terms of single particle trajectories. PACS Number(s):
32.80.RmComment: 10 pages, 16 figures (gzipped postscript), see also
http://www.physik.tu-darmstadt.de/tqe
Angular Distributions from Photoionization of Hââș
A study is made of the differential cross sections arising from the photoionization of H2+. Previous studies indicated surprising differences in the shapes of the angular distributions calculated from exterior complex scaling and 2C methods. To further explore these differences, we have calculated the angular distributions from the photoionization of H2+ using an independent two-body Coulomb function (2C) method and a distorted wave approach. As a final test, we also present calculations using a time-dependent technique. Our results confirm the discrepancies found previously and we present possible reasons for these differences
Evidence for Unnatural-Parity Contributions to Electron-Impact Ionization of Laser-Aligned Atoms
Recent measurements have examined the electron-impact ionization of excited-state laser-aligned Mg atoms. In this work we show that the ionization cross section arising from the geometry where the aligned atom is perpendicular to the scattering plane directly probes the unnatural parity contributions to the ionization amplitude. The contributions from natural parity partial waves cancel exactly in this geometry. Our calculations resolve the discrepancy between the nonzero measured cross sections in this plane and the zero cross section predicted by distorted-wave approaches. We demonstrate that this is a general feature of ionization from p-state targets by additional studies of ionization from excited Ca and Na atoms
Lower entropy bounds and particle number fluctuations in a Fermi sea
We demonstrate, in an elementary manner, that given a partition of the single
particle Hilbert space into orthogonal subspaces, a Fermi sea may be factored
into pairs of entangled modes, similar to a BCS state. We derive expressions
for the entropy and for the particle number fluctuations of a subspace of a
fermi sea, at zero and finite temperatures, and relate these by a lower bound
on the entropy. As an application we investigate analytically and numerically
these quantities for electrons in the lowest Landau level of a quantum Hall
sample.Comment: shorter version, typos fixe
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