1,590 research outputs found
Photon Echoes Produced by Switching Electric Fields
We demonstrate photon echoes in Eu:YSiO by controlling the
inhomogeneous broadening of the Eu
FD optical transition. This transition has
a linear Stark shift and we induce inhomogeneous broadening by applying an
external electric field gradient. After optical excitation, reversing the
polarity of the field rephases the ensemble, resulting in a photon echo. This
is the first demonstration of such a photon echo and its application as a
quantum memory is discussed.Comment: improved introduction, including theoretical outline of the relvant
quantum memory proposa
Radiation Damping in the Photoionization of Fe^{14+}
A theoretical investigation of photoabsorption and photoionization of
Fe^{14+} extending beyond an earlier frame transformation R-matrix
implementation is performed using a fully-correlated, Breit-Pauli R-matrix
formulation including both fine-structure splitting of strongly-bound
resonances and radiation damping. The radiation damping of
resonances gives rise to a resonant photoionization cross section that is
significantly lower than the total photoabsorption cross section. Furthermore,
the radiation-damped photoionization cross section is found to be in good
agreement with recent experimental results once a global shift in energy of
eV is applied. These findings have important implications.
Firstly, the presently available synchrotron experimental data are applicable
only to photoionization processes and not to photoabsorption; the latter is
required in opacity calculations. Secondly, our computed cross section, for
which the L-shell ionization threshold is aligned with the NIST value, shows a
series of Rydberg resonances that are uniformly 3-4 eV
higher in energy than the corresponding experimental profiles, indicating that
the L-shell threshold energy values currently recommended by NIST are likely in
error.Comment: 4 pages, 1 figures, and 2 table
A Comprehensive X-ray Absorption Model for Atomic Oxygen
An analytical formula is developed to represent accurately the
photoabsorption cross section of O I for all energies of interest in X-ray
spectral modeling. In the vicinity of the Kedge, a Rydberg series expression is
used to fit R-matrix results, including important orbital relaxation effects,
that accurately predict the absorption oscillator strengths below threshold and
merge consistently and continuously to the above-threshold cross section.
Further minor adjustments are made to the threshold energies in order to
reliably align the atomic Rydberg resonances after consideration of both
experimental and observed line positions. At energies far below or above the
K-edge region, the formulation is based on both outer- and inner-shell direct
photoionization, including significant shake-up and shake-off processes that
result in photoionization-excitation and double photoionization contributions
to the total cross section. The ultimate purpose for developing a definitive
model for oxygen absorption is to resolve standing discrepancies between the
astronomically observed and laboratory measured line positions, and between the
inferred atomic and molecular oxygen abundances in the interstellar medium from
XSTAR and SPEX spectral models
Muon-fluorine entangled states in molecular magnets
The information accessible from a muon-spin relaxation experiment is often
limited since we lack knowledge of the precise muon stopping site. We
demonstrate here the possibility of localizing a spin polarized muon in a known
stopping state in a molecular material containing fluorine. The muon-spin
precession that results from the entangled nature of the muon-spin and
surrounding nuclear spins is sensitive to the nature of the stopping site and
we use this property to identify three classes of site. We are also able to
describe the extent to which the muon distorts its surroundings.Comment: 4 pages, 3 figure
Nondipole Effects in the Photoionization of Xe 4d5/2 and 4d3/2: Evidence for Quadrupole Satellites
Measurements of nondipole parameters in spin-orbit-resolved Xe 4d photoionization demonstrate dynamical differences arising from relativistic effects. The experimental data do not agree with relativistic random-phase approximation calculations of single ionization dipole and quadrupole channels. It is suggested that the discrepancy is due to the omission of multiple-excitation quadrupole channels, i.e., quadrupole satellite transitions
Magnetic properties of the S=1/2 quasi square lattice antiferromagnet CuF2(H2O)2(pyz) (pyz=pyrazine) investigated by neutron scattering
We have performed elastic and inelastic neutron experiments on single crystal
samples of the coordination polymer compound CuF2(H2O)2(pyz) (pyz=pyrazine) to
study the magnetic structure and excitations. The elastic neutron diffraction
measurements indicate a collinear antiferromagnetic structure with moments
oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60
+/- 0.03 muB/Cu. This value is significantly smaller than the single ion
magnetic moment, reflecting the presence of strong quantum fluctuations. The
spin wave dispersion from magnetic zone center to the zone boundary points (0.5
1.5 0) and (0.5 0 1.5) can be described by a two dimensional Heisenberg model
with a nearest neighbor magnetic exchange constant J2d = 0.934 +/-0.0025 meV.
The inter-layer interaction Jperp in this compound is less than 1.5% of J2d.
The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced
when compared to the (0.5 1 0.5) zone boundary point by ~10.3 +/- 1.4 %. This
zone boundary dispersion is consistent with quantum Monte Carlo and series
expansion calculations which include corrections for quantum fluctuations to
linear spin wave theory.Comment: 7 pages, 6 figure
Singlet levels of the NV−centre in diamond
The characteristic transition of theNV− centre at 637 nm is between A
3
2 and E
3 triplet states. There
are also intermediate A
1
1 and E
1 singlet states, and the infrared transition at 1042 nm between these
singlets is studied here using uniaxial stress. The stress shift and splitting parameters are determined,
and the physical interaction giving rise to the parameters is considered within the accepted electronic
model of the centre. It is established that this interaction for the infrared transition is due to a modification
of electron–electron Coulomb repulsion interaction. This is in contrast to the visible 637 nm
transition where shifts and splittings arise from modification to the one-electron Coulomb interaction.
It is also established that a dynamic Jahn–Teller interaction is associated with the singlet E
1 state,
which gives rise to a vibronic level 115 cm−1 above the E
1 electronic state. Arguments associated with
this level are used to provide experimental confirmation that the A
1
1 is the upper singlet level and E
1 is
the lower singlet level.This work was supported by the Australian Research Council (DP 120102232)
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Calculation of Atomic Data for NASA Missions
The interpretation of cosmic spectra relies on a vast sea of atomic data which are not readily obtainable from analytic expressions or simple calculations. Rather, their evaluation typically requires state-of-the-art atomic physics calculations, with the inclusion of weaker effects (spin-orbit and configuration interactions, relaxation, Auger broadening, etc.), to achieve the level of accuracy needed for use by astrophysicists. Our NASA-supported research program is focused on calculating data for three important atomic processes, 1) dielectronic recombination (DR), 2) inner-shell photoabsorption, and 3) fluorescence and Auger decay of inner-shell vacancy states. Some additional details and examples of our recent findings are given below
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