1,141 research outputs found
Many-body theory calculations of positron binding to negative ions
A many-body theory approach developed by the authors [Phys. Rev. A 70, 032720
(2004)] is applied to positron bound states and annihilation rates in atomic
systems. Within the formalism, full account of virtual positronium (Ps)
formation is made by summing the electron-positron ladder diagram series, thus
enabling the theory to include all important many-body correlation effects in
the positron problem. Numerical calculations have been performed for positron
bound states with the hydrogen and halogen negative ions, also known as Ps
hydride and Ps halides. The Ps binding energies of 1.118, 2.718, 2.245, 1.873
and 1.393 eV and annihilation rates of 2.544, 2.482, 1.984, 1.913 and 1.809
ns, have been obtained for PsH, PsF, PsCl, PsBr and PsI, respectively.Comment: 19 pages, 13 figures, submitted to International Review of Atomic and
Molecular Physic
van der Waals coefficients for positronium interactions with atoms
The random-phase approximation with exchange (RPAE) is used with a -spline
basis to compute dynamic dipole polarizabilities of noble-gas atoms and several
other closed-shell atoms (Be, Mg, Ca, Zn, Sr, Cd, and Ba). From these, values
of the van der Waals constants for positronium interactions with these
atoms are determined and compared with existing data. Our best predictions of
for Ps--noble-gas pairs are expected to be accurate to within 1%, and to
within a few per cent for the alkaline earths. We also used accurate dynamic
dipole polarizabilities from the literature to compute the coefficients
for the alkali-metal atoms. Implications of increased values for Ps
scattering from more polarizable atoms are discussed.Comment: 6 pages, submitted to Physical Review
Positron scattering and annihilation on noble gas atoms
Positron scattering and annihilation on noble gas atoms below the positronium
formation threshold is studied ab initio using many-body theory methods. The
many-body theory provides a near-complete understanding of the
positron-noble-gas-atom system at these energies and yields accurate numerical
results. It accounts for positron-atom and electron-positron correlations,
e.g., polarization of the atom by the incident positron and the
non-perturbative process of virtual positronium formation. These correlations
have a large effect on the scattering dynamics and result in a strong
enhancement of the annihilation rates compared to the independent-particle
mean-field description. Computed elastic scattering cross sections are found to
be in good agreement with recent experimental results and Kohn variational and
convergent close-coupling calculations. The calculated values of the
annihilation rate parameter (effective number of electrons
participating in annihilation) rise steeply along the sequence of noble gas
atoms due to the increasing strength of the correlation effects, and agree well
with experimental data.Comment: 24 pages, 17 figure
Carbohydrate Metabolism in Drought-Stressed Leaves of Pigeonpea (Cajanus cajan)
Pigeonpea is a tropical grain-legume, which is highly dehydration tolerant. The effect of drought stress on the carbohydrate metabolism in mature pigeonpea leaves was investigated by withholding water from plants grown in very large pots (50 kg of soil). The most striking feature of drought-stressed plants was the pronounced accumulation of D-pinitol (1D-3-methyl-chiro-inositol), which increased from 14 to 85 mg g−1 dry weight during a 27 d stress period. Concomitantly, the levels of starch, sucrose and the pinitol precursors myo-inositol and ononitol all decreased rapidly to zero or near-zero in response to drought. The levels of glucose and fructose increased moderately. Drought stress induced a pronounced increase of the activities of enzymes hydrolysing soluble starch (amylases) and sucrose (invertase and sucrose synthase). The two anabolic enzymes sucrose phosphate synthase (sucrose synthetic pathway) and myo-inositol methyl transferase (pinitol synthetic pathway) also showed an increase of activity during stress. These results indicate that pinitol accumulated in pigeonpea leaves, because the carbon flux was diverted from starch and sucrose into polyol
Relativistic coupled-cluster single-double calculations of positron-atom bound states
Relativistic coupled-cluster single-double approximation is used to calculate
positron-atom bound states. The method is tested on closed-shell atoms such as
Be, Mg, Ca, Zn, Cd, and Hg where a number of accurate calculations is
available. It is then used to calculate positron binding energies for a range
of open-shell transition metal atoms from Sc to Cu, from Y to Pd, and from Lu
to Pt. These systems possess Feshbach resonances, which can be used to search
for positron-atom binding experimentally through resonant annihilation or
scattering.Comment: submitted to Phys. Rev.
The Mass Profile and Accretion History of Cold Dark Matter Halos
We use the Millennium Simulation series to study the relation between the
accretion history (MAH) and mass profile of cold dark matter halos. We find
that the mean density within the scale radius, r_{-2} (where the halo density
profile has isothermal slope), is directly proportional to the critical density
of the Universe at the time when the main progenitor's virial mass equals the
mass enclosed within r_{-2}. Scaled to these characteristic values of mass and
density, the mean MAH, expressed in terms of the critical density of the
Universe, M(\rho_{crit}(z)), resembles that of the enclosed density profile,
M(), at z=0. Both follow closely the NFW profile, suggesting that the
similarity of halo mass profiles originates from the mass-independence of halo
MAHs. Support for this interpretation is provided by outlier halos whose
accretion histories deviate from the NFW shape; their mass profiles show
correlated deviations from NFW and are better approximated by Einasto profiles.
Fitting both M() and M(\rho_{crit}) with either NFW or Einasto profiles
yield concentration and shape parameters that are correlated, confirming and
extending earlier work linking the concentration of a halo with its accretion
history. These correlations also confirm that halo structure is insensitive to
initial conditions: only halos whose accretion histories differ greatly from
the NFW shape show noticeable deviations from NFW in their mass profiles. As a
result, the NFW profile provides acceptable fits to hot dark matter halos,
which do not form hierarchically, and for fluctuation power spectra other than
CDM. Our findings, however, predict a subtle but systematic dependence of mass
profile shape on accretion history which, if confirmed, would provide strong
support for the link between accretion history and halo structure we propose
here.Comment: 12 pages, 8 figures, MNRAS 432 1103L (2013
Hyper-Ramsey Spectroscopy of Optical Clock Transitions
We present non-standard optical Ramsey schemes that use pulses individually
tailored in duration, phase, and frequency to cancel spurious frequency shifts
related to the excitation itself. In particular, the field shifts and their
uncertainties of Ramsey fringes can be radically suppressed (by 2-4 orders of
magnitude) in comparison with the usual Ramsey method (using two equal pulses)
as well as with single-pulse Rabi spectroscopy. Atom interferometers and
optical clocks based on two-photon transitions, heavily forbidden transitions,
or magnetically induced spectroscopy could significantly benefit from this
method. In the latter case these frequency shifts can be suppressed
considerably below a fractional level of 10^{-17}. Moreover, our approach opens
the door for the high-precision optical clocks based on direct frequency comb
spectroscopy.Comment: 5 pages, 4 figure
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