184 research outputs found
Collisions of antiprotons with hydrogen molecular ions
Time-dependent close-coupling calculations of the ionization and excitation
cross section for antiproton collisions with molecular hydrogen ions are
performed in an impact-energy range from 0.5 keV to 10 MeV. The
Born-Oppenheimer and Franck-Condon approximations as well as the impact
parameter method are applied in order to describe the target molecule and the
collision process. It is shown that three perpendicular orientations of the
molecular axis with respect to the trajectory are sufficient to accurately
reproduce the ionization cross section calculated by [Sakimoto, Phys. Rev. A
71, 062704 (2005)] reducing the numerical effort drastically. The
independent-event model is employed to approximate the cross section for double
ionization and H+ production in antiproton collisions with H2.Comment: 12 pages, 5 figures, 4 table
Continuum states from time-dependent density functional theory
Linear response time-dependent density functional theory is used to study
low-lying electronic continuum states of targets that can bind an extra
electron. Exact formulas to extract scattering amplitudes from the
susceptibility are derived in one dimension. A single-pole approximation for
scattering phase shifts in three dimensions is shown to be more accurate than
static exchange for singlet electron-He scattering.Comment: 5 pages, 2 figures, J. Chem. Phys. accepte
A numerical study of two-photon ionization of helium using the Pyprop framework
Few-photon induced breakup of helium is studied using a newly developed ab
initio numerical framework for solving the six-dimensional time-dependent
Schroedinger equation. We present details of the method and calculate
(generalized) cross sections for the process of two-photon nonsequential
(direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, a
process that has been very much debated in recent years and is not yet fully
understood. In particular, we have studied the convergence property of the
total cross section in the vicinity of the upper threshold (54.4 eV), versus
the pulse duration of the applied laser field. We find that the cross section
exhibits an increasing trend near the threshold, as has also been observed by
others, and show that this rise cannot solely be attributed to an unintended
inclusion of the sequential two-photon double ionization process, caused by the
bandwidth of the applied field.Comment: 7 pages, 3 figure
Polarization Observables for Two-Pion Production off the Nucleon
We develop polarization observables for the processes
and , using both a helicity and hybrid helicity-transversity
basis. Such observables are crucial if processes that produce final states
consisting of a spin-1/2 baryon and two pseudoscalar mesons are to be fully
exploited for baryon spectroscopy. We derive relationships among the
observables, as well as inequalities that they must satisfy. We also discuss
the observables that must be measured in `complete' experiments, and briefly
examine the prospects for measurement of some of these observables in the near
future.Comment: 20 pages, using revtex
Development of a polarization resolved spectroscopic diagnostic for measurements of the vector magnetic field in the Caltech coaxial magnetized plasma jet experiment
In the Caltech coaxial magnetized plasma jet experiment, fundamental studies are carried out relevant
to spheromak formation, astrophysical jet formation/propagation, solar coronal physics, and
the general behavior of twisted magnetic flux tubes that intercept a boundary. In order to measure the
spatial profile of the magnetic field vector for understanding the underlying physics governing the dynamical
behavior, a non-perturbing visible emission spectroscopic method is implemented to observe
the Zeeman splitting in emission spectra. We have designed and constructed a polarization-resolving
optical system that can simultaneously detect the left- and right-circularly polarized emission. The
system is applied to singly ionized nitrogen spectral lines. The magnetic field strength is measured
with a precision of about Âą13 mT. The radial profiles of the azimuthal and axial vector magnetic
field components are resolved by using an inversion method
Validity of the WKB Approximation in Calculating the Asymptotic Quasinormal Modes of Black Holes
In this paper, we categorize non-rotating black hole spacetimes based on
their pole structure and in each of these categories we determine whether the
WKB approximation is a valid approximation for calculating the asymptotic
quasinormal modes. We show that Schwarzschild black holes with the Gauss-Bonnet
correction belong to the category in which the WKB approximation is invalid for
calculating these modes. In this context, we further discuss and clarify some
of the ambiguity in the literature surrounding the validity conditions provided
for the WKB approximation.Comment: 10 page
Spheroidal galactic halos and mirror dark matter
Mirror matter has been proposed as a dark matter candidate. It has several
very attractive features, including automatic stability and darkness, the
ability to mimic the broad features of cold dark matter while in the linear
density perturbation regime, and consistency with all direct dark matter search
experiments, both negative (e.g. CDMS II) and positive (DAMA). In this paper we
consider an important unsolved problem: Are there plausible reasons to explain
why most of the mirror matter in spiral galaxies exists in the form of gaseous
{\it spheroidal} galactic halos around ordinary matter {\it disks}? We compute
an order-of-magnitude estimate that the mirror photon luminosity of a typical
spiral galaxy today is around erg/s. Interestingly, this rate of
energy loss is similar to the power supplied by ordinary supernova explosions.
We discuss circumstances under which supernova power can be used to heat the
gaseous part of the mirror matter halo and hence prevent its collapse to a
disk. The {\it macro}scopic ordinary-mirror asymmetry plays a fundamental role
in our analysis.Comment: about 6 page
Electronic excitation of carbonyl sulphide (COS) by high-resolution vacuum ultraviolet photoabsorption and electron-impact spectroscopy in the energy region from 4 to 11 eV
The electronic state spectroscopy of carbonyl sulphide, COS, has been investigated using high resolution vacuum ultraviolet photoabsorption spectroscopy and electron energy loss spectroscopy in the energy range of 4.0â10.8 eV. The spectrum reveals several new features not previously reported in the literature. Vibronic structure has been observed, notably in the low energy absorption dipole forbidden band assigned to the (4Ďâ3Ď) (1Îâ1ÎŁ+) transition, with a new weak transition assigned to (1ÎŁââ1ÎŁ+) reported here for the first time. The absolute optical oscillator strengths are determined for ground state to 1ÎŁ+ and 1Î transitions. Based on our recent measurements of differential cross sections for the optically allowed (1ÎŁ+ and 1Î ) transitions of COS by electron impact, the optical oscillator strength f0 value and integral cross sections (ICSs) are derived by applying a generalized oscillator strength analysis. Subsequently, ICSs predicted by the scaling are confirmed down to 60 eV in the intermediate energy region. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of carbonyl sulphide in the upper stratosphere (20â50 km)
Unambiguous comparison of the states of multiple quantum systems
We consider N quantum systems initially prepared in pure states and address
the problem of unambiguously comparing them. One may ask whether or not all
systems are in the same state. Alternatively, one may ask whether or not the
states of all N systems are different. We investigate the possibility of
unambiguously obtaining this kind of information. It is found that some
unambiguous comparison tasks are possible only when certain linear independence
conditions are satisfied. We also obtain measurement strategies for certain
comparison tasks which are optimal under a broad range of circumstances, in
particular when the states are completely unknown. Such strategies, which we
call universal comparison strategies, are found to have intriguing connections
with the problem of quantifying the distinguishability of a set of quantum
states and also with unresolved conjectures in linear algebra. We finally
investigate a potential generalisation of unambiguous state comparison, which
we term unambiguous overlap filtering.Comment: 20 pages, no figure
Spontaneous emission from large quantum dots in nanostructures: exciton-photon interaction beyond the dipole approximation
We derive a rigorous theory of the interaction between photons and spatially
extended excitons confined in quantum dots in inhomogeneous photonic materials.
We show that, beyond the dipole approximation, the radiative decay rate is
proportional to a non-local interaction function, which describes the
interaction between light and spatially extended excitons. In this regime,
light and matter degrees of freedom cannot be separated and a complex interplay
between the nanostructured optical environment and the exciton envelope
function emerges. We illustrate this by specific examples and derive a series
of important analytical relations, which are useful for applying the formalism
to practical problems. In the dipole limit, the decay rate is proportional to
the projected local density of optical states and we obtain the strong and weak
confinement regimes as special cases.Comment: 14 pages, 4 figure
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