2,235 research outputs found
Program to calculate pure angular momentum coefficients in jj-coupling
A program for computing pure angular momentum coefficients in relativistic
atomic structure for any scalar one- and two-particle operator is presented.
The program, written in Fortran 90/95 and based on techniques of second
quantization, irreducible tensorial operators, quasispin and the theory of
angular momentum, is intended to replace existing angular coefficient modules
from GRASP92. The new module uses a different decomposition of the coefficients
as sums of products of pure angular momentum coefficients, which depend only on
the tensor rank of the interaction but not on its details, with effective
interaction strengths of specific interactions. This saves memory and reduces
the computational cost of big calculations signficantly
Scattering of twisted relativistic electrons by atoms
The Mott scattering of high-energetic twisted electrons by atoms is
investigated within the framework of the first Born approximation and Dirac's
relativistic equation. Special emphasis is placed on the angular distribution
and longitudinal polarization of the scattered electrons. In order to evaluate
these angular and polarization properties we consider two experimental setups
in which the twisted electron beam collides with either a single well-localized
atom or macroscopic atomic target. Detailed relativistic calculations have been
performed for both setups and for the electrons with kinetic energy from 10 keV
to 1000 keV. The results of these calculations indicate that the emission
pattern and polarization of outgoing electrons differ significantly from the
scattering of plane-wave electrons and can be very sensitive to the parameters
of the incident twisted beam. In particular, it is shown that the angular- and
polarization-sensitive Mott measurements may reveal valuable information about,
both the transverse and longitudinal components of the linear momentum and the
projection of the total angular momentum of twisted electron states. Thus, the
Mott scattering emerges as a diagnostic tool for the relativistic vortex beams.Comment: 12 pages, 4 figure
Angular distribution studies on the two-photon ionization of hydrogen-like ions: Relativistic description
The angular distribution of the emitted electrons, following the two-photon
ionization of the hydrogen-like ions, is studied within the framework of second
order perturbation theory and the Dirac equation. Using a density matrix
approach, we have investigated the effects which arise from the polarization of
the incoming light as well as from the higher multipoles in the expansion of
the electron--photon interaction. For medium- and high-Z ions, in particular,
the non-dipole contributions give rise to a significant change in the angular
distribution of the emitted electrons, if compared with the electric-dipole
approximation. This includes a strong forward emission while, in dipole
approxmation, the electron emission always occurs symmetric with respect to the
plane which is perpendicular to the photon beam. Detailed computations for the
dependence of the photoelectron angular distributions on the polarization of
the incident light are carried out for the ionization of H, Xe, and
U (hydrogen-like) ions.Comment: 16 pages, 4 figures, published in J Phys
Electron-ion recombination of Si IV forming Si III: Storage-ring measurement and multiconfiguration Dirac-Fock calculations
The electron-ion recombination rate coefficient for Si IV forming Si III was
measured at the heavy-ion storage-ring TSR. The experimental electron-ion
collision energy range of 0-186 eV encompassed the 2p(6) nl n'l' dielectronic
recombination (DR) resonances associated with 3s to nl core excitations, 2s
2p(6) 3s nl n'l' resonances associated with 2s to nl (n=3,4) core excitations,
and 2p(5) 3s nl n'l' resonances associated with 2p to nl (n=3,...,infinity)
core excitations. The experimental DR results are compared with theoretical
calculations using the multiconfiguration Dirac-Fock (MCDF) method for DR via
the 3s to 3p n'l' and 3s to 3d n'l' (both n'=3,...,6) and 2p(5) 3s 3l n'l'
(n'=3,4) capture channels. Finally, the experimental and theoretical plasma DR
rate coefficients for Si IV forming Si III are derived and compared with
previously available results.Comment: 13 pages, 9 figures, 3 tables. Accepted for publication in Physical
Review
Parametrization of the angular correlation and degree of linear polarization in two-photon decays of hydrogen-like ions
The two-photon decay in hydrogen-like ions is investigated within the
framework of second order perturbation theory and Dirac's relativistic
equation. Special attention is paid to the angular correlation of the emitted
photons as well as to the degree of linear polarization of one of the two
photons, if the second is just observed under given angles. Expressions for the
angular correlation and the degree of linear polarization are expanded in terms
of -polynomials, whose coefficients depend on the atomic number and
the energy sharing of the emitted photons. The effects of including higher
(electric and magnetic) multipoles upon the emitted photon pairs beyond the
electric-dipole approximation are also discussed. Calculations of the
coefficients are performed for the transitions ,
and , along the
entire hydrogen isoelectronic sequence ()
Two-photon excitation and relaxation of the 3d-4d resonance in atomic Kr
Two-photon excitation of a single-photon forbidden Auger resonance has been observed and investigated using the intense extreme ultraviolet radiation from the free electron laser in Hamburg. At the wavelength 26.9 nm (46 eV) two photons promoted a 3d core electron to the outer 4d shell. The subsequent Auger decay, as well as several nonlinear above threshold ionization processes, were studied by electron spectroscopy. The experimental data are in excellent agreement with theoretical predictions and analysis of the underlying multiphoton processes
Role of Heterogeneities in Staebler-Wronski Effect
The effect of light soaking (LS) on the properties of hydrogenated amorphous
silicon presents many challenging puzzles. Some of them are discussed here,
along with their present status. In particular the role of the heterogeneities
in LS is examined. We find that for the majority of the solved as well unsolved
puzzles the long range potential fluctuations arising from the heterogeneities
in the films can provide answers which look quite plausible.Comment: 10 pages, 7 figure
Relativistic and retardation effects in the two--photon ionization of hydrogen--like ions
The non-resonant two-photon ionization of hydrogen-like ions is studied in
second-order perturbation theory, based on the Dirac equation. To carry out the
summation over the complete Coulomb spectrum, a Green function approach has
been applied to the computation of the ionization cross sections. Exact
second-order relativistic cross sections are compared with data as obtained
from a relativistic long-wavelength approximation as well as from the scaling
of non-relativistic results. For high-Z ions, the relativistic wavefunction
contraction may lower the two-photon ionization cross sections by a factor of
two or more, while retardation effects appear less pronounced but still give
rise to non-negligible contributions.Comment: 6 pages, 2 figure
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