14 research outputs found
A large-scale R-matrix calculation for electron-impact excitation of the Ne O-like ion
The five J levels within a or ground state complex provide
an excellent testing ground for the comparison of theoretical line ratios with
astrophysically observed values, in addition to providing valuable electron
temperature and density diagnostics. The low temperature nature of the line
ratios ensure that the theoretically derived values are sensitive to the
underlying atomic structure and electron-impact excitation rates. Previous
R-matrix calculations for the Ne O-like ion exhibit large spurious
structure in the cross sections at higher electron energies, which may affect
Maxwellian averaged rates even at low temperatures. Furthermore, there is an
absence of comprehensive excitation data between the excited states that may
provide newer diagnostics to compliment the more established lines discussed in
this paper. To resolve these issues, we present both a small scale 56-level
Breit-Pauli (BP) calculation and a large-scale 554 levels R-matrix Intermediate
Coupling Frame Transformation (ICFT) calculation that extends the scope and
validity of earlier JAJOM calculations both in terms of the atomic structure
and scattering cross sections. Our results provide a comprehensive
electron-impact excitation data set for all transitions to higher shells.
The fundamental atomic data for this O-like ion is subsequently used within a
collisional radiative framework to provide the line ratios across a range of
electron temperatures and densities of interest in astrophysical observations.Comment: 17 pages, 8 figure
K-shell photoionization of ground-state Li-like boron ions [B]: Experiment and Theory
Absolute cross sections for the K-shell photoionization of ground-state
Li-like boron [B(1s2s S)] ions were measured by employing the
ion-photon merged-beams technique at the Advanced Light Source synchrotron
radiation facility. The energy ranges 197.5--200.5 eV, 201.9--202.1 eV of the
[1s(2s\,2p)P]P and [1s(2s\,2p)P] P
resonances, respectively, were investigated using resolving powers of up to
17\,600. The energy range of the experiments was extended to about 238.2 eV
yielding energies of the most prominent
[1s(2\,n)]P resonances with an absolute accuracy
of the order of 130 ppm. The natural linewidths of the [1s(2s\,2p)P]
P and [1s(2s\,2p)P] P resonances were measured
to be meV and meV, respectively, which compare
favourably with theoretical results of 4.40 meV and 30.53 meV determined using
an intermediate coupling R-matrix method.Comment: 6 figures and 2 table
K-shell photoionization of ground-state Li-like carbon ions [C]: experiment, theory and comparison with time-reversed photorecombination
Absolute cross sections for the K-shell photoionization of ground-state
Li-like carbon [C(1s2s S)] ions were measured by employing the
ion-photon merged-beams technique at the Advanced Light Source. The energy
ranges 299.8--300.15 eV, 303.29--303.58 eV and 335.61--337.57 eV of the
[1s(2s2p)P]P, [1s(2s2p)P]P and [(1s2s)S 3p]P
resonances, respectively, were investigated using resolving powers of up to
6000. The autoionization linewidth of the [1s(2s2p)P]P resonance was
measured to be meV and compares favourably with a theoretical result
of 26 meV obtained from the intermediate coupling R-Matrix method. The present
photoionization cross section results are compared with the outcome from
photorecombination measurements by employing the principle of detailed balance.Comment: 3 figures and 2 table
Convergent close-coupling method for positron scattering from noble gases
We present the convergent close-coupling formulation for positron scattering from noble gases (Ne, Ar, Kr and Xe) within the single-center approximation. Target functions are described in a model of six p-electrons above an inert Hartree–Fock core with only one-electron excitations from the outer p6 shell allowed. Target states have been obtained using a Sturmian (Laguerre) basis in order to model coupling to ionization and positronium (Ps) formation channels. Such an approach is unable to yield explicit Ps-formation cross sections, but is valid below this threshold and above the ionization threshold. The present calculations are found to show good agreement with recent measurements
Theory and applications of atomic and ionic polarizabilities
Atomic polarization phenomena impinge upon a number of areas and processes in
physics. The dielectric constant and refractive index of any gas are examples
of macroscopic properties that are largely determined by the dipole
polarizability. When it comes to microscopic phenomena, the existence of
alkaline-earth anions and the recently discovered ability of positrons to bind
to many atoms are predominantly due to the polarization interaction. An
imperfect knowledge of atomic polarizabilities is presently looming as the
largest source of uncertainty in the new generation of optical frequency
standards. Accurate polarizabilities for the group I and II atoms and ions of
the periodic table have recently become available by a variety of techniques.
These include refined many-body perturbation theory and coupled-cluster
calculations sometimes combined with precise experimental data for selected
transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index
measurements in microwave cavities, ab initio calculations of atomic structures
using explicitly correlated wave functions, interferometry with atom beams, and
velocity changes of laser cooled atoms induced by an electric field. This
review examines existing theoretical methods of determining atomic and ionic
polarizabilities, and discusses their relevance to various applications with
particular emphasis on cold-atom physics and the metrology of atomic frequency
standards.Comment: Review paper, 44 page