184 research outputs found
Large-scale Multiconfiguration Hartree-fock and Configuration-interaction Calculations of Isotope Shifts and Hyperfine Structures In Boron
A new isotope shift program, part of the multiconfiguration Hartree-Fock (MCHF) atomic structure package, has been written and tested. The program calculates the isotope shift of an atomic level from MCHF or configuration-interaction (CI) wave functions. The program is specially designed to be used with very large CI expansions, for which angular data cannot be stored on disk. To explore the capacity of the program, large-scale isotope shift calculations were performed for the 1s(2)2s(2)2p 2P, 1s(2)2s(2)3s2S, and 1s(2)2s2p2 2D levels in boron. From the isotope shifts of these levels the transition isotope shifts were calculated for the two resonance transitions. The calculated transition isotope shifts are in very good agreement with experimental shifts. As an additional test of the quality of the CI wave functions, the hyperfine structure was calculated for all levels
The Multi-Configurational Hartree-Fock close-coupling ansatz: application to Argon photoionization cross section and delays
We present a robust, ab initio method for addressing atom-light interactions
and apply it to photoionization of argon. We use a close-coupling ansatz
constructed on a multi-configurational Hartree-Fock description of localized
states and B-spline expansions of the electron radial wave functions. In this
implementation, the general many-electron problem can be tackled thanks to the
use of the ATSP2K libraries [CPC 176 (2007) 559]. In the present contribution,
we combine this method with exterior complex scaling, thereby allowing for the
computation of the complex partial amplitudes that encode the whole dynamics of
the photoionization process. The method is validated on the 3s3p6np series of
resonances converging to the 3s extraction. Then, it is used for computing the
energy dependent differential atomic delay between 3p and 3s photoemission, and
agreement is found with the measurements of Gu\'enot et al. [PRA 85 (2012)
053424]. The effect of the presence of resonances in the one-photon spectrum on
photoionization delay measurements is studied.Comment: 15 pages, 8 figures, 4 table
Exploring Biorthonormal Transformations of Pair-Correlation Functions in Atomic Structure Variational Calculations
Multiconfiguration expansions frequently target valence correlation and
correlation between valence electrons and the outermost core electrons.
Correlation within the core is often neglected. A large orbital basis is needed
to saturate both the valence and core-valence correlation effects. This in turn
leads to huge numbers of CSFs, many of which are unimportant. To avoid the
problems inherent to the use of a single common orthonormal orbital basis for
all correlation effects in the MCHF method, we propose to optimize independent
MCHF pair-correlation functions (PCFs), bringing their own orthonormal
one-electron basis. Each PCF is generated by allowing single- and double-
excitations from a multireference (MR) function. This computational scheme has
the advantage of using targeted and optimally localized orbital sets for each
PCF. These pair-correlation functions are coupled together and with each
component of the MR space through a low dimension generalized eigenvalue
problem. Nonorthogonal orbital sets being involved, the interaction and overlap
matrices are built using biorthonormal transformation of the coupled basis sets
followed by a counter-transformation of the PCF expansions.
Applied to the ground state of beryllium, the new method gives total energies
that are lower than the ones from traditional CAS-MCHF calculations using large
orbital active sets. It is fair to say that we now have the possibility to
account for, in a balanced way, correlation deep down in the atomic core in
variational calculations
Isotope shift on the chlorine electron affinity revisited by an MCHF/CI approach
Today, the electron affinity is experimentally well known for most of the
elements and is a useful guideline for developing ab initio computational
methods. However, the measurements of isotope shifts on the electron affinity
are limited by both resolution and sensitivity. In this context, theory
eventually contributes to the knowledge and understanding of atomic structures,
even though correlation plays a dominant role in negative ions properties and,
particularly, in the calculation of the specific mass shift contribution. The
present study solves the longstanding discrepancy between calculated and
measured specific mass shifts on the electron affinity of chlorine (Phys. Rev.
A 51 (1995) 231)Comment: 18 pages, 2 figures, 7 table
A theoretical study of the C- 4So_3/2 and 2Do_{3/2,5/2} bound states and C ground configuration: fine and hyperfine structures, isotope shifts and transition probabilities
This work is an ab initio study of the 2p3 4So_3/2, and 2Do_{3/2,5/2} states
of C- and 2p2 3P_{0,1,2}, 1D_2, and 1S_0 states of neutral carbon. We use the
multi-configuration Hartree-Fock approach, focusing on the accuracy of the wave
function itself. We obtain all C- detachment thresholds, including correlation
effects to about 0.5%. Isotope shifts and hyperfine structures are calculated.
The achieved accuracy of the latter is of the order of 0.1 MHz.
Intra-configuration transition probabilities are also estimated.Comment: 15 pages, 2 figures, 12 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
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