Search results for "ab initio"
showing 10 items of 990 documents
Hyperfine structure studies in the 6s 6p3P2-level of barium
1991
The hfs of the3P2 state of the 6s 6p configuration of atomic barium has been determined for137Ba, applying the collinear fast-beam laser spectroscopy. The data fill the last gap in the experimental information on low lying 6s2, 6s 6p, 6s 5d and 6p 5d configurations of this alcaline earth system. The results are analyzed in terms for the existing semi-empirical modified Breit-Wills theory (MBWT), working with two basic wave functions, as well as by an extension to six-component wave functions of inclusion of configuration interaction. The semi-empirical approaches lead to a consistent description of the different hyperfine structures; they are compared with the results from recent ab initio …
Average charge states of heavy and superheavy ions passing through a rarified gas: Theory and experiment
2013
The average charge states $\overline{q}$ of heavy and superheavy ions (atomic numbers $Z=80$--114) passing through He gas are studied experimentally and theoretically. Experimental data were measured at the gas-filled recoil separator, i.e., the TransActinide Separator and Chemistry Apparatus (TASCA) at GSI Darmstadt, for ion energies of a few hundred keV/u at gas pressures of 0.2 to 2.0 mbar. An attempt is made to describe experimental $\overline{q}$ values by means of atomic calculations of the binding energies and electron-loss and electron-capture cross sections. The influence of the gas-density effect is included in the calculations. The calculated $\overline{q}$ reproduce the experime…
Aluminum cluster anions: Photoelectron spectroscopy andab initiosimulations
2000
Atomic structures and geometries, electronic structure, and temperature-dependent photoelectron spectra of ${\mathrm{Al}}_{N}^{\ensuremath{-}} (N=19\ensuremath{-}102)$ clusters are studied both theoretically via ab initio local-density-functional simulations, and experimentally with high-resolution measurements. The use of a theoretically well-defined energy shift in conjunction with a generalized Koopmans' theorem enables direct comparisons between the calculated density of states and the experimental photoelectron spectrum. Such comparisons, using photoelectron spectra calculated for various relaxed cluster geometries, enables a determination of the optimal structures of the clusters. The…
Role of Chiral Two-Body Currents in Li6 Magnetic Properties in Light of a New Precision Measurement with the Relative Self-Absorption Technique
2021
A direct measurement of the decay width of the excited ${0}_{1}^{+}$ state of $^{6}\mathrm{Li}$ using the relative self-absorption technique is reported. Our value of ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma},{0}_{1}^{+}\ensuremath{\rightarrow}{1}_{1}^{+}}=8.17(14{)}_{\mathrm{stat}.}(11{)}_{\mathrm{syst}.}\text{ }\text{ }\mathrm{eV}$ provides sufficiently low experimental uncertainties to test modern theories of nuclear forces. The corresponding transition rate is compared to the results of ab initio calculations based on chiral effective field theory that take into account contributions to the magnetic dipole operator beyond leading order. This enables a precision test of the im…
Precision Test of Many-Body QED in theBe+2pFine Structure Doublet Using Short-Lived Isotopes
2015
Absolute transition frequencies of the $2s\text{ }{^{2}S}_{1/2}\ensuremath{\rightarrow}2p\text{ }{^{2}P}_{1/2,3/2}$ transitions in ${\mathrm{Be}}^{+}$ were measured for the isotopes $^{7,9--12}\mathrm{Be}$. The fine structure splitting of the $2p$ state and its isotope dependence are extracted and compared to results of ab initio calculations using explicitly correlated basis functions, including relativistic and quantum electrodynamics effects at the order of $m{\ensuremath{\alpha}}^{6}$ and $m{\ensuremath{\alpha}}^{7} \mathrm{ln} \ensuremath{\alpha}$. Accuracy has been improved in both the theory and experiment by 2 orders of magnitude, and good agreement is observed. This represents on…
Ab initio three-loop calculation of the W -exchange contribution to nonleptonic decays of double charm baryons
2019
We have made an ab initio three-loop quark model calculation of the $W$-exchange contribution to the nonleptonic two-body decays of the doubly charmed baryons $\Xi_{cc}^{++}$ and $\Omega_{cc}^{+}$. The $W$-exchange contributions appear in addition to the factorizable tree graph contributions and are not suppressed in general. We make use of the covariant confined quark model previously developed by us to calculate the tree graph as well as the $W$-exchange contribution. We calculate helicity amplitudes and quantitatively compare the tree graph and $W$-exchange contributions. Finally, we compare the calculated decay widths with those from other theoretical approaches when they are available.
Structural and vibrational study of Bi2Se3under high pressure
2011
The structural and vibrational properties of bismuth selenide (Bi${}_{2}$Se${}_{3}$) have been studied by means of x-ray diffraction and Raman scattering measurements up to 20 and 30 GPa, respectively. The measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Our experimental results evidence a phase transition from the low-pressure rhombohedral ($R$-3$m$) phase (\ensuremath{\alpha}-Bi${}_{2}$Se${}_{3}$) with sixfold coordination for Bi to a monoclinic $C$2/$m$ structure (\ensuremath{\beta}-Bi${}_{2}$Se${}_{3}$) with sevenfold coordination for Bi above 10 GPa. The equation of state and the pressure dependence of the lattice parameters and volume …
Vibrational properties of delafossiteCuGaO2at ambient and high pressures
2005
In this paper we investigate the vibrational properties of $\mathrm{Cu}\mathrm{Ga}{\mathrm{O}}_{2}$ delafossite by means of Raman experiments and ab initio calculations. Both investigations have been performed at ambient pressure and also at high pressure. The two Raman-active modes have frequencies ${w}_{{E}_{g}}=368\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and ${w}_{{A}_{1g}}=729\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and pressure coefficients $2.78\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{GPa}$ $({E}_{g})$ and $4.64\ifmmode\pm\else\text…
Ab initiostudy of compressedAr(H2)2: Structural stability and anomalous melting
2010
We study the structural stability and dynamical properties of $\text{Ar}{({\text{H}}_{2})}_{2}$ under pressure using first-principles and ab initio molecular-dynamics techniques. At low temperatures, $\text{Ar}{({\text{H}}_{2})}_{2}$ is found to stabilize in the cubic C15 Laves structure $({\text{MgCu}}_{2})$ and not in the hexagonal C14 Laves structure $({\text{MgZn}}_{2})$ as it has been assumed previously. Based on enthalpy energy and phonon calculations, we propose a temperature-induced ${\text{MgCu}}_{2}\ensuremath{\rightarrow}{\text{MgZn}}_{2}$ phase transition that may rationalize the existing discrepancies between the sets of Raman and infrared vibron measurements. Our AIMD simulati…
Precision Mass Measurements of Cr58–63 : Nuclear Collectivity Towards the N=40 Island of Inversion
2018
The neutron-rich isotopes $^{58-63}$Cr were produced for the first time at the ISOLDE facility and their masses were measured with the ISOLTRAP spectrometer. The new values are up to 300 times more precise than those in the literature and indicate significantly different nuclear structure from the new mass-surface trend. A gradual onset of deformation is found in this proton and neutron mid-shell region, which is a gateway to the second island of inversion around \emph{N}=40. In addition to comparisons with density-functional theory and large-scale shell-model calculations, we present predictions from the valence-space formulation of the \emph{ab initio} in-medium similarity renormalization…