arXiv subjects

C. Harabati

Publications and source records attributed to C. Harabati.

16 recordsLinked to original sources

arXiv2017

Effect of nuclear quadrupole moment on parity nonconservation in atoms

Nuclei with spin $I \ge 1$ have a weak quadrupole moment which leads to tensor contribution to the parity non-conserving interaction between nuclei and electrons. We calculate this contribution for Yb$^+$, Fr and Ra$^+$ and found it to be small. In contrast, in many lanthanides (e.g., Nd, Gd, Dy, Ho, Er, Pr, Sm) and Ra close levels of opposite parity lead to strong enhancement of the effect making it sufficiently large to be measured. Another possibility is to measure the PNC transitions between the hyperfine components of the ground state of Bi. Since nuclear weak charge is dominated by neutrons this opens a way of measuring quadrupole moments of neutron distribution in nuclei.

physics.atom-ph↗

arXiv2016

Electron recombination with tungsten ions with open f-shells

We calculate the electron recombination rates with target ions W$^{q+}$, $q = 18$ -- $25$, as functions of electron energy and electron temperature (i.e. the rates integrated over the Maxwellian velocity distribution). Comparison with available experimental data for W$^{18+}$, W$^{19+}$, and W$^{20+}$ is used as a test of our calculations. Our predictions for W$^{21+}$, W$^{22+}$, W$^{23+}$, W$^{24+}$, and W$^{25+}$ (where the experimental data are not available) may be used for plasma modelling in thermonuclear reactors. The results for the temperature dependent rates for each ion are fitted with the standard analytical expressions to make them easy to use. All of these ions have an open electron $f$-shell and have an extremely dense spectrum of chaotic many-electron compound resonances which enhance the recombination rates by 2-3 orders of magnitude in comparison with the direct electron recombination. Conventional dielectronic recombination theory is not directly applicable in this case. Instead, we developed a statistical theory based on the properties of chaotic eigenstates. This theory describes a multi-electronic recombination (extension of the dielectronic recombination) via many-excited-electron compound resonances.

physics.atom-ph↗

arXiv2016

Combining configuration interaction with perturbation theory for atoms with large number of valence electrons

A version of the configuration interaction (CI) method is developed which treats highly excited many-electron basis states perturbatively, so that their inclusion does not affect the size of the CI matrix. This removes, at least in principle, the main limitation of the CI method in dealing with many-electron atoms or ions. We perform calculations of the spectra of iodine and its ions, tungsten, and ytterbium as examples of atoms with open $s$, $p$, $d$ and $f$-shells. Good agreement of the calculated data with experiment illustrates the power of the method. Its advantages and limitations are discussed.

physics.atom-ph↗

arXiv2015

Level-resolved quantum statistical theory of electron capture into many-electron compound resonances in highly charged ions

The strong mixing of many-electron basis states in excited atoms and ions with open $f$ shells results in very large numbers of complex, chaotic eigenstates that cannot be computed to any degree of accuracy. Describing the processes which involve such states requires the use of a statistical theory. Electron capture into these 'compound resonances' leads to electron-ion recombination rates that are orders of magnitude greater than those of direct, radiative recombination, and cannot be described by standard theories of dielectronic recombination. Previous statistical theories considered this as a two-electron capture process which populates a pair of single-particle orbitals, followed by 'spreading' of the two-electron states into chaotically mixed eigenstates. This method is similar to a configuration-average approach, as it neglects potentially important effects of spectator electrons and conservation of total angular momentum. In this work we develop a statistical theory which considers electron capture into 'doorway' states with definite angular momentum obtained by the configuration interaction method. We apply this approach to electron recombination with W$^{20+}$, considering 2 million doorway states. Despite strong effects from the spectator electrons, we find that the results of the earlier theories largely hold. Finally, we extract the fluorescence yield (the probability of photoemission and hence recombination) by comparison with experiment.

physics.atom-ph↗

arXiv2015

Effects of Lorentz Symmetry Violation in the Spectra of Rare-Earth Ions in a Crystal Field

We demonstrate that experiments measuring the transition energies of rare-earth ions doped in crystalline lattices are sensitive to violations of Local Lorentz Invariance and Einstein's Equivalence Principle. Using the crystal field of LaCl$_{3}$ as an example, we calculate the frame-dependent energy shifts of the transition frequencies between low-lying states of Ce$^{3+}$, Nd$^{3+}$, and Er$^{3+}$ dopants in the context of the Standard Model Extension, and show that they have high sensitivity to electron anomalies that break rotational invariance.

hep-ph↗

arXiv2014

Effect of atomic electric quadrupole moment on positron binding

Effect of the electric quadrupole moment, $Q$, is studied for positron-atom bound systems. It is demonstrated that for $Q >50$ a.u. the electric quadrupole potential is sufficiently strong to bind positron (or electron) even in the absence of the dipole polarization potential. Such large values of $Q$ are not known for atomic ground states, however, they exist in molecules and excited atoms. In the state $2s2p~^3P^o_2$ of beryllium, the quadrupole contribution makes difference between stable bound state and decay to Be$^+$ ion and positronium. In a majority of atoms the quadrupole contribution is small and can be neglected.

physics.atom-ph↗

arXiv2014

Atoms which can bind positrons

Calculations of the positron binding energies to all atoms in the periodic table are presented and atoms where the positron-atom binding actually exists are identified. The results of these calculations and accurate calculations of other authors (which existed for several atoms only) are used to evaluate recommended values of the positron binding energies to the ground states of atoms. We also present the recommended energies of the positron excited bound levels and resonances (due to the binding of positron to excited states of atoms) which can not emit positronium and have relatively narrow widths. Such resonances in positron annihilation and scattering may be used to measure the positron binding energy.

physics.atom-ph↗

arXiv2013

Electron recombination, photoionization and scattering via many-electron compound resonances

Highly excited eigenstates of atoms and ions with open f shell are chaotic superpositions of thousands, or even millions of Hartree-Fock determinant states. The interaction between dielectronic and multielectronic configurations leads to the broadening of dielectronic recombination resonances and relative enhancement of photon emission due to opening of thousands of radiative decay channels. The radiative yield is close to 100% for electron energy . 1 eV and rapidly decreases for higher energies due to opening of many autoionization channels. The same mechanism predicts suppression of photoionization and relative enhancement of the Raman scattering. Results of our calculations of the recombination rate are in agreement with the experimental data for W20+ and Au25+.

physics.atom-ph↗

arXiv2013

Limits on Violations of Lorentz Symmetry and the Einstein Equivalence Principle using Radio-Frequency Spectroscopy of Atomic Dysprosium

We report a joint test of local Lorentz invariance and the Einstein equivalence principle for electrons, using long-term measurements of the transition frequency between two nearly degenerate states of atomic dysprosium. We present many-body calculations which demonstrate that the energy splitting of these states is particularly sensitive to violations of both special and general relativity. We limit Lorentz violation for electrons at the level of $10^{-17}$, matching or improving the best laboratory and astrophysical limits by up to a factor of 10, and improve bounds on gravitational redshift anomalies for electrons by 2 orders of magnitude, to $10^{-8}$. With some enhancements, our experiment may be sensitive to Lorentz violation at the level of $9\times 10^{-20}$.

hep-ph↗

arXiv2012

Relativistic coupled-cluster single-double calculations of positron-atom bound states

Relativistic coupled-cluster single-double approximation is used to calculate positron-atom bound states. The method is tested on closed-shell atoms such as Be, Mg, Ca, Zn, Cd, and Hg where a number of accurate calculations is available. It is then used to calculate positron binding energies for a range of open-shell transition metal atoms from Sc to Cu, from Y to Pd, and from Lu to Pt. These systems possess Feshbach resonances, which can be used to search for positron-atom binding experimentally through resonant annihilation or scattering.

physics.atom-ph↗

arXiv2012

Chaos-induced enhancement of resonant multielectron recombination in highly charged ions: Statistical theory

A statistical theory of resonant multielectron recombination based on properties of chaotic eigenstates is developed. The level density of many-body states increases exponentially with the number of excited electrons. When the residual electron-electron interaction exceeds the interval between these levels, the eigenstates (called compound states or compound resonances if these states are in the continuum) become "chaotic" superpositions of large numbers of Hartree-Fock configurational basis states. This situation takes place in some rare-earth atoms and many open-shell multiply charged ions excited in the process of electron recombination. Our theory describes resonant multielectron recombination via dielectronic doorway states leading to such compound resonances. The result is a radiative capture cross section averaged over a small energy interval containing several compound resonances. In many cases individual resonances are not resolved experimentally (since the interval between them is small, e.g., $\le 1$ meV, possibly even smaller than their radiative widths), therefore, our statistical theory should correctly describe the experimental data. We perform numerical calculations of the recombination cross sections for tungsten ions W$^{q+}$, $q=18$--25. The recombination rate for W$^{20+}$ measured recently [Phys. Rev. A {\bf 83}, 012711 (2011)] is $10^3$ greater than the direct radiative recombination rate at low energies, and our result for W$^{20+}$ agrees with the measurements.

physics.atom-ph↗

arXiv2012

Relations between matrix elements of different weak interactions and interpretation of the PNC and EDM measurements in atoms and molecules

The relations between matrix elements of different P- and T-odd weak interactions are derived. We demonstrate that similar relations hold for parity nonconserving (PNC) transition amplitudes and electron electric dipole moments (EDM) of atoms and molecules. This allows to express P- and T-odd effects in many-electron systems caused by different symmetry-breaking mechanisms via each other using simple analytical formulas. We use these relations for the interpretation of the anapole moment measurements in cesium and thallium and for the analysis of the relative contributions of the scalar-pseudoscalar CP-odd weak interaction and electron EDM to the EDM of Cs, Tl, Fr and other atoms and many polar molecules (YbF, PbO, ThO, etc.). Model-independent limits on electron EDM and the parameter of the scalar-pseudoscalar CP-odd interaction are found from the analysis of the EDM measurements for Tl and YbF.

physics.atom-ph↗

arXiv2003

Long-time and unitary properties of semiclassical initial value representations

We numerically compare the semiclassical ``frozen Gaussian'' Herman-Kluk propagator [Chem. Phys. 91, 27 (1984)] and the ``thawed Gaussian'' propagator put forward recently by Baranger et al. [J. Phys. A 34, 7227 (2001)] by studying the quantum dynamics in some nonlinear one-dimensional potentials. The reasons for the lack of long time accuracy and norm conservation in the latter method are uncovered. We amend the thawed Gaussian propagator with a global harmonic approximation for the stability of the trajectories and demonstrate that this revised propagator is a true alternative to the Herman-Kluk propagator with similar accuracy.

quant-ph↗

arXiv2001

Electron recombination with multicharged ions via chaotic many-electron states

We show that a dense spectrum of chaotic multiply-excited eigenstates can play a major role in collision processes involving many-electron multicharged ions. A statistical theory based on chaotic properties of the eigenstates enables one to obtain relevant energy-averaged cross sections in terms of sums over single-electron orbitals. Our calculation of the low-energy electron recombination of Au$^{25+}$ shows that the resonant process is 200 times more intense than direct radiative recombination, which explains the recent experimental results of Hoffknecht {\em et al.} [J. Phys. B {\bf 31}, 2415 (1998)].

physics.atom-ph↗

arXiv2000

Calculation of positron binding to silver and gold atoms

Positron binding to silver and gold atoms was studied using a fully {\it ab initio} relativistic method, which combines the configuration interaction method with many-body perturbation theory. It was found that the silver atom forms a bound state with a positron with binding energy 123 ($\pm$ 20%) meV, while the gold atom cannot bind a positron. Our calculations reveal the importance of the relativistic effects for positron binding to heavy atoms. The role of these effects was studied by varying the value of the fine structure constant $α$. In the non-relativistic limit, $α= 0$, both systems $e^+$Ag and $e^+$Au are bound with binding energies of about 200 meV for $e^+$Ag and 220 meV for $e^+$Au. Relativistic corrections for a negative ion are essentially different from that for a positron interacting with an atom. Therefore, the calculation of electron affinities cannot serve as a test of the method used for positron binding in the non-relativistic case. However, it is still a good test of the relativistic calculations. Our calculated electron affinities for silver (1.327 eV) and gold (2.307 eV) atoms are in very good agreement with corresponding experimental values (1.303 eV and 2.309 eV respectively).

physics.atom-ph↗

arXiv1999

Calculation of the positron bound state with the copper atom

A new relativistic method for calculation of positron binding to atoms is presented. The method combines a configuration interaction treatment of the valence electron and the positron with a many-body perturbation theory description of their interaction with the atomic core. We apply this method to positron binding by the copper atom and obtain the binding energy of 170 meV (+ - 10%). To check the accuracy of the method we use a similar approach to calculate the negative copper ion. The calculated electron affinity is 1.218 eV, in good agreement with the experimental value of 1.236 eV. The problem of convergence of positron-atom bound state calculations is investigated, and means to improve it are discussed. The relativistic character of the method and its satisfactory convergence make it a suitable tool for heavier atoms.

physics.atom-ph↗