arXiv subjects

G. F. Gribakin

Publications and source records attributed to G. F. Gribakin.

At least 55 records · Page 3Linked to original sources

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

Dense spectrum of resonances and particle capture in a near-black-hole metric

We show that a quantum scalar particle in the gravitational field of a massive body of radius R which slightly exceeds the Schwarzschild radius r_s, possesses a dense spectrum of narrow resonances. Their lifetimes and density tend to infinity in the limit R -> r_s. We determine the cross section of the particle capture into these resonances and show that it is equal to the absorption cross section for a Schwarzschild black hole. Thus, a non-singular static metric acquires black-hole properties before the actual formation of a black hole.

gr-qc↗

arXiv2012

Effect of positron-atom interactions on the annihilation gamma spectra of molecules

Calculations of gamma spectra for positron annihilation on a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The annihilation gamma spectra characterise the momentum distribution of the electron-positron pair at the instant of annihilation. The contribution to the gamma spectra from individual molecular orbitals is obtained from electron momentum densities calculated using modern computational quantum chemistry density functional theory tools. The calculation, in its simplest form, effectively treats the low-energy (thermalised, room-temperature) positron as a plane wave and gives annihilation gamma spectra that are about 40% broader than experiment, although the main chemical trends are reproduced. We show that this effective "narrowing" of the experimental spectra is due to the action of the molecular potential on the positron, chiefly, due to the positron repulsion from the nuclei. It leads to a suppression of the contribution of small positron-nuclear separations where the electron momentum is large. To investigate the effect of the nuclear repulsion, as well as that of short-range electron-positron and positron-molecule correlations, a linear combination of atomic orbital description of the molecular orbitals is employed. It facilitates the incorporation of correction factors which can be calculated from atomic many-body theory and account for the repulsion and correlations. Their inclusion in the calculation gives gamma spectrum linewidths that are in much better agreement with experiment. Furthermore, it is shown that the effective distortion of the electron momentum density, when it is observed through positron annihilation gamma spectra, can be approximated by a relatively simple scaling factor.

physics.atom-ph↗

arXiv2010

Detection of positron-atom bound states through resonant annihilation

A method is proposed for detecting positron-atom bound states by observing enhanced positron annihilation due to electronic Feshbach resonances at electron-volt energies. The method is applicable to a range of open-shell transition metal atoms which are likely to bind the positron: Fe, Co, Ni, Tc, Ru, Rh, Sn, Sb, Ta, W, Os, Ir, and Pt. Estimates of their binding energies are provided.

physics.atom-ph↗

arXiv2010

Positron-molecule interactions: resonant attachment, annihilation, and bound states

This article presents an overview of current understanding of the interaction of low-energy positrons with molecules with emphasis on resonances, positron attachment and annihilation. Annihilation rates measured as a function of positron energy reveal the presence of vibrational Feshbach resonances (VFR) for many polyatomic molecules. These resonances lead to strong enhancement of the annihilation rates. They also provide evidence that positrons bind to many molecular species. A quantitative theory of VFR-mediated attachment to small molecules is presented. It is tested successfully for selected molecules (e.g., methyl halides and methanol) where all modes couple to the positron continuum. Combination and overtone resonances are observed and their role is elucidated. In larger molecules, annihilation rates from VFR far exceed those explicable on the basis of single-mode resonances. These enhancements increase rapidly with the number of vibrational degrees of freedom. While the details are as yet unclear, intramolecular vibrational energy redistribution to states that do not couple directly to the positron continuum appears to be responsible for these enhanced annihilation rates. Downshifts of the VFR from the vibrational mode energies have provided binding energies for thirty species. Their dependence upon molecular parameters and their relationship to positron-atom and positron-molecule binding energy calculations are discussed. Feshbach resonances and positron binding to molecules are compared with the analogous electron-molecule (negative ion) cases. The relationship of VFR-mediated annihilation to other phenomena such as Doppler-broadening of the gamma-ray annihilation spectra, annihilation of thermalized positrons in gases, and annihilation-induced fragmentation of molecules is discussed.

physics.atom-ph↗

arXiv2010

Many-body theory calculations of positron binding to negative ions

A many-body theory approach developed by the authors [Phys. Rev. A 70, 032720 (2004)] is applied to positron bound states and annihilation rates in atomic systems. Within the formalism, full account of virtual positronium (Ps) formation is made by summing the electron-positron ladder diagram series, thus enabling the theory to include all important many-body correlation effects in the positron problem. Numerical calculations have been performed for positron bound states with the hydrogen and halogen negative ions, also known as Ps hydride and Ps halides. The Ps binding energies of 1.118, 2.718, 2.245, 1.873 and 1.393 eV and annihilation rates of 2.544, 2.482, 1.984, 1.913 and 1.809 ns$^{-1}$, have been obtained for PsH, PsF, PsCl, PsBr and PsI, respectively.

physics.atom-ph↗

arXiv2009

Parity nonconservation in electron recombination of multiply charged ions

We discuss a parity nonconserving asymmetry in the cross section of KLL dielectronic recombination of polarized electrons on the hydrogen-like ions with $Z \lesssim 60$. This effect is strongly enhanced because of the near-degeneracy of doubly-excited $2l2l'$ states of opposite parity in He-like ions. For ions with $Z \sim 30$ the asymmetry is of the order of $10^{-9}$. For $Z \approx 48$ a level crossing takes place, leading to the PNC asymmetry of $\pm 5\times 10^{-9}$, which is $10^8$ times greater than the basic strength of the weak interaction in atoms.

physics.atom-ph↗

arXiv2008

Electron attachment to SF6 and lifetimes of SF6- negative ions

We study the process of low-energy electron capture by the SF6 molecule. Our approach is based on the model of Gauyacq and Herzenberg [J. Phys. B 17, 1155 (1984)] in which the electron motion is coupled to the fully symmetric vibrational mode through a weakly bound or virtual s state. By tuning the two free parameters of the model, we achieve an accurate description of the measured electron attachment cross section and good agreement with vibrational excitation cross sections of the fully symmetric mode. An extension of the model provides a limit on the characteristic time of intramolecular vibrational relaxation in highly-excited SF6-. By evaluating the total vibrational spectrum density of SF6-, we estimate the widths of the vibrational Feshbach resonances of the long-lived negative ion. We also analyse the possible distribution of the widths and its effect on the lifetime measurements, and investigate nonexponential decay features in metastable SF6-.

physics.atom-ph↗

arXiv2006

Positron-molecule annihilation by capture into vibrational Feshbach resonances of infrared-active modes

Enhanced positron annihilation on polyatomic molecules is a long-standing and complex problem. We report the results of calculations of resonant positron annihilation on methyl halides. A free parameter of our theory is the positron binding energy. A comparison with energy-resolved annihilation rates measured for CH$_3$F, CH$_3$Cl, CH$_3$Br [Barnes et al. Phys. Rev. A {\bf 74}, 012706 (2006)] shows good agreement and yields estimates of the binding energies.

physics.atom-ph↗

arXiv2005

Application of the zero-range potential model to positron annihilation on molecules

In this paper we use a zero-range potential (ZRP) method to model positron interaction with molecules. This allows us to investigate the effect of molecular vibrations on positron-molecule annihilation using the van der Waals dimer Kr2 as an example. We also use the ZRP to explore positron binding to polyatomics and examine the dependence of the binding energy on the size of the molecule for alkanes. We find that a second bound state appears for a molecule with ten carbons, similar to recent experimental evidence for such a state emerging in alkanes with twelve carbons.

physics.atom-ph↗

arXiv2005

Positronium formation from valence and inner shells in noble gas atoms

When recent experimental positronium (Ps) formation cross sections in noble gases have been compared with the most up-to date theoretical studies, the agreement is qualitative, but not quantitative. In this paper we re-examine this process and show that at low energies Ps formation must be treated nonperturbatively. We also look at Ps formation with inner shell electrons.

physics.atom-ph↗

arXiv2005

Many-body theory of gamma spectra from positron-atom annihilation

A many-body theory approach to the calculation of gamma spectra of positron annihilation on many-electron atoms is developed. We evaluate the first-order correlation correction to the annihilation vertex and perform numerical calculations for the noble gas atoms. Extrapolation with respect to the maximal orbital momentum of the intermediate electron and positron states is used to achieve convergence. The inclusion of correlation corrections improves agreement with experimental gamma spectra.

physics.atom-ph↗

arXiv2004

Many-body theory of positron-atom interactions

A many-body theory approach is developed for the problem of positron-atom scattering and annihilation. Strong electron-positron correlations are included non-perturbatively through the calculation of the electron-positron vertex function. It corresponds to the sum of an infinite series of ladder diagrams, and describes the physical effect of virtual positronium formation. The vertex function is used to calculate the positron-atom correlation potential and nonlocal corrections to the electron-positron annihilation vertex. Numerically, we make use of B-spline basis sets, which ensures rapid convergence of the sums over intermediate states. We have also devised an extrapolation procedure that allows one to achieve convergence with respect to the number of intermediate-state orbital angular momenta included in the calculations. As a test, the present formalism is applied to positron scattering and annihilation on hydrogen, where it is exact. Our results agree with those of accurate variational calculations. We also examine in detail the properties of the large correlation corrections to the annihilation vertex.

physics.atom-ph↗

arXiv2004

Recombination of low energy electrons with U$^{28+}$

We performed an extensive study of configuration mixing between the doubly excited (doorway) states and complex multiply excited states of U$^{28+}$ near its ionization threshold. The detailed investigation of complex spectrum and analysis of the statistics of eigenstate components show that the dielectronic (doorway) states weakly mixed with each other. However, they show substantial mixing with the complex multiply excited states. This situation explains the mechanism of low energy electron recombination with U$^{28+}$. We calculated the energy averaged capture cross sections as a sum over dielectronic doorway states and found our present calculation interprets well the experimental recombination rates in the energy range of 1 to 100 eV.

physics.atom-ph↗

arXiv2004

The role of inner-shell excitations in electron recombination with multiply charged complex ions

We performed an overview of the inner shell excitation phenomena in the study of electron recombination with multiply charged complex ions such as Au$^{q+}$ (q=49-52) and Pb$^{53+}$. It is found the the inner shell excitations play a significant role in the low energy electron recombination. Taking into account these inner shell excitations we have calculated the energy avaraged capture cross sections. The dielectronic rate coefficients are found to be in good agreement with the experiment. We show that the contribution from the inner-shell enhances the recombinmation rate by an order of magitude. We also made an attempt to identify the resonaces observed by the experiment in Au$^{50+}$ and Pb$^{53+}$. A prediction is made for the rate enhancement in Au$^{52+}$ in which we found the configuration mixing between the doubly and the complex multiply excited states. Analyzing the statistics of eigenstate components we estimated the spreading width is about 0.85 a.u. which defines the energy range within which strong mixing takes place.

physics.atom-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↗

arXiv1999

Mechanisms of positron annihilation on molecules

The aim of this work is to identify the mechanisms responsible for very large rates and other peculiarities observed in low-energy positron annihilation on molecules. The two mechanisms considered are: (i) Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism dominates for atoms and small molecules. I show that its contribution to the annihilation rate can be related to the positron elastic scattering cross section. This mechanism is characterized by strong energy dependence of the annihilation rate at small positron energies if a low-lying virtual level or a weakly bound state exists for the positron. (ii) Resonant annihilation, which takes place when the positron undergoes resonant capture into a vibrationally excited quasibound state of the positron-molecule complex. This mechanism dominates for larger molecules capable of forming bound states with the positron. For this mechanism the energy-averaged annihilation rate is proportional to the level density of the positron-molecule complex, which is basically determined by the spectrum of molecular vibrational states populated in the positron capture. For room-temperature positrons this mechnism can produce annihilation rates up to 5 orders of magnitude greater than the first one. Its contribution is inversely proportional to the positron momentum at small positron energies.

physics.atom-ph↗

arXiv1999

Unusual statistics of interference effects in neutron scattering from compound nuclei

We consider interference effects between p-wave resonance scattering amplitude and background s-wave amplitude in low-energy neutron scattering from a heavy nucleus which goes through the compound nucleus stage. The first effect is in the difference between the forward and backward scattering cross sections. Because of the chaotic nature of the compound states, this effect is a random variable with zero mean. However, a statistical consideration shows that the probability distribution of this effect does not obey the standard central limit theorem. That is, the probability density for the effect averaged over n resonances does not become a Gaussian distribution with the variance decreasing as 1/sqrt(n) (``violation'' of the theorem!). We derive the probability distribution of the effect and the limit distribution of the average. It is found that the width of this distribution does not decrease with the increase of n, i.e., fluctuations are not suppressed by averaging. Furthermore, we consider the correlation between the neutron spin and the scattering plane and find that this effect, although much smaller, shows fluctuations which actually increase upon averaging over many measurements. Limits of the effects due to finite resonance widths are also considered. In the appendix we present a simple derivation of the limit theorem for the average of random variables with infinite variances.

nucl-th↗