arXiv subjects

A. Anikin

Publications and source records attributed to A. Anikin.

At least 19 recordsLinked to original sources

arXiv2026

Long-range magnetic interaction within quantum electrodynamics formalism

Within the framework of quantum electrodynamics, the interaction between two atoms at large distances is analyzed. Using the S-matrix formalism, an expression for the magnetic interaction potential is derived, which agrees with the well-known result of classical electrodynamics. However, quantum electrodynamics goes beyond this conventional result and allows one to treat a wide range of problems related to the structure of atomic energy levels. In particular, it is shown that the asymptotic behavior of the interaction potential can deviate from the classical prediction, depending on the atomic states involved. As an example, dispersion coefficients are calculated for the s-states of hydrogen atoms, where the long-range potential reduces to a spin-spin interaction. The results obtained open up the possibility of a straightforward comparative analysis of long-range interaction potentials between atoms of matter and antimatter. The applicability of this approach is demonstrated for the hydrogen-antihydrogen system.

physics.atom-ph↗

arXiv2026

Reassessment of line profile asymmetry in measurements of the 1s-2s energy interval in hydrogen

Experiments to determine transition frequencies in the hydrogen atom represent some of the most precise spectroscopic measurements and are at a higher level among simple atomic systems. The most persistent measured value in hydrogen is the energy interval corresponding to the $1s-2s$ two-photon transition. The achieved experimental precision is several parts of $10^{-15}$ and has not changed over the last two decades. Although repeated experiments in 2011 and 2013 have improved the accuracy by several times, the frequency value has not changed significantly. On this basis, the frequency of the $1s-2s$ transition holds pivotal for determining physical quantities such as the Rydberg constant and the proton charge radius. Theoretical efforts to study in detail the effects that might influence such precise measurements have not revealed significant contributions. The present work revises the theoretical analysis of the line contour asymmetry and its influence on the determination of the two-photon absorption transition frequency, taking into account the theoretical achievements of recent years in this direction. It is shown that the asymmetry of the observed profile can lead to a $1s-2s$ transition frequency shift at the level of modern experimental accuracy. The found frequency shift is consistent with the line shape model contribution that forms the error budget of the experimental measurements. Adjustment can be carried out on the basis of the asymmetric profile that has become standard in recent years.

physics.atom-ph↗

arXiv2026

Non-adiabatic Effects Induced by Strong Light-Matter Coupling in Cavity QED

We present a systematic study of the diagonal Born-Oppenheimer correction (DBOC) for atoms and molecules embedded in optical cavities and interacting with a quantized electromagnetic field. By explicitly evaluating the nuclear kinetic energy operator, we analyze cavity-induced modifications of DBOC within a quantum electrodynamics configuration-interaction (QED-CI) framework built on quantum electrodynamics Hartree-Fock (QED-HF) and strong-coupling quantum electrodynamics Hartree-Fock (SC-QED-HF) reference states. The analysis covers a diverse set of atomic and molecular systems, including He, H-, Be, H2, LiH, HF, ammonia (NH3), and formaldehyde (CH2O). We show that the presence of the cavity leads to shifts in molecular dissociation energies on the order of a few inverse centimeters. For several atomic systems, the inclusion of the DBOC yields a pronounced effect, with the correction magnitude reaching the experimental resolution. These findings reveal finite nuclear mass effects as an essential component of nuclear dynamics in cavity QED and suggest their relevance for precision analysis in strongly coupled light-matter systems.

physics.atom-ph↗

arXiv2024

Effect of antiprotons on hydrogen-like ions in external magnetic fields

In the present work, quasi-molecular compounds consisting of one antiproton ($\bar{p}$) and one hydrogen-like ion are investigated: $\mathrm{He}^{+} - \bar{p}$, $\mathrm{Li}^{2+} - \bar{p}$, $\mathrm{C}^{5+} - \bar{p}$, $\mathrm{S}^{15+} - \bar{p}$, $\mathrm{Kr}^{35+} - \bar{p}$, $\mathrm{Ho}^{66+} - \bar{p}$, $\mathrm{Re}^{74+} - \bar{p}$, $\mathrm{U}^{91+} - \bar{p}$. For the calculations, the Dirac equation with two-center potential is solved numerically using the dual-kinetically balanced finite-basis-set method adapted to systems with axial symmetry (A-DKB). Adiabatic potential curves are constructed for the ground state of the above quasi-molecular compounds in the framework of the A-DKB approach. Calculations were also performed for the case of an external magnetic field (the field is taken into account non-perturbatively). Zeeman shifts of the quasi-molecular terms are obtained for a homogeneous magnetic field with a strength of the laboratory order (up to 100 Tesla) directed along the axis of the molecule.

physics.atom-ph↗

arXiv2024

Handling the asymmetric spectral line profile

This paper discusses some features of the spectral line profile theory used in the treatment of measured atomic transitions. It is shown that going beyond the established linear approximation for the spectral line contour in the case of its nonresonant extension, the potential for a more accurate extraction of atomic characteristics from experimental data arises. Using the example of the Lyman-$α$ (Ly$_α$) transition in hydrogen, a simple analysis of the observed spectral line distorted by a possible interfering transitions is given. In particular, the results obtained in the present work clearly demonstrate that the processing of the same experimental data at different settings can provide an accurate determination of the transition frequency, the centre of gravity as well as the hyperfine splitting of the ground state in hydrogen-like atomic systems. The latter is especially important for setting up precision spectroscopic experiments on the antihydrogen atom.

physics.atom-ph↗

arXiv2023

Theoretical prerequisites for the upcoming generation of precision spectroscopic experiments

Modern resonant spectroscopic experiments to measure transition frequencies in atoms have reached a level where a meticulous description of all aspects of the processes under study has become obligatory. The precision achieved in the experiments of A. Beyer, et al., Science 358, 79 (2017), has led to the fact that the determination of the transition frequency based on measured data is significantly refined by theoretical treatment of the observed spectral line profile. As it was predicted theoretically, a great impact of effects arising beyond the resonance approximation was found experimentally. These findings marked the beginning of the upcoming epoch in the resonant atomic spectroscopy when many commonly understood ideas became invalid. For example, the atomic transition may be characterized by several different but equally acceptable frequencies. Furthermore, we show that the picture becomes even more complicated when the observed spectral line profile is "identified" with one of the processes - emission or absorption. Precise determination of the transition frequency requires a description of the absorption line profile inseparable from the emission process and vice versa. The theoretical aspects discussed in this work provide prerequisites for more accurate and yet simpler experiments than those reported in Science 358, 79 (2017). Implementing the new physics expected in atomic resonance spectroscopy in the near future beyond the resonance approximation is unfeasible without resolving these issues.

physics.atom-ph↗

arXiv2023

Light one-electron quasi-molecular ions within the finite-basis-set method for the two-center Dirac equation

The electronic spectra of light one-electron quasi-molecular compounds H-H$^+$, He$^+$-He$^2+$ and He$^+$-H$^+$ are analyzed. To this end, the two-center Dirac equation is solved by the dual-kinetically balanced finite-basis-set method for axially symmetric systems termed as A-DKB. This method allows a complete relativistic consideration of these systems at fixed internuclear distances. A comparison of the obtained results with the nonrelativistic and relativistic calculations presented in the literature is performed. The advantages and disadvantages of the approach are discussed in details.

physics.atom-ph↗

arXiv2022

Natural line profile asymmetry

The paper discusses the line profile asymmetry of the photon scattering process that arises naturally in quantum electrodynamics (QED). Based on precision spectroscopic experiments conducted on hydrogen atoms, we focus our attention on the two-photon $1s-2s$ transition. As one of the most precisely determined transition frequencies, it is a key pillar of optical frequency standards and is used in determining fundamental physical constants, testing physical principles, and searching constraints on new fundamental interactions. The results obtained in this work show the need to take into account the natural line profile asymmetry in precision spectroscopic experiments.

physics.atom-ph↗

arXiv2022

Analysis of nonresonant effects in the two-photon spectroscopy of helium

In the present paper, we study nonresonant corrections for experimental measurements of the transition frequencies in the helium atom. Having attracted more attention, such effects can make a significant contribution to experiments based on one- and two-photon atomic spectroscopy. The quantum interference effects in the measurements of $2^3S_1-n^3D_1$ ($ n=3,\,4,\,5 $) transition frequencies based on Doppler-free two-photon spectroscopy, are considered as a possible source of current discrepancy between the experimental and theoretical data. We demonstrate that line profile asymmetry caused by the quantum interference of fine sub-levels of the $ ^3D_{J} $ $ (J=1,2,3) $ state can reach tenths of a megahertz for different experimental conditions. Thus, previously unaccounted nonresonant corrections should be taken in next-generation experimental measurements of transitions frequencies in helium. However, they could not completely eliminate the current imbalance in the study of helium spectra and the question is still open.

physics.atom-ph↗

arXiv2022

Line Profile Asymmetry in Precision Spectroscopy

In this review, we have investigated the asymmetry of the line profile in precision one- and two-photon spectroscopy of hydrogen and helium atoms within the framework of a rigorous QED approach. A detailed analysis of the angular correlations of the quantum interference effect has been carried out using various examples. Nonresonant effects are also considered in relation to some astrophysical problems. In particular, a rigorous QED derivation of the nonresonant extension for the Lorentz line profile is given using the Ly$_α$ transition as an example; such a QED derivation has been lacking in the literature.

physics.atom-ph↗

arXiv2021

Thermal corrections for positronium

Thermal corrections, including relativistic effects, for the positronium atom are discussed. The theoretical description of thermal corrections is carried out within the framework of relativistic quantum electrodynamics. As a result, thermal corrections to atomic energy levels with a fine and hyperfine structure and to the probabilities of annihilation of a positronium atom placed in a thermal environment (blackbody radiation) are taken into account. Numerical results are discussed throughout the paper in view of modern experiments and theoretical searches for verification of fundamental interactions.

physics.atom-ph↗

arXiv2021

Lowest order thermal correction to the hydrogen recombination cross section in presence of blackbody radiation

In the present paper, the correction due to the thermal interaction of two charges to the recombination and ionization processes for the hydrogen atom is considered. The evaluation is based on a rigorous quantum electrodynamic (QED) approach within the framework of perturbation theory. The lowest-order radiative correction to the recombination/ionization cross-section is examined for a wide range of temperatures corresponding to laboratory and astrophysical conditions. The found thermal contribution is discussed both for specific states and for the total recombination and ionization coefficients.

physics.atom-ph↗

arXiv2021

Proton size from precision experiments on hydrogen and muonic hydrogen atoms

The "proton radius puzzle" was recently solved by reducing the four-standard deviation discrepancy between the results for electronic hydrogen ($H$) and muonic hydrogen ($μH$) atoms to $3.3$ value. The value of the root-mean-square radius of the proton ($r_p$), extracted from experiments on measuring the one-photon $2s-4p$ transition and the Lamb shift in hydrogen, is now $0.8335(95)$ fm, that is in good agreement with the muonic hydrogen experiments, $0.84087(39)$ fm. Even so, these values deviate significantly from the CODATA value, which is determined as the average using the results for various spectral lines including two-photon transitions in the hydrogen atom. The solution of the proton radius puzzle was realized by taking into account the influence of interference effect in one-photon scattering processes. The importance of interfering effects in atomic frequencies measurements gives an impetus to the study of experiments based on two-photon spectroscopy with the suchlike thoroughness. It is shown here that the effect of interfering pathways for two-photon $2s-nd$ transitions in a hydrogen atom is also significant in determining the proton charge radius and Rydberg constant.

physics.atom-ph↗

arXiv2020

Angular correlations in two-photon spectroscopy of hydrogen

In the present paper, we consider nonresonant corrections to $ 2s-ns/nd $ transition frequencies in hydrogen for the experiments based on two-photon spectroscopy. A detailed study of angular correlations of quantum interference effects within the framework of rigorous quantum electrodynamics is given. Closed expressions for the resonant two-photon scattering cross sections on an atomic level with dependence on all atomic quantum numbers including fine and hyperfine structure are derived. These expressions are applied for the description of experiments based on two-photon spectroscopy with fixing of incident (outgoing) photon polarizations. We demonstrate that for certain experimental geometry nonresonant corrections could be significant for the determination of Rydberg constant and proton charge radius.

physics.atom-ph↗

arXiv2020

Relativistic corrections to the thermal interaction of bound particles

This paper discusses relativistic corrections to the thermal Coulomb potential for simple atomic systems. The theoretical description of the revealed thermal corrections is carried out within the framework of relativistic quantum electrodymamics (QED). As a result, thermal corrections to the fine and hyperfine strucutres of atomic levels are introduced. The theory presented in this paper is based on the assumption that the atom is placed in a thermal environment created by the blackbody radiation (BBR). The numerical results allow us to expect hteir significance for modern experiments and testing the fundamental interactions.

physics.atom-ph↗

arXiv2020

Vertex-type thermal correction to the one-photon transition rates

Thermal corrections to the one-photon spontaneous and induced transition probabilities for hydrogen and hydrogen-like ions are evaluated. The found thermal corrections are given by the vertex Feynman graph, where the vertex represents the thermal interaction between the bound electron and the nucleus. All derivations of thermal corrections to bound-bound transitions for an atom exposed to blackbody radiation (BBR) are made in a fully relativistic approach within the framework of the adiabatic $S$-matrix formalism. It is found that the vertex-type radiative corrections to the transition rates can be at the level of a few percent to corresponding spontaneous rates for highly excited states in the hydrogen atom. A comprehensive analysis of the vertex-type thermal corrections for hydrogen-like atomic systems is presented.

physics.atom-ph↗

arXiv2020

Ultrafast dynamics in normal and Charge Density Wave phase of 2H-NbSe2

We investigate carrier and collective mode dynamics in 2H-NbSe$_2$ using time-resolved optical pump-probe spectroscopy and compare the results with first-principle calculations. Broadband ultrafast reflectivity studies of 2H-NbSe$_2$ in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons showed CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the CDW transition temperature T$_{CDW}$ with subsequent hardening below T$_{CDW}$. The global fit of the broadband probe data reveals four different relaxation times associated with characteristic electron-electron, electron-phonon and phonon-phonon relaxation processes. From first principle calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time $τ_2$ with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of $τ_3$~100 ps is associated with anharmonicity-driven phonon-phonon scattering. All relaxation processes result from anomalies near the second order CDW phase transition that are reflected in the temperature dependencies of the characteristic relaxation times and amplitudes of optical densities. At highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below T$_{CDW}$.

cond-mat.str-el↗

arXiv2020

Two-photon atomic level widths at finite temperatures

The thermal two-photon level broadening of the excited energy levels in the hydrogen and H-like helium is evaluated via the imaginary part of thermal two-loop self-energy correction for bound electron. All the derivations are presented in the framework of rigorous quantum electrodynamic theory at finite temperatures and are applicable for the H-like ions. On this basis, we found a contribution to the level broadening induced by the blackbody radiation which is fundamentally different from the usual line broadening caused by the stimulated two-photon decay and the Raman scattering of thermal photons. Numerical calculations of the two-loop thermal correction to the two-photon width for the $2s$ state in hydrogen and singly ionized helium atoms show that the effect could significantly exceed the higher-order relativistic and radiative QED corrections commonly included in the calculations. In addition, the thermal two-loop self-energy correction significantly exceeds the "ordinary" stimulated one-photon depopulation rate at the relevant laboratory temperatures. In this work, detailed analysis and the corresponding comparison of the effect with the existing laboratory measurements in H-like ions are carried out.

physics.atom-ph↗