Search

When collecting spectroscopic data on at least four isotopes, nonlinearitiesin the King plot are a possible sign of Physics beyond the Standard Model. Inthis work, an improved approach to the search for hypothetical new interactionswith isotope shift spectroscopy of few-electron ions is presented. Very carefulaccount is taken of the small nuclear corrections to the energy levels and thegyromagnetic factors, which cause deviations from King linearity within theStandard Model and are hence a possible source of confounds. In this newapproach, the experimental King nonlinearity is not compared to the vanishingprediction of the Standard Model at the leading order, but to the calculatedfull Standard Model contribution to King nonlinearity. This makes searching forbeyond-the-Standard-Model physics with King linearity analysis possible in avery-high-precision experimental regime, avoiding confounds. The bounds whichcan be set on beyond-the-Standard-Model parameters remain limited by theuncertainties on the small Standard Model nuclear corrections which cause Kingnonlinearity. Direct comparison between theory and experiment on a single pairof isotopes is advocated as a more suitable approach for few-electron ions.

The nondipole under-the-barrier dynamics of the electron during strong-field tunneling ionization is investigated, examining the role of the Coulomb field of the atomic core. The common analysis in the strong field approximation is consequently generalised to include the leading light-front non-dipole Coulomb corrections and demonstrates the counter-intuitive impact of the sub-barrier Coulomb field. Despite its attractive nature, the sub-barrier Coulomb field increases the photoelectron nondipole momentum shift along the laser propagation direction, involving a strong dependence on the laser field. The scaling of the effect with respect to the principal quantum number and angular momentum of the bound state is found. We demonstrate that the signature of Coulomb induced sub-barrier effects can be identified in the asymptotic photoelectron momentum distribution via a comparative study of the field-dependent longitudinal momentum shift for different atomic species with state-of-the-art experimental techniques of mid-infrared lasers., 6 pages, 4 figures

The problem of Freeman resonances [R. R. Freeman \textit{et al.}, Phys. Rev. Lett. \textbf{59}, 1092 (1987)] when strong field ionization is enhanced due to the transient population of excited states during the ionization, is revisited. An intuitive model is put forward which explains the mechanism of the intermediate population of excited states during nonadiabatic tunneling ionization via the under-the-barrier recollision and recombination. The theoretical model is based on perturbative strong-field approximation (SFA), where the sub-barrier bound-continuum-bound pathway is described in the second-order SFA, while the further ionization from the excited state by an additional perturbative step. The enhancement of ionization is shown to arise due to the constructive interference of contributions into the excitation amplitudes originating from different laser cycles. The applied model provides an intuitive understanding of the electron dynamics during a Freeman resonance in strong-field ionization, as well as means of enhancing the process and possible applications to related processes.

The spontaneous emission of an excited two-level emitter driven by a strong classical coherent low-frequency electromagnetic field is investigated. We find that for relatively strong laser driving, multi-photon processes are induced, thereby opening additional decay channels for the atom. We analyze the interplay between the strong low-frequency driving and the interfering multiphoton decay channels, and discuss its implications for the spontaneous emission dynamics., 8 pages, 4 figures

The procedure for the calculation of the self-energy-corrected wave function of the bound electron in the field of the nucleus is discussed. We present the related formulas and discuss the numerical difficulties and the methods used to overcome them. The results of the calculation are presented for a wide range of ions. Possible applications of the numerically obtained wave functions are discussed.

X-ray assisted nuclear excitation by electron capture (NEEC) into inner-shell atomic holes in a plasma environment generated by strong optical lasers is investigated theoretically. The considered scenario involves the interaction of a strong optical laser with a solid-state nuclear target leading to the generation of a plasma. In addition, intense x-ray radiation from an X-ray Free Electron Laser (XFEL) produces inner-shell holes in the plasma ions, into which NEEC may occur. As case study we consider the $4.85$-keV transition starting from the 2.4 MeV long-lived $^{\mathrm{93m}}$Mo isomer that can be used to release the energy stored in this metastable nuclear state. We find that the recombination into $2p_{1/2}$ inner-shell holes is most efficient in driving the nuclear transition. Already at few hundred eV plasma temperature, the generation of inner-shell holes can allow optimal conditions for NEEC, otherwise reached for steady-state plasma conditions in thermodynamical equilibrium only at few keV. The combination of x-ray and optical lasers presents two advantages: first, NEEC rates can be maximized at plasma temperatures where the photoexcitation rate remains low. Second, with mJ-class optical lasers and an XFEL repetition rate of $10$ kHz, the NEEC excitation number can reach $\sim 1$ depleted isomer per second and is competitive with scenarios recently envisaged at petawatt-class lasers., 9 pages, 4 figures; minor modifications made; accepted for publication in Physical Review A

The local-constant-field approximation (LCFA) is an essential theoretical tool for investigating strong-field QED phenomena in background electromagnetic fields with complex spacetime structure. In our previous work [Phys.~Rev.~A~\textbf{98}, 012134 (2018)] we have analyzed the shortcomings of the LCFA in nonlinear Compton scattering at low emitted photon energies for the case of a background plane-wave field. Here, we generalize that analysis to background fields, which can feature a virtually arbitrary spacetime structure. In addition, we provide an explicit and simple implementation of an improved expression of the nonlinear Compton scattering differential probability that solves the main shortcomings of the standard LCFA in the infrared region, and is suitable for background electromagnetic fields with arbitrary spacetime structure such as those occurring in particle-in-cell simulations. Finally, we carry out a systematic procedure to calculate the probability of nonlinear Compton scattering per unit of emitted photon light-cone energy and of nonlinear Breit-Wheeler pair production per unit of produced positron light-cone energy beyond the LCFA in a plane-wave background field, which allows us to identify the limits of validity of this approximation quantitatively., Comment: 15 pages, 3 figures

The impact of the carrier-envelope phase (CEP) of a long relativistic tightly focused laser pulse on the dynamics of a counterpropagating electron beam has been investigated theoretically in the, so-called, electron reflection regime. Our semiclassical Monte Carlo simulations show that the electrons are reflected at the rising edge of the laser pulse due to the ponderomotive force of the focused laser beam, and an asymmetric electron-momentum distribution emerges along the laser polarization direction, which sensitively depends on the CEP of the driving laser pulse for weak radiative stochasticity effects. The CEP signatures can be determined in the electron-momentum distribution at laser intensities of the order or larger than ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ and for the pulse lengths up to 10 cycles. The CEP detection resolution is proportional to the electron beam density and can achieve approximately $0.{1}^{\ensuremath{\circ}}$ at an electron density of about ${10}^{15}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The method is applicable for currently available ultraintense laser facilities with the laser peak power ranging from tens of terawatt to multipetawatt.

Theoretical and experimental resonance strengths for $KLL$ dielectronic recombination (DR) into He-, Li-, Be-, and B-like mercury ions are presented, based on state-resolved DR x-ray spectra recorded at the Heidelberg electron-beam ion trap. The DR resonance strengths are experimentally extracted by normalizing them to simultaneously recorded radiative recombination signals. The results are compared to state-of-the-art atomic calculations that include relativistic electron correlation and configuration mixing effects. Combining the present data with other existing ones, we derive an improved semiempirical $Z$-scaling law for DR resonance strengths as a function of the atomic number, taking into account higher-order relativistic corrections, which are especially relevant for heavy highly charged ions.

An analytical method for treating Coulomb focusing in strong-field ionization in a linearly polarized laser field has been proposed in the first paper of this series, where analytical formulas have been derived for the Coulomb momentum transfer to the tunneled electron at the tunnel exit and due to rescatterings. In this paper, we analytically investigate a buildup of the Coulomb momentum transfer during multiple laser driven recollisions of the tunneled electron on the parent ion. We find that the momentum transfer at rescattering events, while being a perturbation with respect to the instantaneous electron momentum at the rescattering, induces a significant nonperturbative contribution to the total momentum transfer. Moreover, we show that the higher-order recollisions contribute considerably to the total momentum transfer and cannot be omitted for the proper description of the Coulomb focusing. We also show that our analytical approach to Coulomb focusing is well suitable for proper description of the Coulomb focusing cusps of the photoelectron momentum distribution at energies larger than 50 meV.

Numerous few-electron atomic systems are considered which can be used effectively for observing a potential variation of the fine-structure constant $\ensuremath{\alpha}$ and the electron-proton mass ratio ${m}_{e}/{m}_{p}$. We examine optical magnetic dipole transitions between hyperfine-structure components in heavy highly charged H-like and Li-like ions with observably high sensitivity to a variation of $\ensuremath{\alpha}$ and ${m}_{e}/{m}_{p}$. The experimental spectra of the proposed systems consist of a strong single line, which simplifies significantly the data analysis and shortens the necessary measurement time. Furthermore, we propose systems for an experimental test of the variation of quark masses and discuss the expected level of accuracy in assessing its limitations. Finally, we establish which constraints on the variation of these fundamental constants could be provided by measurements with a hyperfine-structure highly-charged-ion clock and some reference clock, showing that a significant improvement of the current limitations can be reached.

We investigate electron-correlation effects in the $g$-factor of the ground state of Li-like ions. Our calculations are performed within the nonrelativistic quantum electrodynamics (NRQED) expansion up to two leading orders in the fine-structure constant $\alpha$, $\alpha^2$ and $\alpha^3$. The dependence of the NRQED results on the nuclear charge number $Z$ is studied and the individual $1/Z$-expansion contributions are identified. Combining the obtained data with the results of the all-order (in $Z\alpha$) calculations performed within the $1/Z$ expansion, we derive the unified theoretical predictions for the $g$-factor of light Li-like ions., Comment: 9 pages, 4 tables

A weighted difference of the $g$-factors of the Li- and H-like ion of the same element is studied and optimized in order to maximize the cancellation of nuclear effects. To this end, a detailed theoretical investigation is performed for the finite nuclear size correction to the one-electron $g$-factor, the one- and two-photon exchange effects, and the QED effects. The coefficients of the $Z\alpha$ expansion of these corrections are determined, which allows us to set up the optimal definition of the weighted difference. It is demonstrated that, for moderately light elements, such weighted difference is nearly free from uncertainties associated with nuclear effects and can be utilized to extract the fine-structure constant from bound-electron $g$-factor experiments with an accuracy competitive with or better than its current literature value., Comment: 11 pages, 3 figures, 6 tables. arXiv admin note: text overlap with arXiv:1509.08260