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Strong-field Relativistic Processes in Highly Charged Ions

Octavian Postavaru

Published Jan 1, 2010• 0 citations
Publisher

Abstract

In this thesis we investigate strong-field relativistic processes in highly charged ions. In the first part,we study resonance fluorescence of laser-driven highly charged ions in the relativistic regime by solving the time-dependent master equation in a multi-level model. Our ab initio approach based on the Dirac equation allows for investigating highly relativistic ions, and, consequently, provides a sensitive means to test correlated relativistic dynamics, bound-state quantum electrodynamic phenomena and nuclear effects by applying coherent light with x-ray frequencies. Atomic dipole or multipole moments may be determined to unprecedented accuracy by measuring the interference-narrowed fluorescence spectrum. Furthermore, we investigate the level structure of heavy hydrogenlike ions in laser beams. Interaction with the light field leads to dynamic shifts of the electronic energy levels, which is relevant for spectroscopic experiments. We apply a fully relativistic description of the electronic states by means of the Dirac equation. Our formalism goes beyond the dipole approximation and takes into account non-dipole effects of retardation and interaction with the magnetic field components of the laser beam. We predicted cross sections for the inter-shell trielectronic recombination (TR) and quadruelectronic recombination processes which have been experimentally confirmed in electron beam ion trap measurements, mainly for C-like ions, of Ar, Fe and Kr. For Kr30+, inter-shell TR contributions of nearly 6% to the total resonant photorecombination rate were found.

DOI
10.11588/heidok.00011427

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In this thesis we investigate strong-field relativistic processes in highly charged ions. In the first part,we study resonance fluorescence of laser-driven highly charged ions in the relativistic regime by solving the time-dependent master equation in a multi-level model. Our ab initio approach based on the Dirac equation allows for investigating highly relativistic ions, and, consequently, provides a sensitive means to test correlated relativistic dynamics, bound-state quantum electrodynamic phenomena and nuclear effects by applying coherent light with x-ray frequencies. Atomic dipole or multipole moments may be determined to unprecedented accuracy by measuring the interference-narrowed fluorescence spectrum. Furthermore, we investigate the level structure of heavy hydrogenlike ions in laser beams. Interaction with the light field leads to dynamic shifts of the electronic energy levels, which is relevant for spectroscopic experiments. We apply a fully relativistic description of the electronic states by means of the Dirac equation. Our formalism goes beyond the dipole approximation and takes into account non-dipole effects of retardation and interaction with the magnetic field components of the laser beam. We predicted cross sections for the inter-shell trielectronic recombination (TR) and quadruelectronic recombination processes which have been experimentally confirmed in electron beam ion trap measurements, mainly for C-like ions, of Ar, Fe and Kr. For Kr30+, inter-shell TR contributions of nearly 6% to the total resonant photorecombination rate were found.
Resonance fluorescence of laser-driven highly charged ions is investigated with regard to precisely measuring atomic properties. For this purpose an ab initio approach based on the Dirac equation is employed that allows for studying relativistic ions. These systems provide a sensitive means to test correlated relativistic dynamics, quantum electrodynamic phenomena and nuclear effects by applying x-ray lasers. We show how the narrowing of sidebands in the x-ray fluorescence spectrum by interference due to an additional optical driving can be exploited to determine atomic dipole or multipole moments to unprecedented accuracy.
Interactions between light and matter is one way to study both particles whose small spatial sizes render them irresolvable to our eyes and events where the characteristic time scale is too short for conventional recording and measurement techniques. The advent of free electron lasers allow laser pulses of high intensity and small temporal duration such that ultra-fast dynamics in matter systems and resulting non-linear effects can be resolved experimentally. This thesis discusses some theories to mathematically model such light-matter interactions and presents results based both on analytical expressions—where such expressions are available— from the Schrödinger and the Dirac equations as well as numerical calculations. Some models presented are a semi-classical model where light is treated classically and the atomic system quantum-mechanically and a statistical model which describes the light as quantized to feature effects like spontaneous emission and thermal excitation. The main theme of the thesis is relativistic effects and scalings of interaction effects with nuclear charge Z. The results discussed in the work shows that the equivalent characteristics of the dynamics of highly ionized hydrogen-like atoms predicted by the Schrödinger equation fail when relativistic effects are taken into account. This is shown to be a result of the quantities such as energy and mean radial distance from the nucleus not being exactly proportional to powers of Z in Dirac’s description of quantum mechanics. Further, deviations occur due to the necessary detuning of the driving laser field when spin-orbit interactions (a relativistic effect) split states of non-zero orbital momentum. Also, it is shown that as the characteristic time scales of interactions and inherent effects such as spontaneous emission scale differently with Z, the characteristic dynamics are not invariant to atomic number and it is not possible to rescale quantities to make them so. This work provides a guide for future Free-electron laser experiments on highly charged ions at X-ray wave- lengths.
Resonance fluorescence of laser-driven atoms is studied in the relativistic regime by solving the time-dependent Dirac equation in a multi-level model. Electron spin and retardation of the electron–photon interaction give rise to new phenomena such as splitting of sideband peaks and modification of the Rabi oscillator frequencies not explainable in a nonrelativistic theory. In our theoretical investigation, we show how, by applying coherent light with x-ray frequencies, the relativistic fluorescence spectrum can be exploited to determine atomic multipole matrix elements.
On the basis of the analytical solution of the Dirac equation the resonant multiphoton excitation of highly charged hydrogenlike ions in a strong high-frequency laser field is investigated. It is shown that the dynamics of the relativistic system is equivalent to two independent four-level systems (in the electrical-dipole approximation). The time evolution of the system is found using the nonperturbative resonant approach. We predict that due to multiphoton excitation by an appropriate laser pulse one can achieve various coherent superposition states in atomic or ionic systems with a high nuclear charge.
The resonant multiphoton resonances shifts and widths and ionization cross-sections for multielectron atoms in a intense laser radiation field are studied. It is carried out a new consistent approach to atom in a strong realistic laser field, based on the relativistic energy formalism (S-matrix adiabatic formalism), relativistic Dirac equation Green function method and relativistic many-body perturbation theory with the Dirac-Kohn-Sham zeroth approximation. In relativistic theory, the Gell-Mann and Low adiabatic formula for energy shift is connected with electrodynamical scattering matrice, which includes interaction with as a laser field as a photon vacuum field (radiative decay). The optimized basis of the relativistic orbitals is generated with using a minimization procedure for the gauge-non-invariant contribution (the fourth-order of the QED perturbation theory) to the radiation width of atomic state. An approach to treating the multiphoton atomic processes is outlined on example of H, Cs, Kr,Mg etc. Analysis shows that the shift and width of the multi-photon resonance line for the interaction “atom- multimode laser pulse” is greater than the corresponding shift and width for a case of the “atom- single-mode pulse” (the Lorenz pulse model) interaction. From the physical point of view it is naturally provided by action of the photon-correlation effects and influence of the laser pulse multi-modity.
We present the relativistic power spectrum for the interaction of an x-ray field with an highly charged ion, obtained by solving the time-dependent Dirac equation in a two-level approximation [1]. The phenomenon of splitting of sideband peaks is obtained as a consequence of the introduction in theory of the electronic spin. Also this phenomenon together with the retardation of the electron-photon interaction changes the values of Rabi frequencies. After describing the fluorescence spectrum, we show how it can be used to determine atomic multipole matrix elements.