Coherent ionization dynamics induced by intense X-ray free-electron laser pulses
type: CreativeWork
description: In this dissertation, theoretical models of atoms, clusters, and solids irradiated by ultrashort and intense pulses from an (X-ray) free-electron laser are investigated with the objective to find appropriate conditions where coherent dynamics manifests itself in physical observables. The ionization dynamics induced by an X-ray or extreme ultraviolet laser pulse is typically dominated by inner-shell one-photon absorption and subsequent relaxation processes, ultimately leading to sequential multiphoton ionization. If atoms are exposed to laser pulses with a photon energy well separated from absorption edges, a rate-equation description is generally sufficient for describing the temporal evolution of charge states. As this thesis is geared towards coherent ionization dynamics, scenarios have been selected for further analysis where the energy of incident photons is either in the vicinity of or below the most relevant absorption edge. Resonant two-photon-absorption is discussed within a generic model comprising a two-level system plus an energetically separated continuum of states. The two discrete states are in resonance with the electric field of a laser pulse; the ground state energy (relative to the lowest continuum state) is less than twice the photon energy, allowing for an ionization process based on the absorption of two photons. Under these circumstances, the results of two distinct approaches are compared. In the Markovian theory, on the one hand, the coupling to the continuum is reduced to the rate-equation level while possible Rabi cycling between the two discrete states is fully taken into account. The rate-equation approach, on the other hand, further simplifies the situation by treating the two-level system analogous to the Einstein rate equations. It is demonstrated that the ionization probability grows quadratically with increasing number of incident photons for weak pulses, which goes over into a linear scaling behavior for more intense pulses. This fi…
abstract: In this dissertation, theoretical models of atoms, clusters, and solids irradiated by ultrashort and intense pulses from an (X-ray) free-electron laser are investigated with the objective to find appropriate conditions where coherent dynamics manifests itself in physical observables. The ionization dynamics induced by an X-ray or extreme ultraviolet laser pulse is typically dominated by inner-shell one-photon absorption and subsequent relaxation processes, ultimately leading to sequential multiphoton ionization. If atoms are exposed to laser pulses with a photon energy well separated from absorption edges, a rate-equation description is generally sufficient for describing the temporal evolution of charge states. As this thesis is geared towards coherent ionization dynamics, scenarios have been selected for further analysis where the energy of incident photons is either in the vicinity of or below the most relevant absorption edge. Resonant two-photon-absorption is discussed within a generic model comprising a two-level system plus an energetically separated continuum of states. The two discrete states are in resonance with the electric field of a laser pulse; the ground state energy (relative to the lowest continuum state) is less than twice the photon energy, allowing for an ionization process based on the absorption of two photons. Under these circumstances, the results of two distinct approaches are compared. In the Markovian theory, on the one hand, the coupling to the continuum is reduced to the rate-equation level while possible Rabi cycling between the two discrete states is fully taken into account. The rate-equation approach, on the other hand, further simplifies the situation by treating the two-level system analogous to the Einstein rate equations. It is demonstrated that the ionization probability grows quadratically with increasing number of incident photons for weak pulses, which goes over into a linear scaling behavior for more intense pulses. This fi…
author: Brand, André
date published: 2016-01-18
date modified: 2026-07-27T18:49:49Z
provider: Universität Bayreuth
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: Universität Bayreuth