Multiphoton interaction phase shifts in attosecond science

Ab initio simulations of a range of interferometric experiments are used to identify a strong dependence on multiphoton phase shifts in above-threshold ionization. A simple rule of thumb for interaction phase shifts is derived to explain both the conservation of photoelectron yield and its absolute CEP-dependence. For instance, it is found that interferometric above-threshold ionization experiments are shifted by $\pi/4$ relative to RABBIT experiments, and that there is no RABBIT-term in a laser-assisted photoionization experiment with odd and even harmonics. Thus, our work helps to resolve the issues of interpretation of quantum dynamics in attosecond and free-electron laser sciences

Applications of time-dependent configuration-interaction singles for photoelectrons in attosecond and free-electron laser sciences

This thesis addresses the theoretical treatment of photoelectrons in attosecond and free-electron laser sciences. Wehave implemented methods that extract information on the photoionization process, which we use to investigateinterferometric schemes with particular focus on multiphoton ionization and strong-field effects in attosecondmetrology and coherent control of photoelectrons in free-electron laser science.This thesis comprises six papers.In Paper I, we investigate photoelectron angular distributions in laser-assisted photoionization. We confirmearlier predictions of a propensity rule for continuum transitions and discuss interference effects.In Paper II, we interpret experiments from attosecond and free-electron laser sciences in terms of an interactionphase and devise a rule of thumb for predicting general phase and amplitude effects in these experiments.In Paper III, we experimentally determine the one-photon dynamics for each angular momentum channel, as afunction of energy, by studying two-photon photoelectron angular distributions.In Paper Iv, we experimentally and theoretically investigate periodic population transfer between two boundstates in helium at XUV wavelengths by studying the photoelectron spectra.In Paper v, we present a general implementation of time-dependent configuration interaction for atomic sys-tems.In Paper vI, we discuss size-consistency problems in velocity gauge in truncated configuration spaces and pro-pose a correction based on the Thomas–Reiche–Kuhn sum rule. We further predict the occurrence of dynamicalinterference in laser-assisted photoionization

General Time-Dependent Configuration-Interaction Singles II: The Atomic Case

We present a specialization of the grid-based implementation of the time-dependent configuration-interaction singles described in the preceding paper [S. Carlström et al., preceding paper, Phys. Rev. A 106, 043104 (2022)]. to the case of spherical symmetry. We describe the intricate time propagator in detail and conclude with a few example calculations. Among these, of note are high-resolution photoelectron spectra in the vicinity of the Fano resonances in photoionization of neon and spin-polarized photoelectrons from xenon, in agreement with recent experiments

Propensity rules and interference effects in laser-assisted photoionization of helium and neon

We investigate the angle-resolved photoelectron spectra from laser-assisted photoionization for helium and neon atoms using an \textit{ab initio} method based on time-dependent surface flux and configuration interaction singles. We find that the shape of the distributions can be interpreted using a propensity rule, an intrinsic difference in the absorption and emission processes, as well as interference effects between multiple paths to the final angular momentum state. In neon we find that the difference between absorption and emission is hidden in the first sideband due to the multiple competing $m$ channels. Together, this aids the understanding of the formation of minima in the angular distributions, which can be transferred to an improved understanding on photoionization time delays in attosecond science

Thomas--Reiche--Kuhn Correction for Truncated Configuration Interaction Spaces: Case of Laser-Assisted Dynamical Interference

The Thomas--Reiche--Kuhn sum rule is used to form an effective potential that is added to the time-dependent configuration interaction singles (TDCIS) equations of motion in velocity gauge. The purpose of the effective potential is to include virtual coupling from singles to doubles, which is required for size-consistent velocity gauge TDCIS results. The proposed method is compared to length gauge TDCIS results for laser-assisted photoionization. Finally, a novel dynamical interference effect controlled by two-color fields is predicted for atomic targets

Extended analysis of a pseudo-spectral approach to the vortex patch problem

A prestudy indicated superior accuracy and convergence properties of apseudo-spectral method compared to a spline-based method implemented byCòrdoba et al. in 2005 when solving the α-patches problem. In this thesis wefurther investigate the numerical properties of the pseudo-spectral method and makeit more robust by implementing the Nonequispaced Fast Fourier Transform. Wepresent a more detailed overview and analysis of the pseudo-spectral method and theα-patches problem in general and conclude that the pseudo-spectral method issuperior in regards to accuracy in periodic settings

Extended analysis of a pseudo-spectral approach to the vortex patch problem

A prestudy indicated superior accuracy and convergence properties of apseudo-spectral method compared to a spline-based method implemented byCòrdoba et al. in 2005 when solving the α-patches problem. In this thesis wefurther investigate the numerical properties of the pseudo-spectral method and makeit more robust by implementing the Nonequispaced Fast Fourier Transform. Wepresent a more detailed overview and analysis of the pseudo-spectral method and theα-patches problem in general and conclude that the pseudo-spectral method issuperior in regards to accuracy in periodic settings

Extended analysis of a pseudo-spectral approach to the vortex patch problem

A prestudy indicated superior accuracy and convergence properties of apseudo-spectral method compared to a spline-based method implemented byCòrdoba et al. in 2005 when solving the α-patches problem. In this thesis wefurther investigate the numerical properties of the pseudo-spectral method and makeit more robust by implementing the Nonequispaced Fast Fourier Transform. Wepresent a more detailed overview and analysis of the pseudo-spectral method and theα-patches problem in general and conclude that the pseudo-spectral method issuperior in regards to accuracy in periodic settings