During the past decades, the development of laser technology has produced pulses with increasingly higher peak intensities.
These can now be made such that their strength rivals, and even exceeds, the atomic potential at the typical distance of an
electron from the nucleus. To understand the induced dynamics, one can not rely on perturbative methods and must instead
try to get as close to the full machinery of quantum mechanics as practically possible. With increasing field strength, many
exotic interactions such as magnetic, relativistic and higher order electric effects may start to play a significant role. To
keep a problem tractable, only those effects that play a non-negligible role should be accounted for. In order to do this, a
clear notion of their relative importance as a function of the pulse properties is needed.
In this thesis I study the interaction between atomic hydrogen and super-intense laser pulses, with the specific aim to
contribute to the knowledge of the relative importance of different effects. I solve the time-dependent Schrödinger and
Dirac equations, and compare the results to reveal relativistic effects. High order electromagnetic multipole effects are
accounted for by including spatial variation in the laser pulse.
The interaction is first described using minimal coupling. The spatial part of the pulse is accounted for by a series
expansion of the vector potential and convergence with respect to the number of expansion terms is carefully checked.
A significantly higher demand on the spatial description is found in the relativistic case, and its origin is explained. As
a response to this demanding convergence behavior, an alternative interaction form for the relativistic case has been
developed and presented.
As a guide mark for relativistic effects, I use the classical concept of quiver velocity, vquiv, which is the peak velocity of a
free electron in the polarization direction of a monochromatic electromagnetic plane wave that interacts with the electron.
Relativistic effects are expected when vquiv reaches a substantial fraction of the speed of light c, and in this thesis I consider
cases up to vquiv=0.19c. For the present cases, relativistic effects are found to emerge around vquiv=0.16c .

Keywords: Time-dependent Dirac equation, Beyond dipole effects, Relativistic effects, High-Intensity laser-matter
interaction.
Stockholm 2017
http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-147749