title: Reaction microscope studies of relativistic strong-field effects in atomic systems
Creators: name: Haram, Nida
Publisher: name: Griffith University
Year: 2020
Subjects: subject: ultrashort pulsed laser sources
subject: reaction microscope
subject: fundamental strong- eld ionisation mechanisms
subject: few-cycle laser pulses
subject: transverse electron momentum distribution
subject: TEMD
subject: high-resolution photoelectron momentum spectra

description: The ultrashort pulsed laser sources and reaction microscope have been extensively employed to explore the fundamental strong-ﬁeld ionisation mechanisms over the last three decades. The aim of this thesis is to gain insight into the subtleties of the photoelectron dynamics inﬂuenced by the few-cycle laser pulses. In particular, it addresses the possible factors that may aﬀect the photoelectron dynamics. However, these factors are typically ignored under the umbrella of some commonly used approximations, which include the eﬀect of magnetic ﬁeld component of the laser ﬁeld and attractive Coulomb ﬁeld of the parent ion on the photoelectron dynamics. These eﬀects, collectively called relativistic nondipole eﬀects, are known to have subtle inﬂuence on the recollision electron dynamics and may ultimately aﬀect the important strong-ﬁeld processes relying on the recolliding photoelectrons. In this thesis, we explore the relativistic nondipole eﬀects by performing strong-ﬁeld ionisation experiments on the noble gas atoms using linearly-polarised near-infrared few-cycle laser pulses. The high-resolution photoelectron momentum spectra are acquired using a reaction microscope, since these eﬀects often manifest themselves in the transverse electron momentum distribution (TEMD). The ﬁrst study is undertaken at diﬀerent intensities in the range 1014–1015 W/cm2 using Ar as the target gas to investigate the dependence of these nondipole eﬀects on the intensity. The peak of the TEMD is found to be shifted in the direction opposite to the laser propagation (counter-intuitive) direction with increasing laser intensity. The combined eﬀect of the magnetic ﬁeld induced drift and Coulomb focusing of the parent ion accounts for this counter-intuitive peak shift of the TEMD. Owing to the importance of these eﬀects in the strong-ﬁeld ionisation processes, an ab initio fully relativistic 3D model based on the time-dependent Dirac equation (3D-TDDE) is formulated by our theoretical collaborator Igor Ivanov. The theoretical predictions agree quite well with the experimental results over the entire intensity range. Moreover, the simulations based on a Yukawa potential further support the interpretation, which suggests that the Coulomb force is responsible for the counter-intuitive peak shift, since they do not reveal any peak shift. The second study reports on the energy-resolved relativistic nondipole eﬀects, with the experiments performed at 1.5×1015 W/cm2 using Ar and Ne as target gases revealing unique low-energy features in the photoelectron momentum spectra. The experimental and theoretical results show energy-dependent tilting of the TEMD and spectral narrowing for the low-energy photoelectrons. The peak shift of the TEMD for the low-energy photoelectrons is found to be in the counter-intuitive direction, whereas high-energy photoelectrons always exhibit a positive peak shift. To get an intuitive picture of the underlying physical mechanisms, a modiﬁed three step model is proposed. In short, the results of this work need to be taken into account for the most commonly used laser parameters to explain the processes relying on recollision. Also, the theoretical model based on 3D-TDDE is essential to describe the strong-ﬁeld ionisation processes at the truly relativistic intensities i.e. &gt; 1018 W/cm2.