Reaction microscope studies of relativistic strong-field effects in atomic systems

Abstract

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. > 1018 W/cm2.