Strong field sub-femtosecond electronic processes

Abstract

This thesis is comprised of theoretical investigations on several different strong field and/or sub-femtosecond processes resulting from the interaction between atoms and short laser pulses. Said theory is based on the numeric solution of the time-dependent Schro\"odinger equation (TDSE) by high performance computing methods. Specifically, Chapter 2 examines the Attoclock, a strong field problem designed to clock the escape of an electron as it tunnel ionises. In which, we present both the result of a collaboration yielding the first agreement between ab initio theory and experiment [Sainadh et al., Nature 568, 75 (2019)], and a straightforward model based on classical scattering for an idealised version of the problem [Bray et al., Phys. Rev. Lett. 121, 123201 (2018)]. Chapter 3 considers reconstruction of attosecond beating by interference of two-colour transitions (RABBITT) in which an attosecond pulse train 'pump' and infrared pulse 'probe' simultaneously impinge on a target with a precisely controlled delay between them. The oscillating phase of the ionisation probability as a function of this delay yields the angular anisotropy parameter and Wigner time delay for its corresponding energy. We calculate and present these quantities for the valence p-shell of various noble gas atoms [Bray et al., Phys. Rev. A 97, 063404 (2018)] and additionally examine the effect of an encapsulating C60 fullerene cage on the 4d shell of Xe [Bray et al., Phys. Rev. A 98, 043427 (2018)]. In Chapter 4 we look at the effect of electron correlation on high harmonic generation (HHG), the process by which attosecond pulses are produced, from one and two colour fields. We perform single active electron calculations for the 5p shell of Xe and model the correlation as an enhancement factor taken as the ratio between photoionisation cross-sections computed with and without said correlations. Doing so we report solid agreement with experimentally observed spectra for both field setups [Bray et al., Phys. Rev. A 100, 013404 (2019)]. Finally, Chapter 5 investigates the non-dipole problem of the state resolved strong field acceleration of neutral species. This requires the solution of the coupled two-body TDSE of the centre of mass and reduced mass electron, each with three degrees of freedom, in a non-spatially uniform field. Accordingly it necessitates its own dedicated solution method. Developing and applying said method to atomic hydrogen we compute an acceleration for each state consistent with experimental observation [Bray et al., Phys. Rev. Lett., Submitted]. Additionally our method allows us, via comparison with classical expressions, to derive the time at which each excited state was produced and, by similar means, an effective polarisibility for the ground state. Most interestingly this latter value is of opposite sign to the typical +9/2, providing an unambiguous signature of having entered the Kramers-Henneberger regime.