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Modelling experimental conditions for strong field ionisation using R-Matrix with time-dependence calculations Lynda Ruby Hutcheson (24168972) PUREID: 607334382 Attosecond science high harmonic generation attosecond transient absorption spectroscopy atomic physics strong field ionisation Ab initio calculations Attoscience experiments, inherently macroscopic, involve ensembles of atoms or molecules responding to a focused laser beam. However, understanding the electronic response often requires decoupling single-atom behaviour from the macroscopic effects. As experimental techniques advance, measuring ever closer to the single-atom response, detailed calculations are increasingly necessary for accurate predictions and interpretations.<br><br>This thesis bridges the gap between attoscience experiments and theory, particularly in strong field ionisation. Chapter 2 outlines the spectroscopic techniques used; high harmonic spectroscopy (HHS) and attosecond transient absorption spectroscopy (ATAS). Chapter 3 details various theoretical methods for studying attoscience, including the R-Matrix with Time-dependence (RMT) method employed in this thesis.<br><br>Chapter 4 introduces a focal volume averaging method for high harmonic spectra, designed to reduce computational costs, making it feasible for expensive calculations like RMT. This method, validated using two different experimental regimes, helps (1) distinguish features of the experimental spectrum attributable to the single-atom response and (2) predict which single-atom spectral features may be observed experimentally.<br><br>Chapter 5 extends this method to two-colour, pump-probe laser schemes, discussing the additional considerations. It assesses the feasibility of observing an XUV-initiated high harmonic generation (XIHHG) mechanism and applies the method to intensity-average RMT calculations for ATAS.<br><br>Chapter 6 uses ATAS to explore sub-cycle structures in absorption lines in xenon during strong field ionisation. By analysing the phase of the absorption lines (both experimentally and via RMT calculations) alongside depletion of the ground state, a second ionisation pathway, dubbed 'XUV-assisted strong-field ionisation,' is identified as interfering with the direct ionisation pathway. Chapter 7 extends this work to address subtle differences between the experimental and theoretical results, offering an alternative presentation of absorption line-shapes and exploring the nature of the second ionisation pathway introduced in Chapter 6, by linking the observed interference pattern with residual ion populations from RMT calculations. 2026-10-01T16:35:54Z Text Thesis 10.17034/32639919.v1 https://figshare.com/articles/thesis/Modelling_experimental_conditions_for_strong_field_ionisation_using_R-Matrix_with_time-dependence_calculations/32639919 All Rights Reserved