Ultrafast Atomic Physics Between The Perturbative And Strong Field Regimes

Ultrafast Atomic Physics Between The Perturbative And Strong Field Regimes S Marshallsay*, L Hutcheson, H W van der Hart, and A C Brown† Centre For Light Matter Interactions, Queen's University Belfast, BT7 1NN, UK! *[email protected] †andrew.br[email protected] [1] Moore, L. R. et al. Journal of Modern Optics 58, 1132–1140 (2011).! [2] Brown, A. C. et al. Computer Physics Communications 250, 107062 (2020).! [3] Kaldun, A. et al. Science 354, 738–741 (2016).! [4] Ott, C. et al. Science 340, 716–720 (2013).! [5] Hutcheson, L. et al. Phys. Rev. Res. 7, L022074 (2025). Outer Region 0 or 1 ionised electrons.! Exchange effects neglected.! Interacts with residual ion via a long range potentials and boundary conditions. Inner Region Many electrons.! Full multi-electron dynamics. Electric Field Arbitrarily polarised laser pulses.! Strong and weak fields.! Ultrashort pulses. Different numerical schemes tailored to each region. Low Intensity Perturbative Effects High Intensity Strong Field Effects Complex Phenomena We apply RMT to study time resolved autoionisation in Neon with intermediate NIR intensities, finding a complex interplay between the field and the rich atomic structure. ATAS Of Neon Autoionising States -, XUV pump pulse coherently excites from the ground state to a range of above threshold states in neutral Neon.! -, , NIR probe pulse drives electron dynamics within these states.! - Absorption spectra of the XUV induced transitions calculated for a range of pumpprobe time delays, with a focus on the state. 600 as 45.5 eV 17 fs 1.4 eV 1012 Wcm−2 2s 2p63p R-Matrix With Time-Dependence (RMT) - Uses ab initio, non-perturbative techniques [1, 2] to solve for the full time-dependent multielectron wavefunction.! - The full wavefunction gives access to experimental observables:! - Harmonic spectra.! - Photoelectron spectra.! - Absorption spectra. - At negative time delays the NIR acts mostly on the ground state, having little effect on the spectrum.! - At positive time delays fringes from a truncation of the autoionisation process [3] begin to form.! -As the two pulses overlap ( to ) the resonance undergoes a strong phase shift. −4fs 4fs Fitting The Absorption Spectrum - Fitting Fano profiles to the absorption spectrum [4] allows information about interference and population of the given state to be extracted.! - Fitting is robust at negative time delays where resonances in the spectrum are Fano-like.! - At high positive time delays truncation of the autoionisation process causes resonances not to be Fano-like, preventing effective fitting.! - At time delays close to zero, where the most complex interactions are expected to occur, fitting procedures still often fail to find good solutions.! - To extract useful data over the full range of time delays alternative method is required. Direct Access To The Dipole - RMT calculates the full atomic wavefunction, meaning the dipole response can be calculated directly.! - The phase and amplitude information from the dipole response at a given energy contain the same time-delay-dependent information as the absorption spectrum resonances [5].! - Rising amplitude at high time delays caused by the autoionisation process being truncated at later times.! - Some evidence of two-photon coupling from -oscillations in phase.! - Main feature of phase trend is a shift in and return of the phase as the pump and probe overlap. 2ω - Removing nearby thresholds from the calculation basis leads to the phase shift effect disappearing completely.! - To recover the effect all nearby atomic structure must be added back.! - This effect is caused by a complex interplay between the field and the rich structure.