Probing Near-Threshold Electron Dynamics in Neon Using Attosecond Transient Absorption Spectroscopy

In recent work, we used a variant of the Reconstruction of Attosecond Beating by Interference of Two-photon Transitions (RABBITT) technique [1], Under-Threshold RABBITT (uRABBITT) [2], to study photoionization dynamics in neon atoms. uRABBITT experiments involve the use of an IR laser (which acts as a probe) and a time-delayed XUV pulse train (acting as a pump) to generate photoelectron kinetic energy spectra. We found that for a driving IR wavelength of 1006nm and an XUV pulse train containing the 17^th – 33^rd odd harmonics of the IR, the threshold sideband contained contributions from both the 2p⁵4d and 2p⁵6s bound states of neon, which manifest as a double-peak structure, and that the relative heights of these peaks were heavily influenced by the XUV-IR delay in the laser pulse [3, 4]. In order to explore this phenomenon in more detail, we have performed Attosecond Transient Absorption Spectroscopy (ATAS) calculations using the R-Matrix with Time-dependence method [5]. ATAS is a powerful technique for studying the dynamics of the electron on its natural time scale, including near ionization thresholds [6]. In this presentation we will compare the results from our previous uRABBITT investigations with new ATAS calculations, and comment on any similarities and discrepancies in the information obtained from these techniques.

[1] P. Paul et al., Science 292 (2001) 1689

[2] D. M. Villeneuve, P. Hockett, M. J. J. Vrakking, and H. Niikura, Science 356 (2017) 6343

[3] M. Moioli et al., Phys. Rev. Res. 7 (2025) 023034

[4] H. Mekuria et al., Bull. Am. Phys. Soc. (2024) K00.00042

[5] A. C. Brown et al., Comp. Phys. Commun. 250 (2020) 107062

[6] M. Wu et al., J. Phys. B: At. Mol. Opt. Phys. 49 (2016) 062003