ECAMP 15

The RABBITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transitions) technique is a foundational tool in attosecond science, enabling the precise extraction of amplitude and phase information from photoelectron spectra. Traditional RABBITT is limited to weak infrared (IR) fields and two-photon pathways. We have generalised the framework through the RABBITT with Higher Order Processes (RABBITT-HOP) formalism. In this work we show how RABBITT-HOP can explain results from experiments and simulations with helium, and how, even at moderate IR intensities, the signatures of higher-order processes can be extracted from photoelectron spectra.

An attosecond pulse train is constructed to target the $1s4p$ and $1s5p$ states in He, such that the resonant pathways imprint the characteristic `antiresonance' into the phase extracted from the neighbouring sideband [1]. RABBITT-HOP then predicts that the same resonant pathways will contribute to higher-lying sidebands/mainbands via four-photon transitions, and that the antiresonance should manifest in the phase of higher frequency oscillations via four photon transitions.

We verify these predictions with R-matrix with time-dependence [2] simulations of He, and identify the resonant signature in photoelectron signals in multiple sidebands and mainbands. Via analysis of photo-electron angular distributions (PAD) we provide further evidence of these higher-order processes [3]. Delay-resolved anisotropy parameters to reveal the role played by the $g_0$ partial waves (reached by four-photon transitions) to the spectral phase shift in the PAD. Even though the amplitude of the $g_0$ wave is tiny compared to the $s$ and $d_0$ waves, its interference is detectable. This provides an efficient method to visualise partial wave contributions beyond the perturbative regime.

[1] L Drescher et al., Phys. Rev. A 105, L011101 (2022).
[2] A C Brown et al., Comp. Phys. Commun. 250, 107062 (2020).
[3] W Jiang et al., Nat. Comm. 16, 381 (2025).