Under-threshold RABBITT phase evolution in the presence of multiple resonances

The Reconstruction of Attosecond Beating by Interference of Two-photon Transitions (RABBITT) technique is an invaluable tool in atomic and molecular spectroscopy, and has, for example, been extensively applied to the measurement of attosecond photoemission delays in atoms and molecules [1,2]. In one increasingly common variant of the technique, Under-Threshold RABBITT (uRABBITT) [3], the lowest-energy harmonic in the XUV frequency comb is resonant with one or more bound states below the ionization threshold. Consequently, the lowest-energy feature in the RABBITT spectrum, namely the threshold sideband, can contain contributions from bound states of the target atom. This work builds off a recent joint experimental and theoretical study employing uRABBITT schemes in neon [4]. Here it was observed that for a driving IR laser with a wavelength of 1006nm and XUV pulse train containing the 17^th through 33^rd odd harmonics of the IR, contributions from both the 2p⁵4d and 2p⁵6s states manifest in the threshold sideband as a double-peak structure. In this work we use the R-Matrix with Time-dependence method [5] to further explore this phenomenon, particularly at larger XUV-IR delays than in the original work. We observe a time-delay dependent phase behavior across the threshold sideband, with each peak exhibiting different phase behavior.

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

[2] J. M. Dahlström, A. L’Huillier, and A. Maquet, J. Phys. B 45 (2012) 183001

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

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

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