Under-threshold RABBITT in the presence of multiple resonances

The Reconstruction of Attosecond Beating by Interference of Two-photon Transitions (RABBITT) technique has been extensively applied to the measurement of attosecond photoemission delays in atoms and molecules [1,2]. In one 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 is a sideband generated by interference between a bound-bound XUV + bound-free IR transition, and a bound-free XUV + free-free IR transition. In recent experimental work examining uRABBITT schemes in neon [4], contributions from both the 2p⁵4d and 2p⁵6s states are visible in the threshold sideband, with each peak exhibiting different phase behavior. In this work we seek to 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.





[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., Bull. Am. Phys. Soc. 66 (2021) S02.00009

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