Multi-sideband interference structures observed via high-order photon-induced continuum-continuum transitions in helium

As a continuation of our recent work on argon [1], we report a joint experimental and theoretical study of a three-sideband (3-SB) modification of the ''reconstruction of the attosecond beating by interference of two-photon transitions'' (RABBIT) setup [2-4]. The 3-SB arrangement makes it possible to investigate phases resulting from interference between transitions of different orders in the continuum, independent of a chirp in the harmonics, by comparing the RABBIT phases extracted from specific SB groups formed by two adjacent harmonics [5,6]. While experimentally more challenging than argon, using a helium target has the advantage of a well-defined orbital angular momentum after the 1s --> εp step induced by the high-harmonic radiation, which simplifies the analysis of the results. Furthermore, calculations in the single-active electron (SAE) approximation are expected to be of similar quality as predictions based on much more sophisticated approaches such as the multi-electron R-matrix with time dependence method (RMT) [7].

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[3] H. Muller, Appl. Phys. B 74 (2002) S17.

[4] J. Dahlström et al., Chem. Phys. 414 (2013) 53.

[5] A. Harth et al., Phys. Rev. A 99 (2019) 023410.

[6] D. Bharti et al., Phys. Rev. A 103 (2021) 022834.

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