Circular RABBITT goes under threshold

TL;DR

The paper introduces cuRABBITT, a novel circularly polarized attosecond interferometric technique for probing atomic electronic resonances with high spectral and temporal resolution, revealing new insights into resonant dynamics.

Contribution

It presents cuRABBITT as the first method combining circular polarization with broadband spectral analysis to map bound-state resonances and phase information in atoms.

Findings

Revealed strong resonances in helium and argon

Identified a Cooper-like minimum in xenon

Extended Fano's propensity rule into the under-threshold regime

Abstract

We introduce circular under-threshold RABBITT (cuRABBITT) as a new interferometric method to probe discrete electronic excitations in atoms with attosecond resolution. By combining circularly polarized attosecond pulses with broadband (``rainbow'') spectral analysis, we directly access two-photon ionization amplitudes and their relative phases. Time-dependent Schr\"odinger simulations, supported by Green's function theory, reveal strong resonances in helium and argon and a Cooper-like minimum in xenon. These results demonstrate that cuRABBITT provides continuous spectral mapping of bound-state resonances and extends Fano's propensity rule into the under-threshold regime. Our work establishes cuRABBITT as a powerful attosecond metrology technique, opening the way to polarization-resolved studies of resonant dynamics in atoms and molecules.