Energy breakdown of Wigner and Coulomb-laser-coupling time delays

Attosecond pump–probe measurement techniques, such as RABBITT or streaking, typically combine XUV and IR pulses to resolve electron dynamics within atoms and molecules. At sufficiently large kinetic energies, the total measured photoionization delay is often treated as the sum of independent Wigner and Coulomb–laser–coupling (CLC) contributions. While it is known that this assumption is no longer valid as the photoelectron's kinetic energy approaches zero, the precise details for when this breakdown begins have not been widely agreed upon. These questions are becoming increasingly relevant as recent ultrafast experiments in molecules are starting to utilize probe wavelengths shorter than 800 nm to avoid spectral congestion. In this work we present an analytical cut‑off law that specifies the minimum photoelectron energy at which the total delay can be reliably approximated as the sum of Wigner and CLC terms for a given laser probe frequency. Our analysis shows that when the probe wavelength is around 400 nm, the separability between short‑range (Wigner) and long‑range (CLC) delays begins to break down near 23 eV. A result that is significantly higher than what is commonly assumed and has important consequences for the interpretation of photoionization time delay studies using visible-light probing fields.