Residual gaps between measured and computed atomic photoemission delays
In plain words
When light knocks an electron out of an atom, the electron leaves a few attoseconds late, and the delay depends on which shell it came from. For some shells the measured delays still differ from the best calculations by more than the error bars.
Precise statement
Relative photoionization delays $\Delta \tau = \tau(2s) - \tau(2p)$ in Ne at photon energies near $100\,\mathrm{eV}$ and $\tau(3s) - \tau(3p)$ in Ar near the 3s Cooper minimum (the energy where the 3s dipole matrix element passes through zero), measured by attosecond streaking and by RABBITT (two-photon interferometry with an XUV pulse train and an infrared field). Attosecond streaking gave $\Delta \tau(\mathrm{Ne}) = 21 \pm 5\,\mathrm{as}$, about twice the many-body theory values (roughly 6 to 12 as). An answer is a set of streaking, RABBITT and ab initio values (including the measurement-induced continuum-continuum delay and shake-up channels) agreeing within about 2 as for both systems, or an identified physical effect that accounts for the difference.
What would settle it
Streaking and RABBITT measurements on the same Ne and Ar subshells with resolved shake-up satellites, compared with converged many-body calculations of the full two-color process.
Status in the literature
Unverified note
RABBITT measurements resolving shake-up (Isinger et al., Science 2017) agreed with theory for Ne; the original streaking value and the Ar 3s delays near the Cooper minimum remain unreconciled in later theory (e.g. Chernysheva and Yarzhemsky, JETP Letters 2024, https://doi.org/10.1134/s0021364024602148).