Fig. 4: Limits on perturbation theory as a function of photon energy.
From: Above-threshold ionization with X-ray free-electron lasers
Figure depicts two diagrams. a Represents the relationship between photon energy (ℏω) and intensity (I), and b illustrates the correlation between wavelength (λ) and intensity. The intensity values that lie above the curves, which correspond to \({{{\mathcal{Z}}}}=0.1\) (light blue lines) and \({{{{\mathcal{Z}}}}}_{1}=0.1\) (green lines), signify a deviation from the perturbative values. A curve denoted by \({{{\mathcal{Z}}}}\le 1-{{{{\mathcal{Z}}}}}_{0}\) (pink lines) symbolizes the channel closure condition and the maximum intensity permissible by our model. The curve where \({{{{\mathcal{Z}}}}}_{0}=2\) (vertical black lines) marks the wavelength at which \({{{\mathcal{Z}}}}={{{{\mathcal{Z}}}}}_{1}\). Additional curves are incorporated to underscore the characteristic behavior for a strong field at 250 eV (vertical yellow lines) and 800 nm (vertical red lines).
