Fig. 1: Impact of ponderomotive shift on ionization threshold and Rydberg levels.
From: Above-threshold ionization with X-ray free-electron lasers
a Illustrates the processes discussed in this work, where the intensity I is less than the channel closure value I(cl). b Shows processes not covered in this work, where the intensity exceeds I(cl). \({\tilde{\varepsilon }}_{{{{\rm{Ryd.}}}}}\) denotes the energy level of Rydberg states that shift with the ionization threshold. Superscripts 1 and 2 label separate Rydberg levels with energy difference Δ between them.
