Photoelectron signature of dressed-atom stabilization in an intense XUV field

Nonperturbative resonant multiphoton ionization (1+1) is studied using the resolvent operator technique. Scaling parameters for effective two-level Hamiltonians are computed for hydrogen and helium atoms to provide a quantitative description of Rabi oscillations at XUV wavelengths, which were recently observed using a seeded free-electron laser [S. Nandi, Nature (London) 608, 488 (2022)0028-083610.1038/s41586-022-04948-y]. The resulting photoelectron spectra exhibit a range of Autler-Townes doublets, which are studied for different intensities, detunings, and interaction times. We identify a photoelectron signature that originates from stabilization against ionization of helium atoms interacting with intense circularly polarized XUV light. Thus, our study shows how it is possible to test the prediction of dressed-atom stabilization by Beers and Armstrong [B. L. Beers and L. Armstrong, Phys. Rev. A 12, 2447 (1975)0556-279110.1103/PhysRevA.12.2447], without the demanding requirement of atomic saturation in the time domain.