Structures with local minima at low energies in above-threshold ionization electron spectra*

We perform a theoretical and computational study of multiphoton above-threshold ionization (ATI) of several one-electron atomic (H and He⁺) and diatomic (H₂⁺ and HeH²⁺) quantum systems. The driving laser fields are in the near- and mid-infrared frequency range and have moderate peak intensities corresponding to the Keldysh parameter slightly larger than unity. Our calculations reveal structures with local minima in the low-energy part of the ATI electron energy spectra if the targets are initially in the excited electronic states: 2s states for H and He⁺, 1σ_u state for H₂⁺, and 2σ state for HeH²⁺. The effect is non-perturbative and depends on the frequency and intensity of the driving field. The structures in the ATI electron energy spectra have the same nature for all the systems under consideration. Although the ionization process is generally nonresonant, our analysis shows an important role of intermediate electronic energy levels between the initial state and the onset of the continuum. For the hydrogen atom initially in the 2s state, we identify the intermediate states with the principal quantum number n=3 as those responsible for shaping the structure with the local minimum in the low-energy part of the ATI electron energy spectrum.