Minima in low-energy above-threshold-ionization spectra induced by electronic structure

A structure with local minima is found in the low-energy part of the above-threshold-ionization electron spectrum for several one-electron atomic and molecular targets initially in excited electronic states. The ionization regime is neither tunneling nor pure multiphoton with the Keldysh parameter slightly larger than unity. The effect is nonperturbative and depends on the frequency and intensity of the driving field. Although the ionization process is generally nonresonant, our analysis reveals an important role of intermediate bound states in formation of the structure.