Conjoint influence of quantum interference and Freeman resonance on substructures of the photoelectron spectra in above-threshold ionization

Based on the numerical solution of the full-dimensional time-dependent Schrodinger equation, we study the above-threshold ionization of atomic hydrogen by subpicosecond laser pulses at wavelengths ranging from 300 nm to 800 nm, in which regime substructures of the photoelectron energy spectra resulting from quantum interference are entangled with those induced by the Freeman resonance. By analyzing the time-dependent populations and ac Stark energy shifts of the relevant atomic levels, and observing the pulse intensity dependence and the pulse length evolution of the spectra, we clearly identify the origins of the substructures exhibited in the spectra.