Harmonic generation with ionizing two-level atoms

We study the harmonics generated by a two-level system in which we include the effects of ionization and recombination. The ionized wavepackets are treated as free classical particles moving inside the electric field. We recover the basic features of the single-atom harmonic generation in the tunnelling limit, finding a good agreement between our model and the one-dimensional Schrodinger equation results. The model allows us to investigate the role of bound states in the harmonic generation in the tunnelling regime. In particular, we show that the harmonic efficiency in the plateau region is strongly dependent on the strength of the dipole moment of bound-bound transitions.