Importance of excited bound states in harmonic generation

We discuss the mechanism of harmonic generation for a one-dimensional model atom with several bound states in a strong, driving laser field. We extract the bound- and continuum-components of the harmonic radiation and investigate their temporal and spectral characteristics. We compare the bound-bound contributions from a fully numerical solution of the Schrodinger equation with the results from a two-level atom, and study the influence of effective ionization rates in the latter. In the generation of the low harmonics, bound-bound transitions play a decisive role. However, a two-level atom model is not adequate for reproducing their characteristics. High-harmonic generation is almost entirely determined by bound-continuum transitions, the transitions involving the ground state being dominant, [S1050-2947(98)11009-0].