High-order harmonic generation with KrF and Ti:sapphire lasers is analyzed with an emphasis on the contribution of multiply charged ions. It is confirmed that the intensity dependence of strong optical field ionization is explained by the Ammosov-Delone-Krainov (ADK) theory or by the barrier suppression ionization model regardless of the wavelength. The harmonic distributions are compared with the single-atom responses calculated using the short-range potential model with the effective interaction intensity determined by the ADK theory. From this analysis, we propose that for the KrF laser the harmonics above the 21st in Ne and above the 13th in Ar may be due to doubly charged ions.

• Received 21 May 1993

DOI:https://doi.org/10.1103/PhysRevA.49.3881