High-order harmonic generation from atoms and ions in the high intensity regime

We present calculated optical harmonic spectra for atoms and ions in the high intensity regime to current short-pulse experiments. We find that ions can produce harmonics comparable in strength to those obtained from neutrals, and that the emission extends to much higher order. Simple scaling laws for the strength of the harmonic emission and the maximum observable harmonic are suggested. These results imply that the photoemission observed in recent experiments in helium and neon contains contributions from ions as well as neutrals.