INFLUENCE OF THE DENSITY OF A MEDIUM ON HIGH-ORDER HARMONIC-GENERATION BY ATOMS IN STRONG LASER FIELDS

An investigation is reported of the maximum possible increase in the d ensity of a medium needed to increase the efficiency of high-order har monic generation by atoms ionised with high-intensity radiation. This investigation is based on a classical description of the motion of a q uasi-free photoelectron in a continuum and of its perturbation by the surrounding ions. A strong dependence of the optimal concentration of atoms on the pump radiation wavelength is revealed. The spectral chara cteristics of high-order harmonic generation resulting from an increas e in the density of the medium are predicted.