Harmonic generation from ions in underdense aluminum and lithium–fluorine plasmas

High harmonic generation from laser interactions with preformed underdense plasmas was observed with subpicosecond ultraviolet laser pulses focused to intensities up to 1018 W/cm2. The generation of seventh and ninth harmonics from aluminum plasmas was measured as well as harmonics to the nineteenth order from lithium–fluorine plasmas. The harmonic generation efficiency measured in these experiments is less than that from previous experiments that used neutral gases.