Generation of high-order harmonics using laser-produced rare-gas-like ions

We report the first observation of high-order harmonic waves from rare-gas-like ions excited by a subpicosecond KrF excimer laser. At an intensity of 10¹⁵ W/cm², the highest orders of observed harmonics are the ninth (27.6 nm) in Li⁺, the eleventh (22.6 nm) in Na⁺, and the thirteenth (19.1 nm) in K⁺. The advantage of such ions as nonlinear media for extreme ultraviolet generation is discussed in relation to ionization potentials.