Abstract

The recent development of ultrashort, highpower lasers allows the generation of higher-order harmonic waves in the extreme ultraviolet and soft x-ray regions.^[1-3] In the past experiments, a supersonic jet of rare gases was used as a nonlinear medium for the higher-order harmonic generation to suppress self-absorption. For short wavelength pumping sources such as a KrF lasers, however, rare gases are easily ionized by multiphoton absorption of several pump photons. The multiphoton ionization which competes with the harmonic generation decreases the harmonic conversion efficiency especially in the higher orders, because the generation of the higher-order harmonics needs higher pump intensity. Lompré et al reported a linear increase of the highest-order harmonics number with an increase of the ionization potential of rare gases.^[4] If a nonlinear medium with a higher ionization potential than that of rare gases is used, much higher-order harmonics would thus be generated. In this paper, we report the first observation of the higher-order harmonics in rare-gas-like ions that have higher ionization potentials than rare gas atoms. For example, helium like Li ions are isoelectronic to He atoms, while the ionization potential of the Li ion (75. 6eV) is three times larger than He atom (24.6 eV).

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