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A subpicosecond KrF* laser system capable of producing 20 +/- 2-mJ pulses has been developed. The means of producing ultrashort seed pulses for the KrF* amplifier system and characteristics of the full system are described. It is shown that efficient subpicosecond energy extraction is possible.

The availability of extraordinarily bright femtosecond ultraviolet sources is rapidly extending the study of nonlinear atomic responses into an unexplored regime involving intensities in the range of ∼ 10^20 W/cm 2. An estimate is made, covering approximately ten orders of magnitude in intensity, of the effective cross section <σNγ> for nonlinear energy transfer to atoms undergoing subpicosecond irradiation. This treatment, which includes : (a) threshold measurements for low stages of ionization in the low intensity regime (< 10^14 W/cm 2), (b) the experimentally determined average cross section for total energy transfer at an intermediate intensity (∼ 10^16 W/cm 2), and (c) theoretical estimates for the strong field regime (> 10^19 W/cm2), indicates that the cross section for energy transfer in the high intensity (> 10^19 W/cm2) high Z limit falls in a relatively narrow range between simply established upper and lower bounds. The values of these limits are σm = 8 π λ2 c (upper) and the magnitude of the total photoabsorption cross section of Cf at the K edge (lower). Based on this analysis, the maximum cross section for heavy atoms in the high intensity limit is expected to be approximately <σ Nγ> max ∼ 10^-20 cm2, a value which represents an energy transfer rate of ∼ 1 W/atom for an assumed intensity of 10^20 W/cm2. Coupling of this strength would enable the creation of highly energetic and strongly nonequilibrium states of matter and motivates the conclusion that stimulated emission in the X-ray range can be generated by these means

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