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 mechanism of collision-free multiphoton ionization of rare gases irradiated with ultraviolet radiation at an intensity of up to -10 16 W/cm 2 at 248 ran and a pulse length of ~ 0.5 ps and -lO 1^ W/cm 2 at 193 nm and -5ps, respectively, has been studied by observing the electron energy spectra and ion charge state threshold intensities. The formation of multiply, charged ions by a sequential process of ionization has been directly detected in the electron energy spectra in the form of a characteristic pattern of interwoven above threshold ionization (ATI) ladder line series. The threshold intensities for ion production have been compared with the Keldysh model and were found to be in good agreement for light ions (Ne) and consistently lower for heavier ions (Xe), scaling with the atonic number. These measurements, together with estimates of the ion population dynamics during the rise of the laser pulse can be reasonably understood on the basis of a single electron picture.

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A picosecond 248 nm KrF* laser system producing a focal intensity of greater than 1015 W /cm2 has been used to study VUV fluorescence and harmonic generation in H2, He, Ne, Ar, Kr, and Xe.The nonlinear media were found to fall into two distinct classes: the lighter materials, H2, He, and Ne, which readily support the generation of harmonic radiation, but do not fluoresce, and the heavier materials, Ar, Kr, and Xe, which produce significant amounts of fluorescence, but in which the generation of harmonic radiation decreases rapidly with the scattering order.

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The absorption of laser radiation by inhomogeneous plasma is of considerable interest in laser fusion research. One process strongly affecting the absorption efficiency is density profile modification due to ponderomotive or radiation pressure at critical density, where most of the absorption occurs.