Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.

The interaction of intense vacuum-ultraviolet radiation from a free-electron laser with rare gas atoms is investigated. The ionization products of xenon and argon atomic beams are analyzed with time-of-flight mass spectroscopy. At 98 nm wavelength and approximately 10(13) W/cm(2) multiple charged ions up to Xe6+ (Ar4+) are detected. From the intensity dependence of multiple charged ion yields the mechanisms of multiphoton processes were derived. In the range of approximately 10(12)-10(13) W/cm(2) the ionization is attributed to sequential multiphoton processes. The production of multiple charged ions saturates at 5-30 times lower power densities than at 193 and 564 nm wavelength, respectively.

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For the first time, the evolution ofluminescence from rare gases was studied as a function of number density. Synchrotron radiation served as a light source for selective and pulsed excitation of the samples. The excitation spectra confirm previous results on perturbed Rydberg states and exciton appearance in dense media. In time-resolved emission spectra the peak energies and widths of the luminescence bands were followed. The energy separation between the fast and slow components is found to be density independent. A model proposed by Cheshnovsky et al. [Chern. Phys. Lett. 15, 475 (1972)] accounts for the change in peak width with temperature. Both lifetimes decrease with increasing density. The data extrapolate to 3.3 ± 0.1 ns (Ar); 3.4 ± 0.1 ns, 270 ± 5 ns (Kr); 4.5 ± 0.1 ns, 100 ± 5 ns (Xe) for the low density limit. For the solid at the triple point, we obtain 1.3 ± 0.1 ns, 82 ± 5 ns (Kr) and1.1 ± 0.1 ns, 18.5 ± 0.5 ns (Xe). Theories on density dependence oflifetimes give only a qualitative description of the experimental results.

The ionization dynamics of Ar and Xe clusters irradiated with intense vacuum ultraviolet light from a free-electron laser is investigated using photoelectron spectroscopy. Clusters comprising between 70 and 900 atoms were irradiated with femtosecond pulses at 95 nm wavelength (approximately 13 eV photon energy) and a peak intensity of approximately 4 x 10(12) W/cm2. A broad thermal distribution of emitted electrons from clusters with a maximum kinetic energy up to 30-40 eV is observed. The observation of relatively low-energy photoelectrons is in good agreement with calculations using a time-dependent Thomas-Fermi model and gives experimental evidence of an outer ionization process of the clusters, due to delayed thermoelectronic emission.