Experimental Observations of High Intensity UV Femtosecond Laser Interactions with Clusters

High-intensity, ultraviolet (248 nm), femtosecond (about 180 fs) laser radiation was focused into a gas jet to study the spectral shifts and phase modulations at intensities up to $10^{18}W/cm^2$. Forward-scattered laser spectra were measured using a spatially resolved spectrometer; the peak pulse intensity envelope and phase were measured by the FROG (frequency-resolved optical gating) technique. In Ar targets, it was found that the forward Raman spectrum was blue-shifted and its bandwidth moderately broadened. Pulse shortening and phase modulation were also observed.