Ionization-induced blue shift of KrF laser pulses in an underdense plasma

The ionization-induced blue-shifted spectra for helium, neon, and nitrogen have been measured at various gas densities up to 5 x 10(20) cm(-3) at a vacuum intensity of 8 x 10(16) W/cm(2) for picosecond KrF laser pulses at 248 nm. A 1-mm diameter gas jet target was used in the experiment to minimize the refraction of the laser beam and thus higher laser intensities were obtained in the gas than in previously reported experiments. For helium, a distinct shifted peak was observed at intermediate densities which was not seen before. For helium and nitrogen, spectra were also measured of the light scattered outside of the original focal cone angle. In this region there was little signal for electron densities below 2 x 10(20) cm(-3) consistent with limited refraction at lower densities and at higher densities the spectra were predominantly blue shifted. These results indicate the importance of refraction in the correct interpretation of ionization blue-shifted spectra.