OPTICAL IONIZATION AND HEATING OF GASES BY INTENSE PICOSECOND KRF LASER-RADIATION

We report on multiphoton ionization experiments using picosecond (ps) and sub-ps W-laser radiation at focused intensities up to 10(18) W/cm(2). The experiments are concerned with determining the electron temperature of optically ionized gases produced by intense KrF lasers. Thomson scattering, stimulated Raman scattering (SRS), and X-ray emission measurements have been made and compared with modeling calculations of heating. A particular objective is to identify the respective roles of above-threshold ionization, nonlinear inverse bremsstrahlung absorption, and SRS in determining the temperature of the electrons. Results for 350-fs pulses are compared with previous measurements for 12-ps pubes (for which strikingly different behavior is observed). The importance of using sub-ps, short-wavelength lasers to minimize electron temperature is confirmed.