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 temper ature of optically ionized gases produced by intense KrF lasers. Thoms on scattering, stimulated Raman scattering (SRS), and X-ray emission m easurements 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 absorpti on, and SRS in determining the temperature of the electrons. Results f or 350-fs pulses are compared with previous measurements for 12-ps pub es (for which strikingly different behavior is observed). The importan ce of using sub-ps, short-wavelength lasers to minimize electron tempe rature is confirmed.