Optical ionization of gases by intense ps KrF laser radiation

Summary form only given. The authors report on studies of optical ionization of gases produced by 12-ps pulses of focused Raman-shifted KrF laser radiation (/spl gamma/=268 nm) at intensities of 3/spl times/10/sup 17/ W/cm/sup 2/. Measurements of Thomson, stimulated Raman, and stimulated Brillouin scattering have been used to determine the electron energy distribution and temperature in He and Ne (static and pulsed gas jet targets) at pressures of 10/sup -3/ to 1 bar and to investigate the role of parametric instabilities in the laser/plasma interaction. The electron velocity distribution evolves from a multiphoton (tunnel) ionization spectrum at low pressure to a thermal electron distribution at moderate pressure. The ion determined portion of the Thomson-scattered spectrum is highly nonthermal for pressures greater than a few mbar. At high pressure, significant Raman and Brillouin backscattering is observed. Inverse bremsstrahlung absorption is shown to dominate heating from p>10/sup -2/ bar except at the highest pressures where stimulated Raman scattering may also be important.