Quantitative X-ray spectroscopy of the light-absorption region at the surface of laser-irradiated polyethylene

Absolute X-ray spectroscopic measurements of the lines and continua from space-resolved C 6+ , C 5+ , C 4+ , C 3+ ions at the surface of a plane polyethylene target, irradiated by a 1.06 mu m neodymium laser, are interpreted in terms of a collisional-radiative model in which some of the allowed lines are optically thick. The degree of ionization increases monotonically with laser power density over the range 10 11 to 2*10 12 W cm -2 and the electron energy distributions are strictly Maxwellian. The ion population ratios indicate that the ionization time is of the order of 1 to 10 picoseconds and is limited by thermal expansion within a thin layer whose mean density is approximately 3*10 21 cm -3 .