Absorption of laser radiation in a laser-produced plasma of Xe: hydrodynamic effects and nonequilibrium ionization

In this paper, experiments on measuring absorption of infrared laser radiation in the laser-produced plasma of Xe are described. An absorbed fraction of up to 65% was obtained when the gas-jet target was illuminated by a wide, defocused beam, whereas it barely reached 8.5% in the case of a sharply focused beam. The phenomenon is explained on the basis of a hypothesis of the plasma's hydrodynamic expansion according to which the plasma leaves the illuminated area faster the smaller its size. This explains the similarity of extreme ultraviolet output and laser energy absorption as functions of the laser beam diameter. Based on the experimental results, an attempt to estimate the plasma parameters (N, T, ) is undertaken, with the mean ion charge, , being calculated using ionization cross-sections for ions from Xe+7 to Xe+14, which were obtained by means of a quantum-mechanical numeric simulation especially for the present work.