Laser wavelength effects on the charge state resolved ion energy distributions from laser-produced Sn plasma

Abstract

The effects of laser wavelength on the charge state resolved ion energy distributions from laser-produced Sn plasma freely expanding into vacuum are investigated. Planar Sn targets are irradiated at laser wavelengths of 10.6 and 1.064 μm and intensities of 1.8×1010 and 3.4×1011 W/cm2, respectively. These parameters are relevant to the extreme ultraviolet x-ray source application. An electrostatic deflection probe and single channel electron multiplier are used to record the charge state resolved ion energy distributions 100 cm from the laser plasma source. At the longer laser wavelength, higher charge state ions are observed. At both laser wavelengths, the peak ion energies increase approximately linearly as a function of charge state, and all ion energies greatly exceed the initial thermal electron temperature. The differences in the ion energy distributions are attributed to the laser wavelength dependence of the laser energy absorption, the resulting plasma density in the corona, and the subsequent recombination after the laser pulse. Numerical simulations of the plasma expansion from a collisional-radiative steady state model support the experimental results.