Non-equilibrium plasma production by pulsed laser ablation of gold

Abstract

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064\\,\\hbox{nm} wavelength, 9\\,\\hbox{ns} pulse width, 900\\,\\hbox{mJ} maximum pulse energy and a maximum power density of the order of 10^{10}\\,\\hbox{W}/\\hbox{cm}^2 . The laser-target interaction produces a strong gold etching with a production of a plasma in front of the target. The plasma contains neutrals and ions having high charge state. Time-of-flight measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits to measure the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6^+ and 10^+ at a laser fluence of 100\\,\\rm{J/cm}^2 and 160\\,\\rm{J/cm}^2 , respectively. The maximum ion energy reaches about 2\\,\\hbox{keV} and 8\\,\\hbox{keV} at these low and at high laser fluence, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a "shifted Maxwellian distribution", which the authors have corrected for the Coulomb interactions occurring inside the plasma.