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 nm wavelength, 9 ns pulse width, 900 mJ maximum pulse energy and a maximum power density of the order of 10¹⁰ W/cm². The laser–target interaction produces a strong gold etching with production of a plasma in front of the target. The plasma contains neutrals and ions having a high charge state. Time-of-flight (TOF) measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits measurement of 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 J/cm² and 160 J/cm², respectively. The maximum ion energy reaches about 2 keV and 8 keV at these low and high laser fluences, 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.

Keywords:

• Plasma by pulsed laser
• Ion source
• Ion energy distribution
• Ion charge states

Acknowledgements

The authors thank Dr. L. Làska and Dr. J. Kràsa of the Institute of Physics of the Academy of Science of the Czech Republic in Praha for the scientific cooperation with this experiment. Many thanks also to Mr. S. Marletta, Mr. C. Marchetta and Mr. C. Percolla of INFN-LNS of Catania for the useful technical support given to the presented measurements.