Fundamental processes in nanosecond pulsed laser ablation of metals in vacuum

Velocity and kinetic energy distributions (VDs, KEDs) of metal ions generated by nanosecond (ns) pulsed laser ablation under high vacuum have been determined using an electrostatic analyzer plus time-of-flight coupled system. A number of metal and alloy targets, principally involving Fe and Ni, have been studied at different laser fluences. At low fluence, the ion distributions have been shown to fit single Maxwell-Boltzmann-Coulomb (MBC) distributions; for medium and higher fluences, each ion distribution is found to comprise that of the surviving precursor ion, itself, overlapped with sidebands which arise from ion-electron recombination and/or ionization. The so-called surviving precursor ion of a distribution is that which underwent no change of charge. The Coulomb velocities of the surviving precursor ions and those of the ion products resulting from ion-electron collisions have been compared. Ion velocities are correlated with the local electric field resulting from ejection of the photoelectrons following laser ablation. Under identical conditions of laser fluence, the ions are seen to experience an electrical field nearly independent of their charge. The transit times of ions in the plasma have been estimated to be of the order of 1 ps. An overall quantitative mechanism for metal ablation on this basis is presented, including the ejection time for photoelectrons and differences in ion distributions resulting from employing laser pulses in the nanosecond (ns) and femtosecond (fs) regimes.