Ion Kinetic Energy Distributions and Mechanisms of Pulsed Laser Ablation on Al

A detailed investigation on the ion kinetic energy distributions of ions ejected in the nanosecond pulsed laser ablation of aluminum is reported. For laser fluences just over threshold, the emerging ions fit shifted neat Maxwell-Boltzmann-Coulomb (MBC) distributions. For fluences higher than similar to 1.3 J/cm(2), the Al+ distributions split into two MBC contributions peaked at different energies. It is demonstrated that the observed Al+ ion distribution has two components, one fast, correlated with the direct multiphoton laser ionization, and the other slow, associated with electron-Al-0 collisions in the solid. A similar behavior is observed at higher fluences for all Al ion distributions indicating that the electron-impact ionization of Al rate constants is faster than that of recombination and other possible collision channels. In addition, the linear relationship between the Coulomb velocities and the ion charges and the behavior of Coulomb energy of the ions versus the laser fluence suggest the appearance of an electric field within the metal/laser interaction volume that impels the ions up to the high velocities measured. A discussion of the application of this type of mechanisms to other metals is advanced.