Characterization of the self-reversal phenomenon of Ag resonant lines in laser-induced breakdown spectroscopy experiments

In this work, we analyzed the phenomenon of self-reversal (SR) of spectral lines in Laser-Induced Breakdown Spectroscopy (LIBS) spectra through the study of the temporal evolution of self-reversed emission lines in neutral and ionized silver, using different laser energies and lens-to-target distances. In these experiments, the characteristic dip of self-reversed lines corresponding to resonant transitions begins to form on the bremsstrahlung continuum within 20 ns after the laser-induced breakdown. The dip generated in the initial moments presents an inverted Lorentzian shape and, at later times, gradually acquires a Gaussian profile. Additionally, we found that the self-reversal phenomenon in silver can occur in all transitions whose lower levels are below 6 eV and whose transition probabilities exceed 108 s−1. The SR depends weakly on the laser pulse energy but is strongly influenced by the lens-to-target distance. It is possible to minimize the effect by adjusting this distance. The formation of craters on the target also favors the appearance of self-reversal. We propose a model to explain all the observed characteristics of the self-reversal phenomenon in this work, based on the interaction of Ag atoms and ions ejected by the plasma with the shock wave generated during the breakdown process.