Temporal behavior of line-to-continuum ratios and ion fractions as a means of assessing thermodynamic equilibrium in laser-induced breakdown spectroscopy

Two diagnostic approaches, aimed at evaluating the departure from local thermodynamic equilibrium (LTE) in laser-induced plasmas, are discussed in this paper. The first approach is based upon the observed temporal behavior of the ratio between selected atomic transitions and the nearby spectral continuum, while the second approach makes use of the plot of the ion fraction of different elements as a function of their respective ionization potentials. Both approaches are known in atomic emission spectroscopy: the former has been described and used in the case of a high pressure surfatron argon plasma, and the latter follows directly from the classic work dealing with d.c. arc emission in general, and a d.c. copper arc in particular. Such approaches, however, have not yet been applied to laser-induced plasmas. It is shown that the experimental results obtained with both methods agree with those reported in the literature using more conventional approaches. For example, in this work, the well-known outcome that the excitation temperature and the ion (electron) temperature differ substantially at early delays after the formation of the plasma is clearly supported by the line-to-continuum approach applied to atomic and ionic transitions of Cu and Fe. In addition, the agreement between the experimental and theoretical behavior of the ion fraction versus the ionization potential for the elements Cu, Fe, Ni, Mg and Ti improves as the delay time increases. (C) 2011 Elsevier B.V. All rights reserved.