Steps and thresholds in LIBS

The LIBS technique basically involves four steps, namely laser-solid interaction, material removal, plasma formation (also called breakdown ) and analysis of the photons emitted by the plasma formed. The conditions used in each step are usually optimized in relation to the particular application. Two of the previous steps (viz. laser-solid interaction and material removal) are also present in laser ablation (see Section 9.2.2). 

Analysis of the photons emitted by the plasma This step is shared by other atomic emission spectrometries. However, the high electron temperature of laser-induced plasmas requires the use of temporal resolution to improve the detection sensitivity. The electron density of the plasma remains very high during the time the laser pulse interacts with the solid and the vapour. The emission spectrum obtained in this situation is a continuum due to ion-electron recombination. Ionic and atomic lines are superimposed on the emission background. Once the laser pulse has extinguished, the system rapidly loses energy and many atoms are excited. Observation of the plume at this time allows species to be much more easily detected. 

As with laser ablation, a number of borderline situations (defined via the so-called damage threshold , ablation threshold and plasma threshold ) can be considered in describing the interaction of a laser pulse with a surface to induce the ablation and plasma formation associated to laser-induced breakdown. Only the plasma threshold is discussed here, however, as the other two are shared by laser ablation and are dealt with in Section 9.2.2. 

The plasma threshold is the irradiance required to produce the optical breakdown of the vapour. It depends on the nature of the surface (particularly on various optical, thermophysical and thermodynamic properties of the material). The plasma threshold is usually higher than the ablation threshold. Observing optical emission entails applying more energy than that required to reach the plasma threshold. 

Although phase explosion and the factors governing it [17] have not been studied in relation to LIBS, its threshold can be assumed to depend strongly on the laser beam spot size and wavelength. 

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