Numerical simulation of laser-induced breakdown spectroscopy: Modeling of aerosol analysis with finite diffusion and vaporization effects

The application of laser-induced breakdown spectroscopy (LIBS) to aerosol systems has been shown to provide quantitative analysis of particle-derived species; however, the exact nature of the plasma/particle interactions remains to be fully understood. Although the plasma/particle interaction may be idealized within a framework of instantaneous vaporization and analyte diffusion throughout the plasma volume, experimental evidence suggests that these processes actually occur on finite time scales relative to the plasma decay times at which measurements are frequently taken. In the present work, a numerical simulation of the temperature and species concentration fields of a plasma containing a single particle, including dissociation and diffusion on semi-empirical finite time scales, is developed. Using these results, the intensity of analyte emission is calculated as a function of time, and the standard ion/neutral ratios typical of aerosol-derived LIBS signals are calculated. Furthermore, the ratio of ion/neutral ratios for two different species was used to assess the temperature homogeneity of the particle-derived analytes in comparison to the overall plasma temperature field. From this numerical study, it is shown that the finite time scale of evaporation and diffusion of aerosol material results in a non-uniform spatial distribution in concentration. This results, in turn, in temperature and free electron density gradients within the plasma, leading to variation between the local conditions surrounding aerosol mass and the bulk conditions of the plasma as a whole.