The validity of the popular Boltzmann plot method was theoretically tested for the case of a non-homogeneous non-isothermal laser-induced plasma. A collisional-dominated plasma model was employed to generate synthetic spectra by solving the radiative transfer equation. The spectra were processed with homemade software that calculated values for the plasma temperature and concentrations using the Boltzmann plot approach. Both static and dynamic plasmas were investigated at various temperature and density gradients. The plasma parameters obtained from the Boltzmann plots were subsequently compared with the exact parameters of the model. The results are shown to have a direct implication for calibration-free laser-induced breakdown spectroscopy (CF-LIBS). For nearly all tested situations, the Boltzmann plot method was capable of semi-quantitative analysis providing the accurate determination of concentrations for main plasma components and failing to accurately predict the concentrations of minor components and trace elements.

The potential of a diode pumped solid state (DPSS) laser for laser induced breakdown spectroscopy (LIBS) and Raman spectroscopy was investigated. The DPSS laser operating at repetition rates of 1 Hz–200 kHz, pulse energy of ∼1 mJ, and pulse duration of ∼20 ns was added to a conventional LIBS system equipped with the flashlamp-pumped Nd:YAG laser (10 Hz, 400 mJ, 6 ns) to directly compare the two laser systems. Despite the lower mass ablated per laser pulse (∼several nanograms) and a weaker plasma, the total mass ablated per measurement interval (∼1 s) and total emission signal were significantly higher than those obtained with the flashlamp pumped Nd:YAG laser. This resulted in the improved signal-to-noise ratio and better limits of detection (LOD). The best LODs were obtained for Cu, Cr and Ni in ultrapure standard iron samples, 0.7 ppm, 1 ppm and 5 ppm, correspondingly. The plasma from the DPSS laser was characterized in terms of its radiative properties, temperature, and shockwave propagation speed. The suitability of this laser for LIBS-Raman was demonstrated by mapping the elemental and molecular composition of a mineral sample. The results strongly imply that DPSS lasers are the promising source for LIBS, Raman, or combined LIBS-Raman spectroscopy.

A method based on matching synthetic and experimental emissivity spectra was applied to spatially resolved measurements of a laser-induced plasma ignited in argon at atmospheric pressure. The experimental emissivity spectra were obtained by Abel inversion of intensity spectra measured from a thin plasma slice perpendicular to the plasma axis. The synthetic spectra were iteratively calculated from an equilibrium model of plasma radiation that included free–free, free–bound, and bound–bound transitions. From both the experimental and synthetic emissivity spectra, spatial and temporal distributions of plasma temperature and number densities of plasma species (atoms, ions, and electrons) were obtained and compared. For the best-fit synthetic spectra, the temperature and number densities were read directly from the model; for experimental spectra, these parameters were obtained by traditional Boltzmann plot and Stark broadening methods. In both cases, the same spectroscopic data were used. Two approaches revealed a close agreement in electron number densities, but differences in plasma excitation temperatures and atom number densities. The trueness of the two methods was tested by the direct Abel transform that reconstructed the original intensity spectra for comparing them to the measured spectra. The comparison yielded a 9 and 13% difference between the reconstructed and experimental spectra for the numerical and traditional methods, respectively. It was thus demonstrated that the spectral fit method is capable of providing more accurate plasma diagnostics than the Boltzmann plot and Stark broadening methods.

The Radon transform is tested as a method for reconstruction of the emissivity distribution of asymmetric laser induced plasmas. Two types of experiments were carried out. First, the plasma asymmetry is introduced via focusing the laser by a cylindrical lens to create plasma plumes elongated along the symmetry axis of the lens. Second, an asymmetric power distribution across the laser beam is created by reflecting the latter from a damaged mirror. Various effects on the quality of the plasma emissivity reconstructed by the Radon tomography method are investigated. The understanding of these effects appears to be essential to design a proper experimental setup to study LIBS plasmas by the Radon tomography method. It is demonstrated that the Radon tomography can successfully be used for experimental studies of asymmetric LIBS plasmas.