Atomic and molecular emissions in laser-induced breakdown spectroscopy

Abstract

This article summarizes measurements and analysis of hydrogen Balmer series atomic lines following laser-induced optical breakdown. Electron number density on the order of 1×1025m−3 can be measured using Hα Stark width and shift in the analysis of breakdown plasma in 1 to 1.3×105Pa, gaseous hydrogen. The Hβ line can be utilized for electron number density up to 7×1023m−3. The historic significance is elaborated of accurate Hβ measurements. Electron excitation temperature is inferred utilizing Boltzmann plot techniques that include Hγ atomic lines and further members of the Balmer series. Laser ablation of aluminum is discussed in view of limits of application of the Balmer series. Hβ and Hγ lines show presence of molecular carbon in a 2.7 and 6.5×105Pa, expanding methane flow. Diagnostic of such diatomic emission spectra is discussed as well. Laser-induced breakdown spectroscopy historically embraces elemental analysis, or atomic spectroscopy, and to a lesser extent molecular spectroscopy. Yet occurrence of superposition spectra in the plasma decay due to recombination or due to onset of chemical reactions necessitates consideration of both atomic and molecular emissions following laser-induced optical breakdown. Molecular excitation temperature is determined using so-called modified Boltzmann plots and fitting of spectra from selected molecular transitions. The primary interest is micro-plasma characterization during the first few micro-seconds following optical breakdown, including shadowgraph visualizations.