Determinants of laser-induced breakdown spectra in N₂-O₂ mixtures

Potential determinants of the laser-induced breakdown spectra in N₂-O₂ mixtures are investigated with varying species concentration, laser energy, and exposure time (equal to the detector's intensifier gate time). A focused laser beam with pulse energy 29 mJ, 38 mJ, or 46 mJ generates the breakdown plasma in a N₂ mixture containing O₂ from 0% to 21% in mole fraction. Time-resolved measurements of the breakdown emission spectra with 2 or 10 ns exposure are carried out beginning from the arrival of the laser beam at the focal point up to several μs later. These measurements show the temporal evolution of the instantaneous spectrum in the range 550-825 nm that contains multiple N and O atomic/ionic emission lines. Analyses of the baseline-subtracted spectra, normalized by the peak intensity of the N atomic emission line at 747 nm, indicate that the temporal evolution of each emission line is independent of species concentration and laser energy, though the instantaneous emission line intensity is strongly dependent on the species concentration and exposure time. The plasma temperature, estimated using two atomic oxygen lines and assuming local thermal equilibrium, and its temporal evolution are shown to be independent of the species concentration and a weak function of the laser energy at a given gas density, e.g. atmospheric pressure and room temperature. In addition, thermal equilibrium calculations, for predicting atomic and ionic species concentrations in the plasma, are carried out, and the combined results dictate that the emission-line intensity is linear with the atomic/ionic species concentration because of the consistent plasma temperature and its temporal evolution, independent of species concentration.