Intensity Ratios of H Lines: Departures from the Ideal Conditions in the Range of Laser-Induced Breakdown Spectroscopy Experiments

Abstract

In the present paper we analyze the behavior of H line intensity ratios with electron density and electron temperature in intermediate-density plasmas. We analyze the influence on the line intensity ratios of (1) the departures from local thermodynamic equilibrium (LTE) of the level population ratios, (2) the plasma opacity, and (3) the lowering of the ionization potential. We look, particularly, at the lines H_α, H_β, H_γ, and H_δ and the energy levels involved in the corresponding atomic transitions for their use as diagnostics in laser-induced breakdown spectroscopy (LIBS) experiments. One important conclusion is that, for typical values of the plasma dimension and the electron temperature taking place in LIBS, i.e., L = 1 mm and T_e = 10 000 K, respectively, the intensity ratios H_β/H_α, H_γ/H_α, and H_δ/H_α depart from the ideal values by less than 10% in the interval 0.65 × 10¹⁴ part/cm³ ≤ N_HI ≤ 3.6 × 10¹⁷ part/cm³, which means 1 × 10¹⁴ part/cm³ ≤ N_e ≤ 1 × 10¹⁶ part/cm³ for a gas of pure hydrogen. For higher densities, the departures from ideal conditions increase very quickly due to opacity effects.