Comparative investigation of laser ablation plumes in air and argon by analysis of spectral line shapes: Insights on calibration-free laser-induced breakdown spectroscopy

• Investigation of laser ablation plumes by analysis of spectral line shapes
• Simulation of emission spectra from nonuniform laser-produced plasma
• Plasma is more uniform for ablation in argon.
• Plasma nonuniformity mostly affects optically thick lines.
• Calibration-free LIBS may ignore gradients if optically thin lines are chosen.

We investigate the characteristic features of plume expansion in air and argon resulting from ultraviolet laser ablation of solid matter in conditions typically applied in material analysis via laser-induced breakdown spectroscopy (LIBS). Barite crown glass is chosen as a target material for the characteristic emission spectrum suitable for plasma diagnostics. The space-integrated plasma emission spectrum recorded with an echelle spectrometer coupled to a gated detector is compared to the computed spectral radiance of a nonuniform plasma in local thermodynamic equilibrium. In particular, resonance lines of neutral sodium atoms and barium ions are observed to probe gradients of temperature and density within the plume. It is shown that laser ablation in argon leads to an almost uniform plasma whereas gradients of temperature and density are evidenced in ambient air. The discrepancy is attributed to the different physical properties of both gases leading to a stronger vapor–gas energy exchange in the case of air. However, strong gradients occur only in a thin peripheral zone, close to the vapor–gas contact front. The larger plasma core appears almost uniform. The peripheral zone of low temperature mostly contributes to the plasma emission spectrum by absorption and material analysis via calibration-free LIBS in air may ignore the nonuniform character of the plasma if only transitions of small optical thickness are considered.