Time-resolved LIBS of atomic and molecular carbon from coal in air, argon and helium

Abstract

Laser ablation chemical analysis of a coal sample was studied by LIBS (laser-induced breakdown spectroscopy). Ablation was performed using a 266 nm Nd:YAG laser in different gas environments (air, argon and helium) at atmospheric pressure. We present characteristics of spectra measured from coal with special attention to atomic and molecular carbon including CI, C₂ and CN. The influence of the ambient gas on the laser-induced coal plasma was studied by using time-resolved analysis. Atomic iron emission lines were employed to construct Boltzmann plots for the plasma excitation temperature. Computer simulations of C₂ spectra were used to deduce the molecular rotational temperature. Electron density and total atomic and molecular number density are reported to describe emission differences of atomic and molecular carbon in the different gas environments. These data demonstrate that the plasma excitation temperature is the primary factor contributing to differences among the atomic carbon emission in the gas environments. Reactions between the plasma species and ambient gas, and the total molecular number are main factors influencing molecular carbon emission. Finally, the influence of laser energy on the rotational temperature was studied in the air environment to demonstrate that the rotational temperature derived from C₂ band emission can be utilized to correct plasma fluctuations.