Optical emission of graphite plasma generated in ambient air using low-irradiance carbon dioxide laser pulses

Abstract

Optical emission studies of graphite plasma induced by infrared (IR) Transversely Excited Atmospheric carbon dioxide (TEA CO2) laser pulses in ambient air at atmospheric pressure are reported. The plasma was induced at relatively low-irradiance, up to 40 MW cm−2, and the plasma emission was recorded using time-integrated LIBS measurements. The time profile of the 160 mJ laser pulse is composed of a short, 100 ns long initial spike, and a long 2 μs tail. About 60 mJ is contained in the initial pulse, and the 100-mJ tail contribution is favorable for extended plasma absorption that promotes creation of long-lasting highly-excited plasma. With laser pulse focused behind the target surface, recorded spectra consisted of intensive, sharp atomic and single charged ionic spectral lines of carbon, and trace elements, e.g. Ca, Cu, V, Si, and Ti. Good signal to background ratios obtained indicate potential application in the analysis of impurities in graphite, and also elemental analysis of other materials with high carbon content. The average electron number density was determined from Stark-broadened emission profile of C I 247.9 nm line, and the line intensity ratio of CII 250.9 nm/C I 247.9 nm line pair was used for estimation of ionization temperature. Depending on the applied fluence, electron density was in the range 2.6–4.8 × 1017 cm−3, and ionization temperature between 19,000 and 22,000 K. Beside line spectra, intensive and well-developed band spectra of diatomic molecules C2 (Swan system), and CN (violet system) were obtained. Pulse energy threshold for observation of molecular emission was 50 mJ. From the spectroscopic studies of the emission bands, the rotational and vibrational temperatures were estimated by comparing the experimental and simulated emission spectra. Vibrational and rotational temperatures deduced from Δν = 0 sequences of the Swan system of C2 were 3100 K and 3850 K, respectively. The most intense band of the CN violet system showed strong self-absorption and led to overestimated temperature values, Tvib = Trot = 4900 K. © 2019 Elsevier B.V.