A detailed optimization study for laser-produced steel plasmas using time-integrated, spatially resolved emission spectroscopy in the vacuum ultraviolet (VUV) (40–160 nm) is presented. The influences of the laser focusing lens type, laser power density, laser wavelength, laser pulse energy, ambient atmospheres, and pressure, as well as spatial distribution of emitting species, on the emission characteristics of the steel plasmas are investigated. The aim of the work is to improve the detection power of the technique for the quantitative determination of carbon in solid steel alloys. In most of the work, Q-switched Nd:YAG (1064 nm, 820 mJ max. energy) laser pulses were used to create the steel plasmas. For the laser harmonics investigations, a second Q-switched Nd:YAG laser system that generated radiation at the second, third, and fourth harmonics as well as at the fundamental was employed. Air, argon, and helium were used as the surrounding atmospheres, and the pressure was varied from 0.005 mbar to 5.0 mbar depending on the gas composition. A 1 m normal incidence vacuum spectrometer, equipped with a 1200 grooves/mm concave reflective grating, was used to disperse the VUV radiation. The radiation was detected by a back-illuminated, anti-reflection coated, charge-coupled device (CCD) array detector. In general, the emission characteristics of the VUV spectral lines studied are similar to those previously investigated in the UV-visible spectral range. An unprecedented limit of detection for carbon in steels of 1.2 ± 0.2 μg/g was measured in this work.

This paper demonstrates that time-integrated space-resolved laser-induced plasma spectroscopy (TISR-LIPS) is a useful technique in the vacuum ultraviolet (VUV) for the quantitative determination of the carbon content in steels. The standard reference samples used were carbon-iron alloys containing a relatively wide concentration range of carbon (0.041-1.32%). In the experiments the output of a Q-switched Nd:YAG (1064 nm) laser, with approximately a 1 J maximum output pulse energy and approximately a 12 ns temporal pulse width, was focused onto the surface of each sample (under vacuum) in order to produce the emitting plasma. A fore-slit mounted in the target chamber allowed spatially-resolved spectral measurements in the axial direction of the plasma and provided emission lines that were almost free of the background continuum. A 1 m normal incidence vacuum spectrometer, equipped with a 1200 grooves mm −1 concave grating and a micro-channel plate/photodiode array detector combination, was used as the detection system. A particularly interesting feature of this work is the demonstration that VUV spectroscopy allows ionic lines to be used and linear calibration curves were obtained for the five carbon spectral lines (from C + and C 2+ ) under investigation. The limits of detection for all lines were determined; the lowest detection limit (87 ± 10 ppm) was obtained from the C 2+ 97.70 nm line, which compares favourably with the only available value in the literature of 100 ppm.

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