Development of methods and applications of laser-induced plasma spectroscopy in vacuum ultraviolet

Abstract

A novel experimental setup for laser-induced plasma spectroscopy for the measurements in the vacuum ultraviolet (VUV) spectral region has been developed. The method is based on use of a purge gas to eliminate atmospheric absorption of VUV light emitted from plasma. A small Czerny-Turner spectrograph was purged with argon or nitrogen to prevent atmospheric absorption inside the spectrograph. The gas flow through the slit of the spectrograph purged the optical path between the slit and the plasma, which was generated near the spectrograph. Emission lines in the VUV region were detected down to 130 nm with an intensified charge-coupled device equipped with MgF2 input window. The setup for VUV analysis was used in two different applications. The potential of VUV detection in analysis of sulfide minerals was demonstrated with drill core samples. Main minerals at each sampling location were identified from their emission spectra in a fitting procedure, where a set of prerecorded reference spectra served as a data base. It was suggested, that the determined mineral distributions could be used to in situ sorting of the drill core samples in mines. Feasibility of the VUV setup in analysis of chlorine and bromine from halogenated organic compounds was studied. In the purging arrangement strong atmospheric absorption at the wavelengths below 160 nm was observed and therefore a small sample chamber was used in these experiments. Carbon emission lines were used as an internal standard and linear correlation was found between the carbon and halogen emission intensity ratio and the corresponding atomic ratio of the compound. According to the results compounds containing 90 carbon atoms and one bromine atom could be detected in VUV. For chlorine compounds the respective ratio was only 20:1 due to the low transmission efficiency of the experimental setup. In all measurements the plasma was generated with excimer lasers operating at UV wavelengths. Therefore influence of plasma shielding was also studied. It was observed, that at high irradiances (> 1 GW cm-2) from ArF and KrF excimer lasers, emission intensities are significantly attenuated, also at longer delay times, as less energy of the laser pulse can reach the surface of the sample because of the plasma shielding. The influence of the laser repetition rate on shielding was found negligible. In argon atmosphere the attenuation was pronounced when compared to measurements in air, which may be caused by breakdown of argon gas in front of the sample.