Spectroscopic investigations of laser-produced steel plasmas in the vacuum ultraviolet

摘要

The analytical feasibility of laser-produced plasmas in the deep vacuum ultraviolet (VUV) spectral range (40 nm to 160 nm) for the quantitative elemental characterization of solid low-alloy steels is investigated, using time-integrated spatially-resolved optical emission spectroscopy (OES). Carbon was chosen as a ‘test’ element in the present study. In this respect, six prominent carbon spectral lines, covering the spectral range mentioned, and representing three different ionization stages of II, III and IV, have been carefully selected and proved to be spectralinterference free.ududIt has been demonstrated that the spatially-resolved approach followed in this work resulted in significantly enhancing the emission intensity of the spectral lines studied with respect to that of the characteristic intense background continua given off by laserproduced plasmas. Furthermore, an extensive series of optimization studies for various experimental parameters and conditions, including the laser-pulse focusing lens types (spherical vs. cylindrical), laser power density, laser harmonics, laser pulse energy, as well as ambient atmospheres and pressures, has been carried out in order to further improve the spectral lines-to-background ratios achieved.ududIn this way, linear analytical calibration functions have been constructed for the different carbon spectral lines under consideration, in the concentration range 0.001% to 1.32%. Moreover, the ultimate sensitivity of the VUV-based laser-induced plasma spectroscopy (LIPS) technique developed has been considerably improved; an unprecedented limit of detection for carbon in steels of 1.2 f_tg/g ± 15% has been measured from the C III 97.70 nm spectral line.ududSpectroscopic diagnostic techniques in the VUV for the determination of the laserproduced steel plasma physical parameters, namely the electron number density (ne) as well as the excitation (Texc) and ionization (Tjon) temperatures, have also been developed. In addition, criteria regarding the existence of local thermodynamic equilibrium (LTE) state in the plasmas investigated have been discussed.