Curve of growth methodology applied to laser-induced plasma emission spectroscopy

The curve-of-growth (COG) method was applied to a laser-induced plasma. The plasma was produced by a Nd:YAG laser on the surface of steel samples containing 0.007-1.3% of Cr. The emission was collected from the top of the plasma by means of a 45 degrees pierced mirror and aligned onto an intensified charge-coupled device (ICCD) with a gate width of 1 mu s and a variable delay time. The resonance 425.4 nm Cr line was used for construction of the COG. The temperature of the plasma (similar to 8000 K at 5-mu s delay) was determined from a Boltzmann plot. The damping constant a, proportional to the ratio of the Lorentzian to the Doppler line widths, was found from the best fit of a series of calculated COG to the experimental data points and was 0.20 +/- 0.05. The number density of neutral Cr atoms which corresponded to the transition between low and high optical densities, was approximate to 6.5 . 10(12) cm(-3). The cross-section for broadening collisions of Cr atoms with atmospheric species (presumably N-2) was calculated to be (66 +/- 16) Angstrom. The shape of the 425.4-nm Cr line was additionally checked by scanning an ultra-narrow ct Ti:Sapphire laser across the atomic transition and found to be in agreement with preliminary estimates. The potential of the COG method for laser breakdown spectroscopy is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.