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 cont aining 0.007-1.3% of Cr. The emission was collected from the top of the pla sma 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 de lay time. The resonance 425.4 nm Cr line was used for construction of the C OG. The temperature of the plasma (similar to 8000 K at 5-mu s delay) was d etermined from a Boltzmann plot. The damping constant a, proportional to th e ratio of the Lorentzian to the Doppler line widths, was found from the be st fit of a series of calculated COG to the experimental data points and wa s 0.20 +/- 0.05. The number density of neutral Cr atoms which corresponded to the transition between low and high optical densities, was approximate t o 6.5 . 10(12) cm(-3). The cross-section for broadening collisions of Cr at oms 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 Else vier Science B.V. All rights reserved.