Influence of the optical depth on spectral line emission from laser-induced plasmas

The emission from a laser-induced plasma of seven self-absorbed Fe(I) spect ral lines has been studied to investigate the influence of the optical dept h on the line intensity. The plasma was generated using an infrared Q-switc hed Nd:YAG laser in air at atmospheric pressure. The plasma emission was de tected with temporal resolution, using a delay of 5 ps from the laser pulse and a gate width of I Ls. Experimental curves of growth (COGs) were obtain ed by measuring the line intensity for Fe-Ni alloy samples with Fe concentr ations in the range 0.2-95%. Using a simple model for self-absorption, base d in a homogeneous plasma with the absence of matrix effects, theoretical C OGs have been calculated that fit the experimental data with good correlati on. The method used allows prediction of the COG that will be obtained for a given spectral line, starting from its transition parameters (oscillator strength, energy levels and degeneracy of the lower level), the plasma temp erature and the damping constant of the line. The plasma temperature (8200 +/- 100 K) was determined using the Boltzmann plot method. The existence of local thermodynamic equilibrium was verified by estimating the plasma elec tron density (2.6 x 10(16) cm(-3)) using the Stark broadening of an emissio n line. The damping constant (0.9 +/- 0.2) was estimated through the determ ination of the Lorentzian line width from measured line profiles. The densi ty of Fe atoms in the plasma for the sample with 100% Fe (7.3 x 10(15) cm(- 3)) was estimated using all the COGs of the lines studied. The experimental results indicate that matrix effects are not present in the ablation proce ss of the Fe-Ni samples. (C) 2001 Elsevier Science B.V. All rights reserved .