A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples

The ablation of brass samples in argon shield gas by 170 fs and 6 ns laser pulses has been studied by optical emission spectroscopy of the evolving plasmas. Differences observed in the temporal behavior of the spectral line intensities are explained by the shielding effect of the Ar plasma for ns-pulses and the free expansion of the plasma of the ablated material in case of fs-pulses. Brass with different Zn/Cu ratios were used as samples. Different types of crater formation mechanisms in the case of ns- and fs-pulses were observed. At 40 mbar argon pressure the thresholds of ablation were found to be ∼0.1 and ∼1.5 J cm−2 for fs- and ns-pulses, respectively. With an internal standardization of zinc to copper it is possible to correct for differences in the ablation rates and to obtain linear calibration curves. For optimum experimental conditions, narrower confidence intervals for the determination of unknown concentrations were found in case of fs-pulses. Within the range of the laser intensities used, no dependence of the Zn/Cu line intensity ratio on the number of laser pulses applied to the same ablation spot was observed, neither for fs- nor for ns-pulses, which is interpreted as the absence of fractional vaporization.