Ideal radiation source for plasma spectroscopy generated by laser ablation

Laboratory plasmas inherently exhibit temperature and density gradients leading to complex investigations. We show that plasmas generated by laser ablation can constitute a robust exception to this. Supported by emission features not observed with other sources, we achieve plasmas of various compositions which are both uniform and in local thermodynamic equilibrium. These properties characterize an ideal radiation source opening multiple perspectives in plasma spectroscopy. The finding also constitutes a breakthrough in the analytical field as fast analyses of complex materials become possible.

Figure 1

Space-integrated spectra recorded during laser ablation of steel in argon at different times. The strongest transitions are shown to saturate at the blackbody spectral radiance. The temperature was deduced from the intensity distribution of optically thin lines. The excellent agreement with the spectral radiance computed using Eq. (1) shows that the emission originates from a uniform plasma in LTE.

Figure 2

Saha-Boltzmann plot of Fe and ${\mathrm{Fe}}^{+}$ transitions for $t=230\mathrm{ns}$ (squares) and $500\mathrm{ns}$ (triangles).

Figure 3

Ba II 413.06 nm line profile recorded during laser ablation of barite crown glass for $t=500\mathrm{ns}$. Comparison to the spectral radiance computed for a uniform LTE plasma. The blue line indicates the resonance wavelength of the transition.

Figure 4

In I 410.18 nm resonance line recorded during laser ablation of indium for $t=500\mathrm{ns}$. The measurements in argon and air are compared to the spectral radiances of a uniform and a nonuniform LTE plasma, respectively.

Figure 5

Measured (black line) and computed (red line) shapes of an Ar transition toward the metastable state. The recording was performed during ablation of steel for $t=500\mathrm{ns}$. The simulation was obtained by adding two layers to the laser plasma, representing the heated gas at the vapor-gas contact front and the adjacent colder gas. The absorption dip was observed for each investigated material.

Figure 6

Ba II 493.41 nm line profile recorded during laser ablation of barite crown glass in argon and air for different times.

Figure 7

Aluminum resonance lines recorded during laser ablation of an aluminum alloy in argon and air for different times.