Spatial characterization of laser induced plasmas obtained in air and argon with different laser focusing distances

Spatial distributions of spectral line emissivities and plasma characteristics (temperature, electron density and neutral atom relative number density) in laser-induced plasmas have been measured for varying laser focusing distances. Time- and space-resolved emission spectroscopy, combined with a deconvolution procedure of the spectra, has been used to obtain the radial profiles of emissivity starting from the lateral profiles of intensity. The distributions have been obtained for the time window 5–6 μs after the laser pulse. The plasmas have been generated in air and argon at atmospheric pressure by focusing a Nd:YAG laser onto an iron sample. The distributions obtained in both ambient gases have similar features, showing a notable variation in shape and magnitude values with the focusing distance. In air, the Fe emissivity and atom density show maximum values when the focal plane of the lens (128-mm focal length) is placed 10 mm and 12 mm, respectively, below the sample surface, whereas the higher temperature is obtained when the focus is 5 mm below the surface and decreases for deeper focusing positions. This behavior is explained by the plasma shielding effect produced at high laser irradiances. For the plasmas generated in argon, the emissivities and atom densities of Fe, coming from the sample and Ar coming from the ambient gas, are obtained. The results show that, at moderate and high irradiances, the distributions of Fe and Ar atom densities are separated to a large extent due to the existence of a region of minimum Ar atom density, which is interpreted to result from the displacement of the ambient gas by the shockwave propagating away from the sample surface.