Spatial distribution of hydrogen and other emitters in aluminum laser-induced plasma in air and consequences on spatially integrated Laser-Induced Breakdown Spectroscopy measurements

The Stark broadening of hydrogen Balmer α line is often measured in Laser-Induced Breakdown Spectroscopy (LIBS) to evaluate the electron number density of the plasma. When measurements are carried out in air, hydrogen atoms are supposedly supplied by the atmosphere. This hypothesis casts some doubts on the validity of hydrogen as an indicator of the conditions in the inner part of the plasma. In this experiment, we acquired spectrally resolved images of a Laser-Induced Breakdown Spectroscopy plasma generated in air on an aluminum target by irradiation with the Nd:YAG fundamental emission. The evolution of the plasma was studied by acquiring images at several delay times. In particular, we imaged the spatial distribution of plasma emitters along the direction orthogonal to the target surface. The spatial distribution of hydrogen emission was compared to the one of the species ablated from the sample. Moreover, electron density and temperature were evaluated in the time- and space-resolved spectra extracted from the images, obtaining thus a picture of the propagation and internal structure of the plasma. The fluid dynamics implications of the plasma expansion were discussed. The z-resolved characterization was compared with the picture of the plasma obtained by processing space-integrated spectra.