Observation of different regimes of the grain size effect on plasma emission induced by laser ablating granular materials

Abstract

The grain size effect on the plasma emission, induced by a 1064 nm laser pulse of 7 ns duration ablating the surface of a randomly packed granular (RPG) copper material in air at atmospheric pressure, was experimentally investigated in the laser fluence range of 12.4 to 35.4 J cm⁻². Four sieved granular copper samples with discrete grain sizes of 297 ± 20, 218 ± 15, 132 ± 10, and 109 ± 5 μm were used in this study. The measured trends of the spectral line intensity emitted by copper versus the laser fluence for the four samples suggest the identification of three laser fluence ranges, where the size effect plays a different role. Combining the plasma parameters calculated through spectroscopic analysis, we interpreted the phenomenon of plasma emission by introducing one new kind of size effect stemming from the mechanical performance of RPG materials to support the shock pressure imparted by the energetic processes of plasma generation and expansion. Such an interpretation was also supported by additional photographic measurements of the particle ejecta and granular crater after laser ablation. The study has practical significance in developing the technology for direct analysis of RPG materials using laser-induced breakdown spectroscopy.