Please read carefuly that paper. If they have used argon gas in the experiment. They must have explained the reason for using that gas. However in general argon gas is used in LIBS experiment to enhance the signal (sensitivity) of the LIBS. The shock wave generated at the boundary of plasma and argon gas compresses the plasma, which increases the plasma temperature and density resulting in comparatively higher LIBS signal.
Argon is used basically to increase signal.The cause of the overall signal enhancement is due to the confinement of the laser-induced plasma in the presence of argon. Due to the heavier density and lower thermal conductivity of argon relative to air, the overall plasma is confined near the sample surface in argon relative to air. The confined plasma increases the number of collisions that might be responsible for more excitation of species in the laser-induced plasma and leads to the enhancement of the overall LIBS signal in an argon atmosphere.