Broadening and shift of the spectral lines of hydrogen atoms and silicon ions in laser plasma

We report an experimental investigation of the broadening and shift of discrete lines in the plasma spectrum produced in the laser ablation of silicon in a broad pressure range ($10^2$–$10^7$ Pa) of the ambient gas (Ar, He, H$_2$). The broadening and line shifts are measured in relation to the distance from the target and initial gas pressure. The threshold nature of the resulting dependences is found to be related to the formation of virtual percolation clusters proceeding in the hot dense plasma.