Shock wave effect on emission spectra of laser plasma induced on the surface of solid targets in gas atmosphere

Laser plasma generated on the surface of solid targets located in gas atmosphere leads to the formation of a strong shock wave, which changes thermodynamic and spectral plasma characteristics accompanied by Stark effect. The registration of Stark shift and broadening of emission spectral lines allowed us to determine electron temperature and electron density for the ambient gas pressure range of 200 - 1200 Torr without considering presence of local thermodynamic equilibrium in the studied volume of aluminum plasma. The electron temperature and density of plasma increased along with gas pressure, which was explained by the formation of the more intensive shock wave. The velocity of laser plasma expansion in vacuum was obtained by measuring Doppler shift of weakly broadened lines of emission spectra, collected at two different angles from the normal of target surface. The measurement of line broadening, caused by square Stark effect, gave us an estimate of electric field in aluminum plasma. For Q- switched Nd:YAG laser operating at a 1.064-micrometers wavelength with a pulse energy of 0.6 J and a time duration of the laser pulse of 20 ns, the electric field in the studied plasma volume was 18 kV/cm for ambient gas (helium) pressure of 760 Torr.