Emission spectra of a magnesium plasma created in vacuum by short laser pulses

An investigation was made of the emission spectra of a plasma formed on the surface of a magnesium barrier in a vacuum by nanosecond laser pulses of wavelengths 1.06 and 0.532 $\mu$ m. The intensity of the radiation incident on the magnesium target was $(1-8)\times10^{12}$ W cm$^{-2}$. Photographic and photoelectric methods were used to record the emission spectra in the visible and near-ultraviolet parts of the spectrum. The ratio of the intensities of the ionic magnesium lines at $\lambda=448.1,292.8$, and $293.6$ nm was used to determine the electron temperature of the plasma during the cooling stage ($20-50$ ns after the beginning of irradiation). The measurements yielded $kT_{\mathrm e}>2$ eV near the target, but at distances exceeding $3\mu$ m from the target the corresponding value was $kT_{\mathrm e}\gtrsim1$ eV. Broadening of the ionic line of magnesium with $\lambda=448.1$ nm was used to determine he maximum electron density (during the existence of the plasma); at $0.5-1$ mm from the target it was $N_{\mathrm e}\approx10^{18}$ cm$^{-3}$. Closer to the target only the continuum was emitted. The results were compared with the parameters of a magnesium plasma determined earlier on irradiation of a target with quasi-cw neodymium laser radiation of relatively low intensity of $10^6-10^7$ W cm$^{-2}$.