X-ray emission spectra of the plasma produced by an ultrashort laser pulse in cluster targets

The first observation of x-ray emission spectra of multiply charged ions in the plasma produced by a 35-fs laser pulse with an intensity up to 10¹⁷ W cm^–2 in CO₂ and Kr gas jet targets is reported. The emission in the wavelength ranges of the 1snp — 1s²(n = 3 — 6) transitions of O VII ions and the Ly_α line of O VIII ions, as well as of the (2s_1/22p⁶3p_3/2)₁ — 2s²2p^{6 1}S₀ and (2s_1/22p⁶3p_1/2)₁ — 2s²2p^{6 1}S₀ lines of Ne-like Kr XXVII ions testifies that the highly ionised plasma is formed by collision processes in clusters. Modelling the shape of the spectral lines of oxygen ions by including the principal mechanisms of broadening and absorption in optically dense plasmas reveals that the main contribution to the time-integrated intensity is made by the plasma with the parameters N_e =(2 – 20)×10²⁰ cm^–3 and T_e = 100 – 115 eV.