High-resolution x-ray spectrum of a laser-produced barium plasma in the 9.10–9.36-Å wavelength range

A highly stripped barium plasma is produced by 120-fs laser pulses irradiating a ${\mathrm{BaF}}_2$ target. The spectrum emitted by the plasma in the 9.10–9.36-Å wavelength range is recorded using a high-resolution spherically bent mica crystal. On the basis of the HULLAC atomic code, a level-by-level collisional-radiative model including autoionization and dielectronic capture processes is developed to calculate the wavelengths and intensities of the spectral lines emitted by each of the Cu-, Zn-, and Ga-like barium ions. $3d-nf$ $\mathrm{}(n=6,7\mathrm{})\mathrm{}$ spectral lines with different spectator electrons, previously observed only as unresolved transition arrays, are resolved. The theoretical results agree reasonably well with experiment. Best agreement is obtained for electron density and temperature of $5\times {10}^{21}{\mathrm{cm}}^{\mathrm{-}3}$ and 120 eV, respectively. The intensity ratios of the resolved Cu-like barium lines are shown to be useful tools for electron density and temperature diagnosis. This diagnostic method was not possible with low-resolution spectroscopy. It is found that at relatively low temperature and high density as in the present experiment, the relative intensities of lines within each ionization state are independent of the ion density ratio of adjacent ionization states.