High-resolution x-ray spectrum of a laser-produced barium plasma in the 9.10 – 9.36-Å wavelength range R. Doron, E. Behar, M. Fraenkel, P. Mandelbaum, A. Zigler, and J. L. Schwob Racah Institute of Physics, The Hebrew University, 91904 Jerusalem, Israel A. Ya. Faenov and T. A. Pikuz Multicharged Ion Spectra Data Center, VNIIFTRI, Mendele`evo, Moscow Region 141570, Russia ~Received 7 November 1997! A highly stripped barium plasma is produced by 120-fs laser pulses irradiating a 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 includ- ing 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. 3 d - nf ( n 56,7) 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 5 310 21 cm 23 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. @S1050-2947~98!01209-8# PACS number~s!: 32.30.Rj, 32.70.2n, 52.70.2m I. INTRODUCTION The research on plasmas of highly ionized heavy elements produced by subpicosecond lasers irradiating solid targets has greatly progressed in the last few years. Recent improve- ment in laser power densities together with the development of very high resolution x-ray crystal spectrometers have pro- vided new insights into the nature of such plasmas. Still, many of the characteristics of these plasmas are believed to be strongly dependent upon the time and space profile of the laser pulse and are yet to be fully understood. Specifically for barium, an experiment with powerful ultrashort laser pulses irradiating on barium compound targets was already per- formed by Zigler et al. @1#, and the emitted x-ray spectrum was recorded. This spectrum was analyzed by Goldstein et al. @2# in the framework of the local thermodynamic equi- librium ~LTE! model. Recently, another experiment for mea- suring x-ray emission from a laser-produced barium plasma over a relatively wide wavelength range from 8 to 14 Å has been carried out @3#. The spectrum was found to consist mainly of unresolved transition arrays ~UTA! emitted by Fe- to Ge-like barium ions. In the present work, we present a different measurement of the same x-ray emission using a spherically bent mica crystal @4# with very high spectral resolution, which enables better insight into the unresolved spectral features. The mica crystal technique has already proven to be a powerful tool for spectroscopic investigations @5,6#. The price for the high resolution is the limited spectral region, which in the present case is 9.10–9.36 Å. This wavelength range corresponds mainly to the 3 d -6 f transitions of Cu- and Zn-like barium, and to the 3 d -7 f transitions of Ga-like barium @3#. In addi- tion to the precise wavelength measurements, the highly re- solved spectrum allows the identification of new lines and opens possibilities for new plasma diagnostic methods. The plasma investigated in Refs. @1# and @2# was produced by intense 650-fs laser pulses of power density 10 17 W/cm 2 at a wavelength of 0.248 mm. Transparent BaF 2 targets were used to eliminate the prepulse effect. The x-ray spectrum was recorded and identified as 3 d -4 f transitions emitted by Sc- to Ni-like barium. The spectrum showed strong continuum features and therefore was modeled using the super transition array ~STA! theory @7# assuming an optically thin LTE plasma. The STA model indicated near solid density, and a weighted electron temperature distribution of seven different temperatures in the 200–350-eV range was assumed in order to reproduce the space- and time-integrated experimental spectrum. In the present experiment, although the targets are also BaF 2 , the laser wavelength is much longer, the pulse dura- tion is much shorter, and the laser power density is some- what lower. The analysis of the present spectrum clearly in- dicates that at least the part of the plasma emitting the 3 d - nf ( n 56,7) lines departs from the pure LTE regime. Therefore, here one must construct a detailed collisional-radiative model. Since the upper levels of the 3 d - nf radiative transi- tions are autoionizing, the model includes autoionization and dielectronic capture processes. The intensities of spectral lines arising from autoionizing levels can strongly depend upon the plasma electron density n e and temperature T e . Therefore, these lines are candidates for plasma diagnostic tools. Particularly, the use of 3 d - nf ( n 54,5) lines emitted by ions isoelectronic to CuI, ZnI, and GaI, where the upper levels are autoionizing, has been sug- gested by Bauche-Arnoult et al. @8# and by Mandelbaum et al. @9# for diagnostic purposes. In the present work, 3 d -6 f lines, emitted by Cu-like barium with different electron spec- tators, are employed. For fully modeling the plasma and the PHYSICAL REVIEW A SEPTEMBER 1998 VOLUME 58, NUMBER 3 PRA 58 1050-2947/98/58~3!/1859~8!/$15.00 1859 © 1998 The American Physical Society