Charge-exchange-induced two-electron satellite transitions from autoionizing levels in dense plasmas F. B. Rosmej, 1, * H. R. Griem, 2 R. C. Elton, 2 V. L. Jacobs, 3 J. A. Cobble, 4 A. Ya. Faenov, 5 T. A. Pikuz, 5 M. Geißel, 1 D. H. H. Hoffmann, 1 W. Su ¨ ß, 1 D. B. Uskov, 6 V. P. Shevelko, 6 and R. C. Mancini 7 1 GSI-Darmstadt, Plasmaphysik, Planckstrasse 1, D-64291 Darmstadt, Germany 2 Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742-3511 3 Center for Computational Materials Science, Materials Science and Technology Division, Naval Research Laboratory, Washington, D.C. 20375-5345 4 Los Alamos National Laboratory, Los Alamos, New Mexico 87545 5 Multicharged Ions Spectra Data Center of VNIIFRI, Russian Committee of Standards, Mendeleevo, 141570 Moscow Region, Russia 6 Lebedev Physical Institute, Moscow, Russia 7 Department of Physics, University of Nevada, Reno, Nevada 89557-0058 Received 29 May 2002; published 11 November 2002 Order-of-magnitude anomalously high intensities for two-electron dielectronic satellite transitions, origi- nating from the He-like 2 s 21 S 0 and Li-like 1 s 2 s 22 S 1/2 autoionizing states of silicon, have been observed in dense laser-produced plasmas at different laboratories. Spatially resolved, high-resolution spectra and plasma images show that these effects are correlated with an intense emission of the He-like 1 s 3 p 1 P –1 s 21 S lines, as well as the K  lines. A time-dependent, collisional-radiative model, allowing for non-Maxwellian electron- energy distributions, has been developed for the determination of the relevant nonequilibrium level populations of the silicon ions, and a detailed analysis of the experimental data has been carried out. Taking into account electron density and temperature variations, plasma optical-depth effects, and hot-electron distributions, the spectral simulations are found to be not in agreement with the observations. We propose that highly stripped target ions e.g., bare nuclei or H-like 1 s ground-state ions are transported into the dense, cold plasma predominantly consisting of L- and M-shell ions near the target surface and undergo single- and double- electron charge-transfer processes. The spectral simulations indicate that, in dense and optically thick plasmas, these charge-transfer processes may lead to an enhancement of the intensities of the two-electron transitions by up to a factor of 10 relative to those of the other emission lines, in agreement with the spectral observations. DOI: 10.1103/PhysRevE.66.056402 PACS numbers: 52.70.La, 32.70.Fw, 32.80.Dz, 34.70.+e I. INTRODUCTION Following the publication of the first monograph on plasma spectroscopy 1, spectroscopic methods have pro- vided essential information about basic plasma parameters and relevant physical processes. The accessible parameter range covers orders of magnitude in temperature and espe- cially density, because practically all elements of particular, selected isoelectronic sequences can be used for diagnostic investigations. These elements may occur as intrinsic impu- rities or may be intentionally injected in small amounts. De- tailed reviews of spectroscopic methods have been published subsequently 2–4. In addition to the traditionally used resonance lines, di- electronic satellite spectra, which arise from radiative transi- tions from autoionizing states, have been successfully ex- ploited for diagnostic investigations 5. With the development of high-intensity lasers and the associated in- vestigation of laser-produced plasmas, dielectronic-satellite transitions have become of increasing importance for the fundamental understanding of atomic radiation processes in plasmas. The primary application of dielectronic-satellite spectra has been as a temperature diagnostic. In the low- density coronal-model approximation, the intensity ratio of the dielectronic-satellite transition following radiationless electron capture—and the corresponding resonance-line transition—is predicted to be a function of the electron tem- perature and independent of the electron density 5. Density and opacity effects can become important only in high- density plasma environments. In laser-produced plasmas, satellite features near the H-like Ly  lines have been observed which could not be interpreted in terms of temperature variations alone. Conse- quently, these satellite features have been characterized as exhibiting anomalous satellite intensities. It has been pre- dicted 6,7 that collisionally induced transitions among the autoionizing states can lead to significant modifications of the autoionizing-level populations in dense plasmas, result- ing in corresponding density-dependent deviations from the coronal-model values of the satellite intensities. At low den- sities, the satellite intensities had been shown to be propor- tional to the satellite-intensity Q factor introduced by Gabriel 5 and defined as follows: Q ij = g j  j , g A ji  k A jk +  l  jl . 1 Here g j is the statistical weight of the autoionizing level j,  j , g is the autoionizing rate for the transition from the level j to the ground level g of the residual ion, and A ji is the *Electronic mail: rosmej@yahoo.de PHYSICAL REVIEW E 66, 056402 2002 1063-651X/2002/665/05640216/$20.00 ©2002 The American Physical Society 66 056402-1