This work presents a theoretical analysis of experimental results for the hydrogen Balmer-alpha line in dense plasmas, with electron densities between 2x10(18) and 9x10(18) e/cm(3) A simulation of both electrons and ions is employed to produce reliable theoretical widths. These results are essentially in agreement with standard theory results and, for the most part, disagree with the experimental results. Consequently, either mechanisms not accounted for in the theoretical results (such as quadrupoles) are more important than previously thought at these densities, or else there is a problem in the experimental data (such as a possible reabsorption, which is not ruled out by the experimental data).

Electron terms in the field of two stationary Coulomb centres (TCCs) of charges Z and Z separated by a distance R are a fundamental problem of quantum mechanics, presenting fascinating atomic physics: the terms show crossings and avoided crossings. In the latter situation, the electron has a much larger probability of tunnelling from one well to the other (i.e. of charge exchange) than in the absence of such degeneracy. These rich features of the TCC problem are also manifest in different areas of physics such as plasma spectroscopy. Recently it was shown experimentally that charge exchange, enhanced by the encounter of two TCC terms, can result in an unusual structure (a dip) in the spectral line profile emitted by a Z ion from a plasma consisting of both Z and Z ions. In this paper we present a detailed quantitative theory of this phenomenon, in which its origin is directly traced to the avoided crossings of terms in the TCC problem. We show that our theory explains quantitatively all the results of the above experiment where such a dip was observed in a hydrogen line. We also consider in detail several prospective 'radiator-perturber' pairs for observing these signatures of charge exchange in lines of hydrogen-like ions. Further experimental studies of such dips would serve to produce not-yet-available fundamental data on charge exchange between multicharged ions, virtually inaccessible by other experimental methods.

The paper deals with a frequently encountered situation where the energy difference between the terms involved in a radiative transition, being plotted versus the radiator-perturber separation, shows extrema. The paradigm, based on 30 years of theoretical and experimental studies, is that the extrema in the transition energy result in satellites in spectral line profiles. In this Rapid Communication we show that this paradigm breaks down: the extrema in the transition energy can also result in dips in spectral line profiles. Moreover, we demonstrate that if the extremum in the transition energy is due to the charge exchange, its spectral signature most probably should be a dip rather than a satellite.

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We report the first experimental observation of charge-exchange-caused dips (also called x dips) in spectral lines of multicharged ions in laser-produced plasmas. Specifically, in the process of a laser irradiation of targets made out of aluminum carbide, we observed two x dips in the Ly(gamma) line of Al XIII perturbed by fully stripped carbon. From the practical point of view, this opens up a way to experimentally produce not-yet-available fundamental data on charge exchange between multicharged ions, virtually inaccessible by other experimental methods. From the theoretical viewpoint, the results are important because the x dips are the only one signature of charge exchange in profiles of spectral lines emitted by plasmas and they are the only one quasimolecular phenomenon that could be observed at relatively "low" densities of laser-produced plasmas.

A formula was obtained that describes asymptotically forbidden quasimolecular optical transitions in the frame of the semiclassical approach. It is particularly relevant for the weak extrema in the difference between the ground-and excitedstate interaction potentials. When averaged over impact parameters and velocity distribution the formula agreed reasonably well with the recent experimental data for the Ca(4 1 S → 3 1 D) + He transition.

A model aimed at describing the electronic structure of hot dense plasmas is presented. This model is first used to study the effect of the nearest neighbor interaction on the photoabsorption K-edge position in a dense fluorine plasma. Changes to the ionization potential lowering from the well-known ion sphere model ͓H. Nguyen et al., Phys. Rev. A 33, 1279 ͑1986͔͒ are obtained by the present approach for plasma density above solid. We believe that this effect could be of importance in the calculation of the opacity of dense materials. The present model also provides an alternative to the treatment of line broadening in very dense plasmas where the average interionic spacing can be of the order of the spatial extent of the excited-state orbitals. To illustrate this point, we present F Lyman-␤ line shapes for various density conditions.

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