We report laboratory studies of the role played by multiple-electron capture (MEC) in solar wind-induced cometary X-ray emission. Collisions of Ne 10ϩ with He, Ne, Ar, CO, and CO 2 have been investigated by means of the traditional noncoincident-singles X-ray spectroscopy in addition to the triple-coincidence measurements of X-rays, scattered projectile, and target recoil ions for the atomic targets. The coincidence measurements enable one to reduce the singles X-ray spectra into partial spectra originating in single-electron capture (SEC) and MEC collisions. The measurements provide unequivocal evidence of the significant role played by MEC and strongly suggest that models based solely on SEC are bound to yield erroneous conclusions on the solar wind composition and velocities and on cometary atmospheres. The experimental relative importance of MEC collisions is compared with the molecular classical-over-the-barrier model, the classical trajectory Monte Carlo technique, and the multichannel Landau-Zener method, calculations that can qualitatively reproduce the experimental trends.

Recent X-ray observations of star-forming galaxies such as M82 have shown the Lyβ/Lyα line ratio of Ne X to be in excess of predictions for thermal electron impact excitation. Here we demonstrate that the observed line ratio may be due to charge exchange and can be used to constrain the ion kinetic energy to be 500 eV/u. This is accomplished by computing spectra and line ratios via a range of theoretical methods and comparing these to experiments with He over astrophysically relevant collision energies. The charge exchange emission spectra calculations were performed for Ne 10+ + H and Ne 10+ + He using widely applied approaches including the atomic orbital close coupling, classical trajectory Monte Carlo, and multichannel Landau-Zener (MCLZ) methods. A comparison of the results from these methods indicates that for the considered energy range and neutrals (H, He) the so-called "low-energy ℓ-distribution" MCLZ method provides the most likely reliable predictions.

Aims. Electron capture in collisions of highly charged O6+ ions with ground-state hydrogen atoms is a very important process in solar wind X-ray studies. Methods. In the present study, the full quantum-mechanical molecular-orbital close-coupling method is employed to study electron capture reactions in collisions of O6+ ion with ground-state atomic hydrogen in the energy region from 10−4 keV u−1 to 5 keV u−1. The ab initio multi-reference single- and double-excitation configuration interaction (MRD-CI) method is used to calculate the potential and coupling data used in the QMOCC calculations. Results. Total and state-selective cross sections for the dominant and subdominant reaction channels are calculated and compared with the available experimental and theoretical data. The branching ratios for Lithium-like O5+ excited ions are used to calculate the contribution of cascade radiative transitions from n = 5 levels to the population of 4l states. From the calculated cross sections, reaction rate coefficients are obtained for temperatures between 1000 and 1 × 109 K and compared with other calculations.