We present experimental data on the dissociative recombination ͑DR͒ and the dissociative excitation ͑DE͒ of O 2 ϩ in its electronic and vibrational ground state using a heavy ion storage ring. The absolute DR cross section has been determined over an electron collision energy range from 1 meV to 3 eV. The thermal DR rate coefficient is derived; ␣(T e )ϭ2.4ϫ10 Ϫ7 (300/T e ) 0.70Ϯ0.01 cm 3 s Ϫ1 , for TϾ200 K. The threshold for DE was observed near its energetic threshold of 6.7 eV. The DE cross section curve has a maximum of 3ϫ10 Ϫ16 cm 2 near 15 eV. We have determined the branching fractions to the different dissociation limits and present atomic quantum yields for the DR process between 0 to 300 meV collision energy. The quantum yield of O( 1 D) is found to be 1.17Ϯ0.05, largely independent of the electron energy. Arguments are presented that the branching fraction to O( 3 P)ϩO( 1 S) is negligible. The branching fraction to the O( 1 S)ϩO( 1 D) is smaller than 0.06 and varies strongly as a function of collision energy. The O( 1 S) quantum yield is a strong function of electron temperature. Hence, the relative strength of the green, O( 1 S), and the red, O( 1 D), airglows may be used as a measure of the electron temperature of the upper atmosphere. A qualitative explanation is given of the consequences of nonadiabatic interactions in the dissociation step of the DR process.

A multistate Landau–Zener method is set up for the calculation of atomic ion–ion mutual neutralization total cross sections. The results of the calculations are compared with experimental results for O++O−, N++O−, He++D−, He++H−, and H++H−. The energy range scanned depends on the system but varies between about 0.1 and 10 000 eV. The agreement between theory and experiment is usually within a factor of 2.

We present a combined analysis of the X-ray emission of the Capella corona obtained with XMM-Newton RGS and Chandra HETGS and LETGS. An improved atomic line database and a new differential emission measure ( DEM ) deconvolution method are developed for this purpose. Our new atomic database is based on the Astrophysical Plasma Emission Database and incorporates improved calculations of ionization equilibrium and line emissivities for L-shell ions of abundant elements using the Flexible Atomic Code. The new DEM deconvolution method uses a Markov Chain Monte Carlo (MCMC) technique that differs from existing MCMC or 2 -fitting-based methods. We analyze the advantages and disadvantages of each individual instrument in determining the DEM and elemental abundances. We conclude that results from either RGS or HETGS data alone are not robust enough due to their failure to constrain the DEM in some temperature region or the lack of significant continuum emission in the wavelength band of the spectrometers, and that the combination of HETGS and RGS produces more stringent constraints on the DEM and abundance determinations. Using the LETGS data, we show that the recently discovered inconsistencies between the EUV and X-ray lines of Fe xviii and xix also exist in more highly charged iron ions, up to Fe xxiii, and that enhanced interstellar absorption due to partially ionized plasma along the Capella line of sight may explain some, but not all, of these discrepancies.