X-ray emission cross sections following charge exchange by multiply charged ions of astrophysical interest

Abstract

State selective $nl$-electron capture cross sections are presented for highly charged ions with $Z=6--10$ colliding with atoms and molecules. The energy range investigated was from $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}(v=0.006\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})\text{to}\phantom{\rule{0.3em}{0ex}}100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}(v=2.0\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})$. The energy dependence of the $l$-level populations is investigated. The $K$ shell x-ray emission cross sections are determined by using the calculated state-selective electron capture results as input and then applying hydrogenic branching and cascading values for the photon emission. A major shift in the line emission from being almost solely Lyman-$\ensuremath{\alpha}$ transitions at the highest collisions energies to strong high-$n$ to $1s$ transitions at the lowest energies is observed. The calculated cross sections are in reasonable accord with measurements made by Greenwood et al. [Phys. Rev. A 63, 062707 (2001)], using ${\mathrm{O}}^{8+}$ and ${\mathrm{Ne}}^{10+}$ on various targets at $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}$. The calculations are also in accord with x-ray emission cross section data obtained on the EBIT machine at Lawrence Livermore National Laboratory (LLNL) where ${\mathrm{O}}^{8+}$ and ${\mathrm{Ne}}^{10+}$ high resolution measurements were made at a temperature of $10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$ for a series of targets with varying ionization potentials. The ${\mathrm{Ne}}^{10+}$ data clearly shows the contribution from multiple capture followed by Auger autoionization in the line emission spectra. Our calculated line emission cross sections are used to provide an ab initio determination of the soft x-ray spectrum of comet C/Linear 1999 S4 that was observed on the Chandra X-ray Observatory. The calculations show that the spectrum is due to the charge exchange of the neutral gases in the comet's coma with the ions of the slow solar wind.