Experimental investigation of the processes determining x-ray emission intensities from charge-exchange collisions

Abstract

Absolute cross sections have been measured for single and double charge exchange and x-ray line emission for highly charged ions of C, N, O, and Ne colliding with He, ${\mathrm{H}}_{2},$ ${\mathrm{CO}}_{2},$ and ${\mathrm{H}}_{2}\mathrm{O}$ at collisions energies of $7q\mathrm{keV}.$ Present results of charge exchange in He and ${\mathrm{H}}_{2}$ compare favorably with previous results. For ${\mathrm{CO}}_{2}$ and ${\mathrm{H}}_{2}\mathrm{O},$ where prior work is scarce, the classical overbarrier model is found to overestimate results by up to a factor of 3. An analysis of the relative intensities of the observed Lyman x-ray transitions indicates that capture into l states is not statistical, as collision velocities are insufficient to populate the highest angular-momentum states. The importance of autoionization following multiple capture is highlighted, and enhanced radiative stabilization following double capture is observed and compared to other studies. Present results are also discussed in terms of mechanisms likely to generate x-ray emission in comets.