On the Significance of the Contribution of Multiple-Electron Capture Processes to Cometary X-Ray Emission

We report laboratory studies of the role played by multiple-electron capture (MEC) in solar wind-induced cometary X-ray emission. Collisions of Ne¹⁰⁺ with He, Ne, Ar, CO, and CO₂ 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.