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

Abstract

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.

X-Ray Signature of Charge Exchange in L-Shell Sulfur Ions

“…This is in agreement with the low ion temperature in EBIT, about 10-20 eV amu −1 , which provides the low-collision energies that favor the population of low-l states during CX (Beiersdorfer et al 2000). It also suggests that high-l capture into n c followed by cascades, which is mainly a result of high-collision energy, is less likely than doubleelectron capture, as the fraction of double-electron captures can approach almost half of the total (Ali et al 2005). However, detailed radiative cascade models are needed to discern which of these processes is more probable.…”

“…Another possible process in CX reactions is doubleelectron capture. This is followed by autoionization, where one electron drops to a lower n level while the other gets ionized (Ali et al 2005). If this occurs, one would see an enhancement in the flux from that lower n level, e.g., n = 3, 4, 5, 6 → n = 2 ).…”

Spectral analysis of theChandracomet survey

“…An important difference between reactions with atomic hydrogen and the other species is the presence of multiple electrons, hence allowing for multiple (mostly double) electron transfer. It has been demonstrated both experimentally and theoretically that double electron capture can be an important reaction channel in multi-electron targets and that after autoionization to an excited state it may contribute to the X-ray emission (Ali et al 2005;Hoekstra et al 1989;Beiersdorfer et al 2003;Otranto et al 2006;Bodewits et al 2006). Unfortunately, experimental data on reactions with species typical for cometary atmospheres, such as H 2 O, atomic O and CO are at best scarcely available.…”

Energy dependence of angular momentum capture states in charge exchange collisions between slow highly charged argon ions and argon neutrals

“…Secondly, the CTMC data from [13] are for single electron capture, whereas in slow HCI-atom collisions the cross sections for multiple electron capture are in fact significant [26]. Indeed it has been shown that multiple electron capture plays an important role in the shaping of charge exchange emission spectra [12,27]. However, the overall magnitude of the hardness ratio might not be changed, since multiple capture events mainly rearrange the np → 1s x-ray emission to lower n levels.…”