Amplification of critical velocity ionization by associative ionization

Abstract

Space experiments to verify the critical ionization velocity (CIV) theory have for the most part involved the release of barium in the ionosphere. It is suggested in this paper that associative ionization (AI) reactions involving barium (i.e., Ba + O → BaO+ + e) may occur in these experiments, and since the products would be indistinguishable from those generated by electron impact ionization (CIV), it is likely that AI‐generated ions may be mistaken for those generated by CIV. The electrons formed in AI and by electron impact may be energized by the ion beams, generated by both CIV and AI. The energized electrons may ionize, enhancing the CIV process. In this way, the AI‐CIV double process could amplify CIV. The amplification is especially important for sustaining CIV when the CIV energy budget is tight or when the ionization rate due to CIV alone is too low to satisfy Townsend's criterion. The result may have significant implications for the results observed in recent CIV space experiments using barium jets. The AI‐CIV double process may offer a plausible explanation for the observed long distance of Ba+ ionization. Any other ion beam formation mechanism, such as charge exchange, stripping and reflected ambient ions, can also amplify CIV in a manner similar to AI, although comparison of the relative magnitudes of several of these processes indicates that AI may be the most significant. We suggest an AI‐assisted experiment in which a samarium beam could undergo AI, promoting CIV in an accompanying xenon beam.