Fingerprints of charge exchange between He2+ and water molecules

We have measured state selective cross sections for single and double charge exchange of He2+ and water molecules at velocities of 500-1000 km/s. The cross sections for single electron capture into the HeII(2p) state are much larger than those for double electron capture into the HeI(1s2p) state. Electron capture into the HeI(1s2p) state appears to be more or less independent of the ion's velocity above 500 km/s, while capture into the HeII(2p) however is strongly dependent on the collision energy. We present a recommended helium line emission ratio for the velocity range typical for the solar wind (200-1000 km/s), that can greatly add to the analysis of cometary X-ray and far-ultraviolet emission in terms of solar wind and coma characteristics. (c) 2005 Elsevier B.V. All rights reserved.