Measurements of absolute, single charge-exchange cross sections of H+, He+and He2+ with H2O and CO2

Absolute measurements have been made of single-electron charge-exchange cro ss sections of H+, He+, and He2+ in H2O and CO2 in the energy range 0.3-7.5 keV amu(-1). Collisions of this type occur in the interaction of solar win d ions with cometary gases and have been observed by the Giotto spacecraft using the Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) d uring a close encounter with comet Halley in 1986. Increases in the He+ ion density, and in the He2+ to H+ density ratio were reported by Shelley et a l, and Fuselier et al. and were explained by charge exchange. However, the lack of reliable cross sections for this process made interpretation of the data difficult. New cross sections are presented and discussed in relation to the Giotto observations.