Production of negatively charged fragments in collisions of swift positive hydrogen ions with molecules

A first detection and analysis of negatively charged fragments produced in collisions of fast (20–150 keV) positive hydrogen ions (H⁺, H ₂ ⁺ and H ₃ ⁺ ) with gas-phase molecules is presented. The fragments and their abundances were determined by means of a time-of-flight mass spectrometer. Negative ions did emerge from every investigated target molecule species, such as halomethanes, sulfur hexafluoride, propane and propene, but in all cases with distinctly lower probability (cross sections in the range 10^?20?10^?18 cm²) than positively charged fragments (approximately on the scale 10^?3 or even less). Another essential result is that stable collisionally induced negative fragments are mostly monatomic ions, whereas positive fragments are in their majority more complex polyatomic ions. Furthermore, we observed a direct electron capture from a positively charged but not totally stripped projectile (here: H ₂ ⁺ and H ₃ ⁺ ) into stable or very longlived states of the molecular ions SF ₆ ^? and O ₂ ^? , the latter with the largest cross section (10^?18?10^?17 cm²) found up to now.