Mass spectrometry of anions and cations produced in 1–4 keV H−, O−, and OH− collisions with nitromethane, water, ethanol, and methanol

Interactions between 1 and 4 keV anions (H−, O−, and OH−) and gas-phase molecules (nitromethane, water, ethanol, and methanol) have been studied using quadrupole mass spectrometry of the product anions and cations. The low collision velocities (0.07–0.40 vBohr) provide favourable conditions for electron transfer from the anion projectile to the neutral target molecule yielding negative ion formation, while strong competition with cation formation is also observed. Relative production of fragment cations increases with H− impact energy and with projectile mass when energy is constant. Considered together, these results suggest a momentum dependence on collisional energy deposition. As far as negative ion production is concerned, comparisons with previous free electron attachment studies are drawn as a starting point for the interpretation of the anion fragmentation channels. For nitromethane and water, the present anion fragmentation patterns are substantially different to the free electron attachment data. Conversely the fragmentation channels of ethanol and methanol anions only show clear dependence on the electron attachment/transfer process in terms of the relative anion yields.

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► 1–4 keV anions (H−, O−, and OH−) interactions with gas-phase molecules.
► Negative and positive ion formation.
► Dipole-bound states.
► Charge transfer to shape resonance.