Anion emission from water molecules colliding with positive ions: Identification of binary and many-body processes

It is shown that negative ions are ejected from gas-phase water molecules when bombarded with positive ions at keV energies typical of solar-wind velocities. This finding is relevant for studies of planetary and cometary atmospheres, as well as for radiolysis and radiobiology. Emission of both ${\mathrm{H}}^{-}$ and heavier (${\mathrm{O}}^{-}$ and $\mathrm{O}{\mathrm{H}}^{-}$) anions, with a larger yield for ${\mathrm{H}}^{-}$, was observed in $6.6\text{−}\mathrm{keV}{}^{16}{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}$ collisions. The experimental setup allowed separate identification of anions formed in collisions with many-body dynamics from those created in hard, binary collisions. Most of the anions are emitted with low kinetic energy due to many-body processes. Model calculations show that both nucleus-nucleus interactions and electronic excitations contribute to the observed large anion emission yield.

Figure 1

Scheme of the electrostatic analyzer equipped with magnetic filters at its entrance and exit. Electrons are deflected so that only anions reach the detector.

Figure 2

Full curves: DDCS for anion emission in $6.6\text{−}\mathrm{keV}{}^{16}{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}$ collisions at the indicated observation angles. For graphical reasons, each spectrum is multiplied by the indicated factor. Anions formed in many-body collisions are observed in the slowly decreasing “continuous” part of the spectra (full and dashed curves in black). Anions created in binary-encounter collisions appear in peak structures. The peak centroids are shifted towards lower energies as the angle is increased (graphically emphasized by the dash-dotted curves). The points at lower energies give the relative contributions due to ${\mathrm{H}}^{-}$ (red circles) and ${\mathrm{O}}^{-}/\mathrm{O}{\mathrm{H}}^{-}$ (green diamonds).

Figure 3

(a) SDCS for ${\mathrm{H}}^{-}$ (open circles) and ${\mathrm{H}}^{+}$ (full circles) emission via the binary-encounter process as a function of the observation angle. Only relative error bars due to statistical uncertainties are shown, except the rightmost points, where a larger uncertainty stems from the overlap of peak structures. Red curves: calculated SDCS for two-body elastic recoil of H atoms by $6.6\text{−}\mathrm{keV}$ oxygen impact, multiplied by factors representing the fraction of the different charge-state components. (b) Ratio between the experimental ${\mathrm{H}}^{-}$ and ${\mathrm{H}}^{+}$ SDCS reported in (a).

Figure 4

Open circles: Simulated energy distribution of the ${\mathrm{H}}^{-}$ anions at different angles with KER. Dotted curve: Same without KER. Full curve: Experimental DDCS for anion emission. Red, full circles: Experimental ${\mathrm{H}}^{-}$ contribution from TOF measurements. Each spectrum is multiplied by the indicated factor.