Electron-Catalyzed Mutual Neutralization of Various Anions with ${\mathrm{Ar}}^{+}$: Evidence of a New Plasma Process

The mutual neutralization of anions with ${\mathrm{Ar}}^{+}$ has been studied by variable electron and neutral density attachment mass spectrometry. Evidence of a previously unobserved plasma loss process, electron-catalyzed mutual neutralization (ECMN), e.g., ${\mathrm{SF}}_6^{-}+{\mathrm{Ar}}^{+}+e^{-}\to \mathrm{\text{neutrals}}+e^{-}$, is reported. Results for 10 species suggest that ECMN occurs generally and significantly affects the total ion-loss rate in plasmas with electron densities exceeding ${10}^{10}{\mathrm{cm}}^{-3}$. ECMN is discussed in the context of other known three-body plasma processes, the mechanisms for which appear insufficient to explain the observed effect. A mechanism for ECMN involving an incident electron facilitating energy transfer to the internal modes of the anion is proposed.

Figure 1

A sketch of the FALP apparatus. Gas flow is from left to right. Locations of the helium, argon, and neutral attaching gas inlets are indicated.

Figure 2

Relative product anion abundances [e.g., $[{\mathrm{SF}}_6^{-}]/([{\mathrm{SF}}_6^{-}]+[{\mathrm{Cl}}^{-}])$] after 4.5 ms reaction time on addition of $1.9\times {10}^9{\mathrm{cm}}^{-3}$ ${\mathrm{SF}}_6$ and the indicated amounts of ${\mathrm{CCl}}_4$ to the flowing afterglow as a function of electron density at the neutral injector ($[e^{-}{]}_{t=0}$). Thick solid lines are best-fit modeled abundances. Dashed lines in (b) are the best-fit modeled abundances assuming the range of $k_{\mathrm{MN}}$ determined in (a) and excluding ECMN; the solid lines in (b) are the best-fit modeled abundances including ECMN. The primary difference between (a) and (b) is the amount of electron depletion due to attachment to ${\mathrm{CCl}}_4$; the degree of this difference is indicated by the modeled average electron density throughout the full reaction time ($⟨[e^{-}]⟩$) as a function of $[e^{-}{]}_{t=0}$ (solid lines, right axis) under each condition.

Figure 3

Schematic representation of the proposed ECMN mechanism. An electron passing between the charged species (shown at three discrete time steps) shields the positive charge, inducing an electromagnetic fluctuation [$V(t)$]. The corresponding distortion in the potential seen by the anion [$V(r)$] results in a changing dipole ($D$), allowing for coupling between the kinetic energy of the system and low energy vibrations of the anion.

Figure 4

Comparison of experimental ECMN rate constants for various anions with ${\mathrm{Ar}}^{+}$ at 300 K with calculated low energy IR intensity. Fit is a linear regression to the experimental points.