Reexamining if long-lived ${\mathrm{N}}^{-}$ anions are produced in fast dissociative electron-capture collisions

The existence of long-lived states of ${\mathrm{N}}^{-}$ has been a topic investigated with conflicting experimental results. Highly excited spin-aligned states, however, were predicted to have long lifetimes and even be stable against autodetachment. We repeated the measurements of ${\mathrm{N}}^{-}$ formation in $0.9\text{MeV}$ ${\mathrm{N}}_2+\text{Ar}$ charge-exchange collisions and found that the ions reported previously as ${\mathrm{N}}^{-}$ [Heber et al., Phys. Rev. A 38, 4504 (1988)] are an ${\mathrm{O}}^{-}$ fragment from a $0.9\text{MeV}$ ${\mathrm{H}}_2{\text{NO}}^{+}$ impurity beam. This result adds to the comulating data indicating that ${\mathrm{N}}^{-}$ is not going to affect carbon dating measurements using accelerator mass spectrometry.

Figure 1

The intensity of ${\mathrm{C}}^{-}$, apparant ${\mathrm{N}}^{-}$ $\left(\text{denoted as}{\mathrm{N}}^{-}\text{in the figure}\right)$, and ${\mathrm{O}}^{-}$ ions detected by the narrowly collimated detector as a function of the deflector voltage.

Figure 2

The energy spectra of negative ions on the expected ${\mathrm{N}}^{-}$ trajectory with (open squares) and without (triangles) foil and mesh. Note that only a shift to lower energy as expected for an atomic ion can be seen.

Figure 3

The energy spectra of (a) negative ions on the expected ${\mathrm{N}}^{-}$ trajectory, (b) ${\mathrm{N}}^{+}$ ions, (c) $450\text{keV}$ ${\mathrm{O}}^{-}$ ions from a $844\text{keV}$ ${\text{NO}}^{+}$ beam suspected to be the impurity generating the “${\mathrm{N}}^{-}$” signature (see text), and (d) $450\text{keV}$ ${\mathrm{O}}^{+}$ from the dissociation of a $900\text{keV}$ $\mathrm{O}_2{}^{+}$ beam. Note that the energy spectrum of the ions suspected as ${\mathrm{N}}^{-}$ match the energy spectrum of the $450\text{keV}$ ${\mathrm{O}}^{-}$ and ${\mathrm{O}}^{+}$, but it does not match the energy spectrum of the ${\mathrm{N}}^{+}$ fragments (see text).