  Anions are formed by electron attachment to helium nanodroplets doped with
N$_2$. The most prominent ion series is due to odd-numbered N$_m$$^{-}$ with 3
$\le$ m < 140. Neither N$^{-}$ nor N$_2$$^{-}$ are observed. An appearance
energy of 11.5 $\pm$ 0.5 eV is measured for N$_3$$^{-}$. The yield of
N$_m$$^{-}$ averaged over 5 $\le$ m $\le$ 21 shows an appearance energy of 9
eV, just above the estimated thermodynamic threshold for formation of
N$_3$$^{-}$ from electron attachment to small N$_2$ clusters embedded in helium
droplets. These findings support the notion that odd-numbered N$_m$$^{-}$
cluster ions are best characterized as N$_2$ van der Waals clusters with an
azide anion chromophore but they are at odds with some theoretical reports.
N$_3$$^{-}$(N$_2$)$_4$ and N$_3$$^{-}$(N$_2$)$_{11}$ form local maxima in the
abundance distribution, suggesting that these ions are particularly stable. The
yield of even-numbered N$_m$$^{-}$ ions (m $\ge$ 4) is two orders of magnitude
lower, barely exceeding the background level. Several other cluster anion
series are observed that involve impurities.
