Nitrogen Cluster Anions

Anions are formed by electron attachment to helium nanodroplets doped with N2. The most prominent ion series is due to odd-numbered N m – with 3 ≤ m < 140. Neither N– nor N2 – are observed. An appearance energy of 11.5 ± 0.5 eV is measured for N3 –. The yield of N m – averaged over 5 ≤ m ≤ 21 shows an appearance energy of 9 eV, just above the estimated thermodynamic threshold for formation of N3 – from electron attachment to small N2 clusters embedded in helium droplets. These findings support the notion that odd-numbered N m – cluster ions are best characterized as N2 van der Waals clusters with an azide anion chromophore, but they are at odds with some theoretical reports. N3 –(N2)4 and N3 –(N2)11 form local maxima in the abundance distribution, suggesting that these ions are particularly stable. The yield of even-numbered N m – ions (m ≥ 4) is 2 orders of magnitude lower, barely exceeding the background level. Several other cluster anion series are observed that involve impurities.