Nitrogen Cluster Anions

TL;DR

This study investigates nitrogen cluster anions formed via electron attachment to helium nanodroplets doped with N₂, revealing the prominence of odd-numbered N_m^- ions, their formation energies, and stability, challenging some theoretical predictions.

Contribution

It provides experimental data on nitrogen cluster anions, especially odd-numbered N_m^- ions, and discusses their stability and formation energies, offering insights into their structure and properties.

Findings

Odd-numbered N_m^- ions are predominant with m between 3 and 140.

N₃⁻ has an appearance energy of 11.5 eV, while N_m^- (5 ≤ m ≤ 21) appears just above 9 eV.

N₃⁻(N₂)₄ and N₃⁻(N₂)₁₁ are particularly stable, forming local maxima.

Abstract

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.