Fast-particle bombardment of solid nitrogen

Abstract

The events accompanying an impact of a keV-energy particle on a frozen gas sample are surveyed briefly. The specific case of solid nitrogen is treated in detail. Experimental evidence has been obtained by mass spectrometry of the ejected material, where N⁺, N⁺₂, N⁺₃(N₂)_n and N⁺₄(N₂)_n, as well as a trace of N^–₃, were detected as products, and by e.s.r. and optical spectroscopies of the residual matrix, with N, N₃ and N^–₃ as products. The vibrational spectrum of N₃ has been recorded for the first time; a previous tentative assignment was incorrect. This radical undergoes a facile photodissociation to N₂ and N(²D). Isotopic-labelling data suggest that it is originally formed by an end-on attack by N on N₂. The photodetachment thresholds for free and ‘ion-paired’ matrix-isolated N^–₃ have been measured. Isotopic-labelling results require a scrambling mechanism, operating either after an initial formation of linear N^–₃ followed by cyclic isomerization in the hot product, or more likely during a side-on attack by N^– on N₂. The latter possibility is supported by computational evidence for a possible transient existence of an cyclic form of N^–₃, the simplest ‘antiaromatic’ triplet.