Energetic Processing of Solid Nitrogen: Infrared Identification of N₄⁺ and Correlated γ-Line Emission under Electron...

We investigate the energetic processing of solid N₂ at 3 K under 300 eV electron impact and extreme-UV irradiation at 58.4 and 68.8 nm by combining in situ IR absorption spectroscopy with simultaneous UV-visible-near-IR luminescence measurements. Electron bombardment produces clear infrared signatures of N₃, N₃^-, and N₄⁺, whereas extreme-UV irradiation predominantly yields neutral products, with N₃ as the major nitrogen-bearing species and only weak N₄⁺ absorption. The accompanying emission spectra reveal the formation of excited N₂ and N atoms under both excitation modes, together with a weak near-IR γ-line. Controlled warm-up of the electron-irradiated samples shows that the γ-line intensity increases sharply between 13 and 18 K, coincident with the most rapid decrease of the N₄⁺ infrared band, while the N₃ absorption remains nearly unchanged. This correlated behavior is consistent with the γ-line emission being associated with a neutralization-driven relaxation pathway involving an N₄⁺-related precursor, although the microscopic emitting state remains to be identified. Preliminary vacuum-ultraviolet photoionization mass spectrometry performed during desorption at 25 K further suggests that irradiated solid N₂ contains metastable higher-order nitrogen reservoirs beyond simple N₂ molecules. These results provide direct spectroscopic constraints on the formation, stabilization, and relaxation of metastable polynitrogen species in condensed nitrogen and show how irradiated solid N₂ can transiently store and redistribute electronic energy through coupled radiative and nonradiative channels.