Energetic Processing of Solid Nitrogen: Infrared Identification of N ₄ ⁺ and Correlated γ-Line Emission under Electron Impact and Extreme-UV Irradiation

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.