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 N 2 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 N 3, N 3 –, and N 4 +, whereas extreme-UV irradiation predominantly yields neutral products, with N 3 as the major nitrogen-bearing species and only weak N 4 + absorption. The accompanying emission spectra reveal the formation of excited N 2 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 N 4 + infrared band, while the N 3 absorption remains nearly unchanged. This correlated behavior is consistent with the γ-line emission being associated with a neutralization-driven relaxation pathway involving an N 4 + -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 N 2 contains metastable higher-order nitrogen reservoirs beyond simple N 2 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 N 2 can transiently store and redistribute electronic energy through coupled radiative and nonradiative channels.