Luminescence Measurements of the Hyperthermal Reactions of N/N⁺ + NH₃

Abstract

Chemiluminescence from a system of collisions, N + / N/Kr + /Kr/Xe + /Xe + NH 3 , at collision energies between 10 and 170 eV (center of mass, COM), was measured in the spectral range 300−1000 nm. The energy dependence of the emission excitation cross sections was quantified, and molecular signatures were fit to known spectroscopic constants to determine vibrational-state populations. For both N and N + collision species, the strongest features were assigned to emissions from NH (A−X) and the atomic hydrogen Balmer series. For each of the spectra resulting from collisions with primary cations, the NH (A−X) emissions had the largest cross sections reaching values of (1.0−1.5) × 10 −18 cm 2 by 100 eV. Additional features originating from atomic nitrogen and NH (c−a) emissions were also observed. The NH (c−a) emissions accounted for about 8%, 13%, and 15% for total excited populations in collisions with Xe + , N + , and Kr + , respectively. These transitions were consistent with short-range interactions resulting in collision-induced dissociation of the NH 3 molecule with apparent energy thresholds between 20 and 30 eV and emission cross sections decreasing with ion mass. Evidence of charge exchange in the N + + NH 3 collisions was observed in the resulting spectra as broad transitions between 420 and 480 nm and were assigned to NH + emitting from the (B) state. Differences between the spectra were observed as changes in the emission signal with the neutral collisions producing only 30% or 65% of the NH (A−X) emission cross sections compared to the cation results for xenon and krypton, respectively. For N and N + , NH (A) was created in equal amounts at lower collision energies, but the emission for the neutral system increases above that of the cation at collision energies greater than 80 eV COM. In both cases, the threshold energy for appearance was below 10 eV, suggesting an additional pathway for NH (A) formation, namely, hydrogen abstraction or charge exchange and abstraction for the N and N + , respectively. In all cases, the neutral NH (c−a) emission intensity was similar between neutral and cation pairs. The H-α emission line (n = 3−2) decreased to about 10%, 33%, and 50% of the corresponding cation spectra for xenon, krypton, and nitrogen, respectively.