2001ApJ...548..836S

We present 2338-2322 cm^–1 (4.277-4.307 µm) infrared spectra of a number of N₂-containing mixed molecular ices and demonstrate that the strength of the infrared ``forbidden'' band due to the NN stretch near 2328 cm^–1 (4.295 µm) is extremely sensitive to the composition of the ice. The strength of the 2328 cm^–1 N₂ fundamental is significantly enhanced relative to that of pure N₂ ice when NH₃, H₂O, or CO₂ are present, but it is largely unaffected by the presence of CO, CH₄, or O₂. We use the laboratory data in conjunction with Infrared Space Observatory (ISO) data that probe several lines of sight through dense molecular clouds to place limits on the abundance of interstellar solid phase N₂ and the composition of the ices. Deriving upper limits is complicated by the presence of overlapping absorptions due to CO₂ gas in the clouds and, in some cases, to photospheric CO in the background star. These upper limits are just beginning to be low enough to constrain interstellar grain models and the composition of possible N₂-bearing interstellar ices. We outline the search criteria that will need to be met if solid interstellar N₂ is to be detected in the future. We also discuss some of the implications of the presence of warm CO₂ gas along the lines of sight to embedded protostars and demonstrate that its presence may help resolve certain puzzles associated with the previously derived gas/solid CO₂ ratios and the relative abundances of polar and nonpolar ices toward these objects. Finally, we briefly comment on the possible implications of these results for the interpretation of N₂ detections on outer solar system bodies.