Infrared spectra of an H₂O:NH₃ 1:3 ice mixture in the far-IR region at 8 K and annealed at 120 K (solid lines). The H₂O:NH₃ ratio of the annealed mixture is greater than 1 due to ammonia desorption (see text). Spectra are offset for clarity. The dashed line represents the calculated spectrum of an ice mixture of the same composition, which is in better agreement with the spectrum of the annealed ice.