(a) Infrared band profile of the CH₃ rock vibrational mode of CH₃NH₂, around 1159 cm⁻¹, in different ices at different temperatures, from left to right: pure CH₃NH₂, CH₃NH₂:H₂O:CH₄ (1:5:5), CH₃NH₂:H₂O:NH₃ (1:5:5), CH₃NH₂:CH₄:NH₃ (1:5:5), and CH₃NH₂:H₂O:CH₄:NH₃(3:10:10:10). The panels are displayed at a different Y-scale. In the last three panels, the CH₃NH₂ feature appears as a shoulder in the ammonia ν₂ band (umbrella mode). (b) Left panel: peak position vs. FWHM for the CH₃ rock band in the ice mixtures displayed in a. Different mixtures are indicated by different colors, and the different temperatures are marked in the graph. Right panel: relative band strength of the CH₃ rock vibrational mode at 15 K.