  A combination band due to a mechanism whereby a photon excites two or more
vibrational modes ({\it e.g.} a bend and a stretch) of an individual molecule
is commonly seen in laboratory and astronomical spectroscopy. Here, we present
evidence of a much less commonly seen combination band $-$ one where a photon
simultaneously excites two adjacent molecules in an ice. In particular, we
present near-infrared spectra of laboratory CO/N$_2$ ice samples where we
identify a band at 4467.5 cm$^{-1}$ (2.239 $\mu$m) that results from single
photons exciting adjacent pairs of CO and N$_2$ molecules. We also present a
near-infrared spectrum of Neptune's largest satellite Triton taken with the
Gemini-South 8.1 meter telescope and the Immersion Grating Infrared
Spectrograph (IGRINS) that shows this 4467.5 cm$^{-1}$ (2.239 $\mu$m) CO-N$_2$
combination band. The existence of the band in a spectrum of Triton indicates
that CO and N$_2$ molecules are intimately mixed in the ice rather than
existing as separate regions of pure CO and pure N$_2$ deposits. Our finding is
important because CO and N$_2$ are the most volatile species on Triton and so
dominate seasonal volatile transport across its surface. Our result will place
constraints on the interaction between the surface and atmosphere of Triton. 1
