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CO$_2$ Infrared Phonon Modes in Interstellar Ice Mixtures

CO$_2$ ice is an important reservoir of carbon and oxygen in star and planet forming regions. Together with water and CO, CO$_2$ sets the physical and chemical characteristics of interstellar icy grain mantles, including desorption and diffusion energies for other ice constituents. A detailed understanding of CO$_2$ ice spectroscopy is a prerequisite to characterize CO$_2$ interactions with other volatiles both in interstellar ices and in laboratory experiments of interstellar ice analogs. We report laboratory spectra of the CO$_2$ longitudinal optical (LO) phonon mode in pure CO$_2$ ice and in CO$_2$ ice mixtures with H$_2$O, CO, O$_2$ components. We show that the LO phonon mode position is sensitive to the mixing ratio of various ice components of astronomical interest. In the era of JWST, this characteristic could be used to constrain interstellar ice compositions and morphologies. More immediately, LO phonon mode spectroscopy provides a sensitive probe of ice mixing in the laboratory and should thus enable diffusion measurements with higher precision than has been previously possible.