Ice mixtures of CO
2 and H
2O are studied using Fourier transform reflection-absorption infrared (RAIR)spectroscopy. Mixtures are prepared by sequential deposition or co-deposition of the two components fromthe gas phase onto an Al plate kept at 87 K inside a low-pressure chamber. Two CO
2 structures are found inmost experiments: a crystalline form similar to pure CO
2, which e
vaporates when warming at 105 K, and anoncrystalline species which remains embedded in amorphous water ice after warming. Significant spectral
variations are found depending on the deposition method and the thickness of the solid. Features characteristicof the RAIR technique appear in the spectral regions of the normal modes of the bending and asymmetricstretching CO
2 vibrations. Simulations using Fresnel theory and Mie scattering are carried out with acceptableagreement with the experimental spectra of solids of
variable thickness, from ~1
m to the limit ofnanoparticles. Theoretical calculations of a pure CO
2 crystal are performed. The relaxed geometry of thesolid and its
vibrational spectrum are determined and compared to the experimental results.