Top: spectra collected at different CO ice deposition temperatures with a spectral resolution of 1 cm^-1 and a 45° incidence angle. Bottom: central peak positions of the CO bands corresponding to warm-up of ice deposited at 14 K, spectral resolution of 1 cm^-1 and 45° incidence angle (crosses, red trace), the deposition experiments performed at different temperatures with a spectral resolution of 1 cm^-1 and 45° incidence angle (asterisks, black trace), and deposition experiments at different temperatures with high spectral resolution of 0.1 cm^-1 and 15° incidence angle (diamonds, blue trace). The observed frequency shifts were reproducible within small errors of 0.04 cm^-1, corresponding to the wavenumber accuracy, in the low-resolution experiments. Frequency shifts in the high-resolution experiments reported in Lasne et al. (2015) are ±0.01 and ±0.02 cm^-1 for the LO and TO modes, respectively.