  Diffusion of species in icy dust grain mantles is a fundamental process that
shapes the chemistry of interstellar regions; yet measurements of diffusion in
interstellar ice analogs are scarce. Here we present measurements of CO
diffusion into CO$_2$ ice at low temperatures (T=11--23~K) using CO$_2$
longitudinal optical (LO) phonon modes to monitor the level of mixing of
initially layered ices. We model the diffusion kinetics using Fick's second law
and find the temperature dependent diffusion coefficients are well fit by an
Arrhenius equation giving a diffusion barrier of 300 $\pm$ 40 K. The low
barrier along with the diffusion kinetics through isotopically labeled layers
suggest that CO diffuses through CO$_2$ along pore surfaces rather than through
bulk diffusion. In complementary experiments, we measure the desorption energy
of CO from CO$_2$ ices deposited at 11-50 K by temperature-programmed
desorption (TPD) and find that the desorption barrier ranges from 1240 $\pm$ 90
K to 1410 $\pm$ 70 K depending on the CO$_2$ deposition temperature and
resultant ice porosity. The measured CO-CO$_2$ desorption barriers demonstrate
that CO binds equally well to CO$_2$ and H$_2$O ices when both are compact. The
CO-CO$_2$ diffusion-desorption barrier ratio ranges from 0.21-0.24 dependent on
the binding environment during diffusion. The diffusion-desorption ratio is
consistent with the above hypothesis that the observed diffusion is a surface
process and adds to previous experimental evidence on diffusion in water ice
that suggests surface diffusion is important to the mobility of molecules
within interstellar ices.
