  We present Temperature Programmed Desorption (TPD) experiments of CO and N2
ices in pure, layered and mixed morphologies at various ice "thicknesses" and
abundance ratios as well as simultaneously taken Reflection Absorption Infrared
Spectra (RAIRS) of CO. A kinetic model has been developed to constrain the
binding energies of CO and N2 in both pure and mixed environments and to derive
the kinetics for desorption, mixing and segregation. For mixed ices N2
desorption occurs in a single step whereas for layered ices it proceeds in two
steps, one corresponding to N2 desorption from a pure N2 ice environment and
one corresponding to desorption from a mixed ice environment. The latter is
dominant for astrophysically relevant ice "thicknesses". The ratio of the
binding energies, Rbe, for pure N2 and CO is found to be 0.936 +/- 0.03, and to
be close to 1 for mixed ice fractions. The model is applied to astrophysically
relevant conditions for cold pre-stellar cores and for protostars which start
to heat their surroundings. The importance of treating CO desorption with
zeroth rather than first order kinetics is shown. The experiments also provide
lower limits of 0.87 +/- 0.05 for the sticking probabilities of CO-CO, N2-CO
and N2-N2 ices at 14 K. The combined results from the desorption experiments,
the kinetic model, and the sticking probability data lead to the conclusion
that these solid-state processes of CO and N2 are very similar under
astrophysically relevant conditions. This conclusion affects the explanations
for the observed anti-correlations of gaseous CO and N2H+ in pre-stellar and
protostellar cores.
