Dust grains in molecular clouds are covered by ice mantles composed
of complex ``cocktail mixtures'' of molecules frozen onto the grains.
Solid CO in the ice mantles is identified by an absorption feature
at 4.67mu m, corresponding to the stretch of the CO molecule. The
observed features generally show two components: a narrow dominant one
corresponding to CO in a nonpolar or weakly polar mixture,
and a broader one appearing as a long-wavelength wing corresponding to
CO in a polar mixture. The comparison of the observed CO--ice bands
with laboratory astrophysical ice analogs might be used as a powerful
probe of the astrophysical conditions in the coolest regions of the
cloud.
In this work, spectra of the solid CO feature are presented for embedded Young Stellar Objects towards nearby molecular clouds. The likely composition of the CO ices is analysed by fitting the observations with laboratory data. The statistical significance of the results is highlighted in the form of a reduced--chi2 ``map'' for one of the objects, and in the histograms of the comparative performance of the laboratory nonpolar mixtures. The results of the fits very strongly point to ice mantles that suffered energetic processing by ion bombardment. The origin of the ion irradiation may be in flares in the embedded objects, or in the form of low energy cosmic rays in the interstellar medium, or by other processes, such as X-ray or UV processing, that modify the mantle structure.