  We study the ellipticity of high-order harmonics emitted from CO$_2$ molecule
driven by linearly polarized laser fields using numerical simulations within
the time-dependent density functional theory. We find that the overall
ellipticity of the harmonics is small, which is in agreement with experimental
data. On the other hand, our analysis of the numerical results indicates that
several valence orbitals contribute significantly to the harmonic emission and
some of these contributions show a strong ellipticity of the harmonics. The
small ellipticity in the total harmonics signal arises from a combination of
factors, namely, the fact that harmonic emission from different orbitals is
strongest at different alignment angles of the molecular axis with respect to
the laser polarization direction, as well as interference effects and a strong
laser coupling between several inner valence orbitals.

Atomic and Molecular Clusters