  We present a theoretical investigation into the two-center interference in
aligned H$_2^+$. The influence of the laser field on the recombination step is
investigated by comparing laser-induced harmonic generation with harmonic
generation from field-free collisions of Gaussian wave packets with the core.
We find that for different Gaussian wave packets colliding with the molecule,
the interference minimum occurs at the same alignment angle. The same result is
obtained for the laser-induced spectrum when only a single electronic
trajectory per harmonic contributes. When multiple electronic trajectories
contribute, we find an effect on the minimum position because the interference
between short and long trajectories is alignment-dependent. The two-center
interference and the influence of the Coulombic potential are clearly seen not
only in the harmonic intensity and phase but also in the polarization direction
and ellipticity. We observe significant ellipticity of the emitted radiation
around the two-center interference minimum.
