  Accurate molecular imaging via high-order harmonic generation relies on
comparing the harmonic emission from a molecule and an adequate reference
system. However, an ideal reference atom with the same ionization properties as
the molecule does not always exist. We show that for suitably designed, very
short laser pulses, a one-to-one mapping between high-order harmonic
frequencies and electron momenta in above-threshold ionization exists.
Comparing molecular and atomic momentum distributions then provides the
electron return amplitude in the molecule for every harmonic frequency. We show
that the method retrieves the molecular recombination transition moments highly
accurately, even with suboptimal reference atoms.
