  We develop the strong-field approximation for high-order harmonic generation
in hydrogen molecules, including the vibrational motion and the laser-induced
coupling of the lowest two Born-Oppenheimer states in the molecular ion that is
created by the initial ionization of the molecule. We show that the field
dressing becomes important at long laser wavelengths ($\approx 2 \mu$m),
leading to an overall reduction of harmonic generation and modifying the ratio
of harmonic signals from different isotopes.
