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.
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