  We report the first experimental observation of frequency shift in high order
harmonic generation (HHG) from isotopic molecules H2 and D2 . It is found that
harmonics generated from the isotopic molecules exhibit obvious spectral red
shift with respect to those from Ar atom. The red shift is further demonstrated
to arise from the laser-driven nuclear motion in isotopic molecules. By
utilizing the red shift observed in experiment, we successfully retrieve the
nuclear vibrations in H2 and D2, which agree well with the theoretical
calculations from the time-dependent Schrodinger equation (TDSE) with
Non-Born-Oppenheimer approximation. Moreover, we demonstrate that the frequency
shift can be manipulated by changing the laser chirp.
