High-order harmonic generation from H2+ with different vibrational states has been theoretically investigated. The results show that the red-shifts of the harmonics can be found in the lower vibrational states. As the vibrational state increases, the red-shifts of the harmonics are decreased even disappeared. Through studying the time-frequency analyses of the harmonics and the contributions of the harmonics from the two-H nuclei, the reason behind the frequency modulation of harmonics can be given.
