  We present a new theory that the molecular high order harmonic generation in
an intense laser field is determined by molecular internal symmetry and
momentum distribution of the tunneling-ionized electron. The molecular internal
symmetry determines the quantum interference form of the returning electron
inside the molecule. The electron momentum distribution determines the relative
interference strength of each individual electron de Broglie wave. All
individual electron de Broglie wave interferences add together to collectively
modulate the molecular high harmonic generation. We specifically discuss the
suppression of the generation on adjacent harmonic orders and the dependence of
molecular high harmonic generation on laser intensities and molecular axis
alignment. Our theoretical results are in good consistency with the
experimental observations.
