  The first high harmonic spectrum, containing only the odd orders, was
observed in experiments 30 years ago (1987). However, a spectrum containing
pure even harmonics has never been observed. We investigate the generation of
pure even harmonics from oriented CO molecules in linearly polarized laser
fields employing the time-dependent density-functional theory. We find that the
even harmonics, with no odd orders, are generated with the polarization
perpendicular to the laser polarization, when the molecular axis of CO is
perpendicular to the laser polarization. Generation of pure even harmonics
reveals a type of dipole acceleration originating from the permanent dipole
moment. This phenomenon exists in all system with permanent dipole moments,
including bulk crystal and polyatomic molecules.
