We show that, by controlling the alignment of molecules, we can influence the high harmonic generation process. We observed strong intensity modulation and spectral shaping of high harmonics produced with a rotational wave packet in a low-density gas of N₂ or O₂. In N₂, where the highest occupied molecular orbital (HOMO) has σg symmetry, the maximum signal occurs when the molecules are aligned along the laser polarization while the minimum occurs when it is perpendicular. In O₂, where the HOMO has πg symmetry, the harmonics are enhanced when the molecules are aligned around 45° to the laser polarization. The symmetry of the molecular orbital can be read by harmonics. Molecular wave packets offer a means of shaping attosecond pulses.
Physical Review Letters94, no. 12, 123902 (31 March 2005).