| We have studied the high-order harmonic emission from small linear molecules (N2, CO2) aligned with respect to the laser polarization. We used the nonadiabatic alignment technique: a rotational wavepacket is created by a strong enough and short aligning pulse, so that a field-free alignment is obtained at the revival (a few ps after the aligning pulse). We measured the amplitude and phase of harmonics generated at the revival, through the photoionization of a target gas by the harmonic beam in presence of a sufficiently intense "dressing" laser beam (RABITT technique). This very demanding experiment thus needs three laser beams (aligning, generating and dressing beams) whose intensities, delays and polarization should be controlled independently. For the first time, we have measured a phase jump in the harmonic relative phases, located at the position in the harmonic spectrum where the amplitude goes through a minimum. This demonstrates that the latter is indeed associated to a destructive interference in the recombination process. This phase jump contains important information on the interference process and is directly linked to the molecular structure; it is thus a key element for an accurate tomographic reconstruction of the molecular orbital that radiates. From the measured harmonic amplitudes and phases, we reconstructed the temporal profile of the attosecond emission. The phase jump induces a strong distortion of the emitted attosecond pulses. However, in CO2, when the molecules are aligned at 90 degrees of the generating laser polarization, the phase jump disappears resulting in attosecond pulses very similar to that of the atomic ¡partner¢ krypton (same ionization potential). A control of the attosecond emission can thus be performed by varying the orientation of the molecule with respect to the laser polarization. |
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