H. Merdji, W. Boutu, R. Fitour, P. Monchicourt, P. Bréger, B. Carré and P. Salières

Service des Photons, Atomes et Molecules, CEA-Saclay, 91191 Gif-sur-Yvette, France

Recently, a number of papers have demonstrated the interest of high-order harmonic generation (HHG) from molecules aligned with respect to the laser polarization. Itatani et al. (Nature 432, 867 (2004)) have shown that a precise characterization of the harmonic emission allows performing a tomographic reconstruction of the molecular orbitals that radiate. Kanai et al. (Nature 435, 470 (2005)) have evidenced quantum interferences in the recombination process of HHG that are directly related to the molecular structure. In all of these papers, only the HHG intensity was measured. The relative harmonic phase, though more difficult to measure, contains important information on the interference process, and is needed for an ab initio tomographic reconstruction. Finally, while the attosecond emission from atoms has been thoroughly studied, in particular by our group (Mairesse et al., Science 302, 1540 (2003)), it has not been investigated in molecules.

In a recent experiment, we have measured for the first time the harmonic amplitude and relative phase for aligned molecules. In order to align the molecules, we used the so-called nonadiabatic 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). The measurement of phase locking between neighboring harmonics was performed through the photoionization of a target gas by the harmonic beam in presence of a sufficiently intense "dressing" laser beam (RABITT technique).

The harmonic emission times measured when the CO2 molecules are aligned parallel to the generating laser polarization (at the revival of the rotational wavepacket, 22 ps after the aligning laser pulse) are significantly different from the Krypton case (which has a similar ionization potential). Up to sideband 22, their behavior is extremely similar, with a constant time shift between consecutive harmonic (~100 as). From sideband 22 to 24 this time shift suddenly increases to 350 as around harmonic 23 in the CO2 case, while it remains the same for the krypton. Finally a saturation for the last sideband is observed. The observed time shift around harmonic 23 corresponds to a shift of the harmonic phase of roughly 2 radians. This is close to the phase jump of pi that is predicted when destructive interference occurs in the recombination process. Our measurement would thus be consistent with that of Kanai et al. (Nature 435, 470 (2005)), where the quantum interference in the harmonic amplitude was observed around order 23.

The temporal profiles of the generated attosecond pulses has been reconstructed for both gases. They reveal the complex electron dynamics involved in the recombination process. While the trains reconstructed from harmonics 15 to 21 (i.e. before the phase jump) are quite similar, their comportment differs when one selects harmonics 23 to 29 due to the phase shift.

Corresponding author: Hamed Merdji, merdji@drecam.cea.fr