Molecular orbital imaging using attosecond pulses generated in N2

Complete text of publication follows. The strong interaction of a molecule with a laser field frees by tunnel ionization an attosecond electron wave packet that probes its bound state half a laser cycle later as it re-collides with the core. Rich information on ths (possibly transient) electronic and nuclear configuration is encoded in the attosecond XUV burst emitted during recombination, a process called high-order harmonic generation (HHG). Complete characterization (intensity, phase and polarization) of this observable gives access to the transition dipole moment over a large momentum span. This transition dipole may allow direct imaging of the radiating molecular orbital using a tomographic procedure. For the first time we succeeded to characterize the intensity, phase and polarization of the XUV emission in aligned N2 molecules. Our measurements evidence multi-orbital contributions to the attosecond emission and also reveal the ellipticity of the harmonics. Recent experimental and theoretical studies have revealed that molecules could be tunnel ionized from several orbitals simultaneously. These different orbitals lead to interfering contributions in the attosecond emission. We were able to separate these contributions and by using the tomographic molecular orbital reconstruction technique, HOMO and HOMO-1 orbitals were reconstructed in N2. These reconstructions show remarkable agreement with theoretical simulations and also provide us with the sign changes in the orbital wave functions. An investigation was addressed to the validity of the plane wave approximation in our calculation. The coherent superposition of the HOMO and HOMO-1 orbitals provides time-resolved experimental images of the wave packet ('hole') left empty after coherent tunnel ionization from both orbitals. The recombining electron wave packet probes the 'hole' at the instant of recombination providing information about the electronic structure of the molecule at that moment. This imaging of a dynamical wavepacket serves as a test of feasibility for intra-molecular imaging with Angstroem spatial resolution and attosecond temporal resolution.