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Tomographic imaging of molecular orbitals

Abstract

Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunction of molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To observe these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficult to observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate that the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmonics generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molecular alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also allows us to follow the attosecond dynamics of an electron wave packet.

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Figure 1: Illustration of the tunnel ionization process from an aligned molecule.
Figure 2: Illustration of a dipole induced by the superposition of a ground-state wavefunction ψg and a re-colliding plane wave packet ψc.
Figure 3: High harmonic spectra were recorded for N2 molecules aligned at 19 different angles between 0 and 90° relative to the polarization axis of the laser.
Figure 4: Molecular orbital wavefunction of N2.
Figure 5: A one-dimensional Schrödinger calculation shows that attosecond electronic wave-packet motion is resolved in the high harmonic spectra.

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Acknowledgements

In addition to the NRC, we acknowledge financial support from the National Science and Engineering Research Council, Photonic Research Ontario, the Canadian Institute for Photonic Innovation, the Alexander von Humboldt-Stiftung and the Japan Society for the Promotion of Science. We thank M. Yu. Ivanov, M. Spanner, J. P. Marangos, M. Lein, P. H. Bucksbaum, I. A. Walmsley, D. Jonas, J. Tse and J. G. Underwood for discussions.

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Correspondence to D. M. Villeneuve.

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Itatani, J., Levesque, J., Zeidler, D. et al. Tomographic imaging of molecular orbitals. Nature 432, 867–871 (2004). https://doi.org/10.1038/nature03183

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