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Resolving the time when an electron exits a tunnelling barrier


The tunnelling of a particle through a barrier is one of the most fundamental and ubiquitous quantum processes. When induced by an intense laser field, electron tunnelling from atoms and molecules initiates a broad range of phenomena such as the generation of attosecond pulses1, laser-induced electron diffraction2,3 and holography2,4. These processes evolve on the attosecond timescale (1 attosecond ≡ 1 as = 10−18 seconds) and are well suited to the investigation of a general issue much debated since the early days of quantum mechanics5,6,7—the link between the tunnelling of an electron through a barrier and its dynamics outside the barrier. Previous experiments have measured tunnelling rates with attosecond time resolution8 and tunnelling delay times9. Here we study laser-induced tunnelling by using a weak probe field to steer the tunnelled electron in the lateral direction and then monitor the effect on the attosecond light bursts emitted when the liberated electron re-encounters the parent ion10. We show that this approach allows us to measure the time at which the electron exits from the tunnelling barrier. We demonstrate the high sensitivity of the measurement by detecting subtle delays in ionization times from two orbitals of a carbon dioxide molecule. Measurement of the tunnelling process is essential for all attosecond experiments where strong-field ionization initiates ultrafast dynamics10. Our approach provides a general tool for time-resolving multi-electron rearrangements in atoms and molecules11,12,13—one of the key challenges in ultrafast science.

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Figure 1: Electron trajectories contributing to the recollision process.
Figure 2: Schematic description of the two-colour gates.
Figure 3: Reconstruction of the ionization and recollision times.
Figure 4: Gating two-channel tunnel ionization in aligned carbon dioxide molecules.


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N.D. acknowledges the Minerva Foundation, the Israeli Science Foundation and the Crown Center of Photonics for financial support. N.D. and O.S. acknowledge the German-Israeli Foundation, and D.S. acknowledges support from the Converging Technologies Fellowship of the Israeli Ministry of Science. H.S. is supported by the Adams Fellowship Program of the Israel Academy of Sciences and Humanities. O.S. acknowledges the support of the DFG grant Sm292/2-1, and M.Yu.I. acknowledges the support of the EPSRC programme grant EP/I032517/1. Y.M. acknowledges financial support from the ANR (ANR-08-JCJC-0029 HarModyn) and the Conseil Regional d’Aquitaine (20091304003 AttoMol). M.I. acknowledges support of the EPSRC Programme Grant EP/I0325171/1.

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D.S., H.S., B.D.B., M.D. and N.D. designed, performed and analysed the experiment. Y.M. contributed to the analysis. M.Yu.I. and O.S. developed the theory and performed the calculations of harmonic spectra. S.P. provided the quantum chemistry input for the calculations. All authors contributed to writing the manuscript.

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Correspondence to Nirit Dudovich.

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The authors declare no competing financial interests.

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Shafir, D., Soifer, H., Bruner, B. et al. Resolving the time when an electron exits a tunnelling barrier. Nature 485, 343–346 (2012).

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