When a strong laser pulse induces the ionization of an atom, momentum conservation dictates that the absorbed photons transfer their momentum to the electron and its parent ion. The sharing of the photon momentum between the two particles and its underlying mechanism in strong-field ionization, occurring when the bound electron tunnels through the barrier created by the superposition of the atomic potential and the electric laser field, are still debated in theory1,2,3,4 after 30 years of research. Corresponding experiments are very challenging due to the extremely small photon momentum and their precision has been too limited, so far, to ultimately resolve this debate5,6,7,8. By utilizing an experimental approach relying on two counter-propagating laser pulses, we present a detailed study of the effects of the photon momentum in strong-field ionization. The high precision of the method and the intrinsically known zero momentum allow us to unambiguously demonstrate the action of the light’s magnetic field on the electron while it is under the tunnel barrier, which has only been theoretically predicted so far1,2,3,9, thereby disproving opposing predictions5,10,11. Our results deepen the understanding of, for example, molecular imaging12,13 and time-resolved photoelectron holography14.
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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.
The code that supports the theoretical plots within this paper and other findings of this study is available from the corresponding authors upon reasonable request.
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A.H., K.F. and K.H. acknowledge support by the German National Merit Foundation. We acknowledge support from Deutsche Forschungsgemeinschaft via Sonderforschungsbereich 1319 (ELCH) and by the DFG Priority Programme ‘Quantum Dynamics in Tailored Intense Fields’.
The authors declare no competing interests.
Peer review information Nature Physics thanks Alexandra Landsman, Catherina Vozzi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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