Abstract
Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms1. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected2. Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly polarized radiation3. In contrast, it has recently been suggested that intense field ionization may be accompanied by a two-stage 'shake-up' reaction4. Here we report a unique combination of experimental techniques5,6,7,8 that allows us to accurately measure the tunnel ionization probability for argon exposed to 50-fs laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry7,8, equivalent to a homogenous electric field. Furthermore, circularly polarized radiation negates recollision. The present measurements indicate that tunnel ionization results in simultaneous excitation of one or more remaining electrons through shake-up9. From an atomic physics standpoint, it may be possible to induce ionization from specific states, and will influence the development of coherent attosecond extreme-ultraviolet-radiation sources10. Such pulses have vital scientific and economic potential in areas such as high-resolution imaging of in vivo cells and nanoscale extreme-ultraviolet lithography.
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Acknowledgements
This work is funded by the Engineering and Physical Sciences Research Council (EPSRC), UK. Research studentships are acknowledged by J.W., E.M.L.E. and S.L.S. (EPSRC), J.M. (DEL) and M.S. (IRCEP at QUB). The authors gratefully acknowledge A. S. Kornev and B. A. Zon from Voronezh State University, Russia for fruitful discussions, suggestions and electronically communicating their theoretical data.
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Bryan, W., Stebbings, S., McKenna, J. et al. Atomic excitation during recollision-free ultrafast multi-electron tunnel ionization. Nature Phys 2, 379–383 (2006). https://doi.org/10.1038/nphys310
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DOI: https://doi.org/10.1038/nphys310
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