1. Department für Physik, Ludwig-Maximilians-Universität, Am Coulombwall 1
   
2. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany
   
3. Fakultät für Physik, Universität Bielefeld, Universitätsstrasse 25, D-33615 Bielefeld, Germany
   
4. Technische Universität Wien, Gusshausstrasse 27, A-1040 Vienna, Austria
   
5. FOM-Instituut voor Atoom- en Molecuulfysica (AMOLF), Kruislaan 407, 1098 SJ, Amsterdam, The Netherlands
   
6. Institute of Nuclear Physics, Moscow State University, Moscow 119992, Russia
   
7. Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, D-22671 Hamburg, Germany

Correspondence to: M. Uiberacker^1,2F. Krausz^1,2,4 Correspondence and requests for materials should be addressed to M.U. (Email: matthias.uiberacker@mpq.mpg.de) or F.K. (Email: ferenc.krausz@mpq.mpg.de).

Abstract

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.