Observing electron tunneling in real time

To escape an atom, an electron usually has to gain enough energy to climb out of the atom’s Coulomb well. However, if the atom is bathed in an intense laser pulse, the pulse’s strong electric field can lower the confining potential enough to allow the electron to tunnel to freedom. That hypothesis, proposed in 1965 by Leonid Keldysh, has now been verified, thanks to physicists’ recent ability to control light on the time scale of electronic motion in atoms: attoseconds. Matthias Uiberacker of the Ludwig Maximilians University of Munich, Germany, and his collaborators blasted a gas of neon atoms with an attosecond UV pulse followed by an intense femtosecond red pulse. The attosecond pulse ripped off an inner electron and excited a second electron to the atom’s periphery. Then the red pulse, consisting of just a few wave cycles with precisely controlled phase, could free the outlying electron by lowering...