Electron Tunneling in Atoms Has Now Been Observed in Real Time

Electron tunneling in atoms has now been observed in real time by a German-Austrian-Dutch team (Ferenc Krausz, Max Planck Institute of Quantum Optics and Ludwig Maximilians University Munich, ferenc.krausz@mpq.mpg.de) using light pulses lasting only several hundred attoseconds (billionths of a billionth of a second), providing new glimpses into an important ultrafast process in nature.

An electron bound to an atom is at the bottom of a sort of energy hill. Escaping the atom usually requires the electron to get enough energy to roll over this hill. So for example, hitting an atom with a light pulse delivering photons of sufficient energy can allow the electron to escape.

However, if an atom is bathed in a shower of lower-energy photons, there is the chance that an electron, if located at the periphery of the atom, can escape even though it doesn't have quite enough energy. This is through the phenomenon of quantum tunneling, in which there is a small chance that the electron can in effect burrow through the energy hill.

The tunneling process is responsible for the operation of certain electronic components, such as scanning tunneling microscopes, Esaki (tunneling) diodes, and quantum-cascade lasers. And in nuclear fission, alpha particles (two protons plus two neutrons) are believed to escape the fracturing nucleus through tunneling. Yet the tunneling process occurs so quickly, on the scale of attoseconds, that it has not been possible to observe directly. With the recent ability to create attosecond-scale light pulses--pioneered by Krausz and others--this is now possible.

In the new experiment, a gas of neon atoms is exposed to two light pulses. One is an intense pulse containing low-energy red photons. The second pulse is an attosecond-length pulse of ultraviolet light. This ultraviolet attosecond pulse delivers photons so energetic that they can rip off an electron and promote a second one to the periphery of the atom, into an excited quantum state.

Then, the intense red pulse, consisting of just a few wave cycles (peaks and valleys), has a chance to liberate the outlying electron via light-field-induced tunneling. Indeed, the researchers saw this phenomenon, predicted theoretically forty years ago but only verified now for the first time experimentally in a direct time-resolved study. As each wave crest in the few-cycle red pulse coursed through the atoms, the electrons each time upped their probability of escaping through tunneling until it reached about 100%.

The data indicate that, in this particular system, the electrons escape via tunneling in three discrete steps, synchronized with the three most intense wave crests at the center of the few-cycle laser wave. Each step lasts less than 400 attoseconds. (Uiberacker et al, Nature, 5 April 2007; also see press release with figures and more information at www.mpq.mpg.de)