To break free from an atom, the negatively charged electron typically has to absorb a high-energy photon, such as that from the ultraviolet (UV) or x-ray spectrum. The electron then gets excited enough to overcome the electrostatic attraction holding it to the positively charged nucleus and escapes, a process called ionization. A German-Dutch team has for the first time provided direct proof of an alternative mechanism. Powerful electric fields from a laser pulse can momentarily weaken the electrostatic bonds and enable the electron to quantum-mechanically tunnel away from the atom.
Leonid Keldysh, now at the Lebedev Physics Institute in Moscow, predicted the effect in 1964, and experiments have already proved that such unusual ionization can occur. But only with the advent of laser pulses lasting just a few hundred attoseconds can physicists observe the phenomenon. (One attosecond is a billionth of a billionth of a second.) Attosecond laser pulses have already made it possible to probe the motion of electrons in atoms and molecules, and improved versions will allow researchers to track electron movements that occur, for instance, during chemical reactions.