EXPERIMENTAL SETUP.During photoionization, helium atoms pass through the focus of an ultra-intense laser beam (purple). The inset shows the actual apparatus, in which the laser beam appears green. The pulse duration is around 220 femtoseconds, and the wavelength is 800 nanometers.


ELECTRON EMISSION. In a helium atom, the electrons (green) exit together once stimulated by a fast pulse of laser light (top). In fact, the researchers propose that one electron is initially liberated and then accelerated back toward the helium atom by the light field. This rescattering event ensures that both electrons are emitted in a correlated fashion (bottom).

It seems that electrons prefer to work in pairs--particularly when they are breaking loose from the confines of rare gas nuclei. Indeed, two multi-institutional German research teams have recently shown that when helium and neon atoms are shot through a powerful laser beam, their electrons do not pop off one by one--as might be expected. Instead they exit at exactly the same moment.

Since the beginning of the 1990s, physicists have studied these so-called double ionizations, hoping to discern the mechanism behind them. They assumed, based on the ionization yields from different atoms, that electrons excited by intense light did not leave atoms sequentially. But at the same time, they couldn't be certain that the electrons actually left in tandem.

To find out, they needed sophisticated equipment: a state-of-the-art Ti-sapphire amplifier laser beam, which pelted their gas atoms with 220-femtosecond pulses of light having a wavelength of 800 nanometers, reached intensities of more than 100 terawatts per centimeter squared. And a four-year-old "momentum microscope," called COLTRIMS, recorded the resulting three-dimensional momentum values for singly and multiply ionized gases.

Image: ROBERT MOSHAMMER, University of Freiburg

RESCATTERING. In this scenario, the laser's electric field strips one electron from the atom at a maximum value (left), hits zero (second from left) and then reaches a reverse maximum (second from right), shooting the electron back at the ion and dislodging another one (right).

One group, led by Reinhard Doerner of the University of Frankfurt and Harald Giessen of the University of Marburg, applied this approach to helium. The other, headed by Robert Moshammer of the University of Freiburg and colleagues at the Max Born Institute in Berlin, focused their work on neon.

Both published their findings in the January 17th issue of the journal Physical Review Letters. And both saw the same results. Had the electrons acted independently, the momentum data would have looked like a single ionization happening twice. Instead the teams witnessed ions with zero momentum--indicating some sort of correlated electron emission.

As for how and why this happens, the groups are still at work. The Frankfurt-Marburg team proposes, among other theories, that perhaps a quantum-mechanical effect is responsible. In this case, the fast change of the combined nuclear Coulomb potential and light potential might jettison the second electron.

The Freiburg scientists favor a "rescattering" model: They suggest that when the laser pulse first hits the atom, the electrons experience the electric field at a maximum, which removes one of the electrons. As the pulse passes through the atom, though, the electric field briefly reaches zero and then hits a maximum in the reverse direction--which shoots the ejected electron back at the ion, where it knocks out another one. Either way, their discoveries mark a leap forward in understanding strongly reacting electrons in mighty light fields.