Lasers act on cue in electron billiards

A sharp burst of laser light striking an atom can yank away an electron, ionizing the atom. If the laser pulse is extremely intense, the rapid oscillations of its electric field pull off multiple electrons, one after another. In the mid-1980s, however, amazed researchers discovered that moderately intense laser beams dislodge multiple electrons at a rate up to a million times higher than expected and, seemingly, in groups of two or more. Since then, researchers have furiously debated explanations for those findings.

In rescattering, free electrons move in a laser’s electric field like balls on a tilted table. Funnel shapes portray confinement of electrons by a charged nucleus. Left to right: The maximum positive field first pulls a weakly bound electron away from the nucleus. Pushed forward and back by the oscillating field (yellow curve), this electron knocks away another. Moshammer/U. Freiburg

This week, two German research teams independently report experimental results that favor one of three models advanced in the debate. The need to understand multiple ionizations has lately grown urgent as scientists increasingly use intense lasers in fundamental physics experiments (SN: 12/19&26/98, p. 390: and to pursue a range of applications, such as nuclear fusion (SN: 3/27/99, p. 196: and particle acceleration (SN: 12/4/99, p. 367).

In what is called the rescattering model, which the new data support, laser beams play “a type of billiard game,” says Reinhard Dörner of the University of Frankfurt, leader of a team that studied laser ionization of helium atoms.

In that game, the laser beam’s potent and fluctuating electric field tugs an electron some 100 atomic diameters away from its parent atom and then shoots it back at the atom like a cue ball, bashing away one or more additional electrons.

“For the first time, we really know what mechanism leads to these doubly and triply ionized ions,” says Robert Moshammer of the University of Freiburg. He and his colleagues studied ionization in neon. Both groups describe their experiments in the Jan. 17 Physical Review Letters.

The new helium experiment “really starts shutting down the controversy,” comments Louis F. DiMauro of Brookhaven National Laboratory in Upton, N.Y. The neon findings are also “important,” he says, but of less immediate consequence to theorists. Analysts have done more ionization calculations for the helium atom, which has only two electrons, than for 10-electron neon.

Fathoming multiple ionizations may lead to new understanding of so-called many-body interactions, researchers say. Such interactions among at least three particles—an ion and two or more electrons, for instance, in the new studies—are common but enormously difficult to analyze mathematically (SN: 1/1/00, p. 4: Computers Crunch Quantum Collisions).

In the new studies, researchers made ions by using moderately intense laser beams, several hundred trillion to a quadrillion watts per square centimeter. They fired the lasers at jets of cold atoms for up to 220 femtoseconds (10-15 second).

After each pulse, a weak electric field accelerates ions toward a detector that recorded ion position and time of arrival. This enabled scientists to calculate momentum. In the two alternatives to the rescattering model, the laser beam imparts no momentum to the ions. Yet both research teams found a range of momenta attributable to the strong electric fields of the beams.

Moshammer says the findings decisively rule out both alternatives to rescattering—two or more electrons jumping ship simultaneously by the quantum-mechanical trick called tunneling or an atom spitting out additional electrons as an adjustment to the initial loss of an electron.

Dörner argues, however, that the data are not definitive, although rescattering is probably correct. Further measurements by both teams of electron, as well as ion, momenta may soon provide a more complete picture.

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