Hydrogen reunion mediated by light.

Two groups of researchers have independently succeeded in using laster light to coax electrons and protons in overlapping beams to reunite and form hydrogen atoms. The achievement represents an important step toward creating the antimatter counterpart of hydrogen by merging beams of antiprotons and antielectrons, or positrons.

These experiments give researchers a precise idea of the crucial role that laser light may eventually play in the production of antihydrogen, says physicist Farook B. Yousif of the University of Western Ontario in London.

Yousif's team and a group of physicists at the University of Heidelberg in Germany describe their observations of laser-induced recombination in separate papers in the July 1 PHYSICAL REVIEW LETTERS.

A close encounter between an electron and a free proton can sometimes produce a state in which the electron briefly moves in some kind of high-energy orbit around the proton. A hydrogen atom results if, during that short time, the electron can slip to a lower orbit and the system can emit a photon to carry away the excess energy. The electron then has insufficient energy to escape from the proton.

However, because the proton-electron interaction time is generally much shorter than the time typically required for the emission of radiation, collisions between electrons and free protons rarely result in the formation of hydrogen atoms. Under normal circumstances, the electrons nearly always escape and continue on their way.

More than a decade ago, theorists predicted that the probability of electron capture would increase significantly if such collisions took place in an intense laser beam of an appropriate wavelength. The new experimental results confirm this prediction.

"We found out that the [rate of hydrogen production] was several thousand times bigger when a laser was used than when the experiment was done without a laser," Yousif says.

To get this result, he and his co-workers directed light from a carbon dioxide laser, operating at a wavelength of 10.6 microns, across a merged beam of electrons and protons. The Heidelberg team performed the same feat using merged beams of protons, electrons and laser photons in an apparatus known as an ion storage ring. Both groups found that they could adjust experimental conditions, such as the initial speed of the electrons, to obtain recombined hydrogen atoms in well-defined final states, in which electrons occupy specific orbits.