Researchers have again advanced our understanding of laser ionization with the observation of correlated momenta in electron pairs that are liberated from their atoms. The finding further supports the rescattering theory, which describes the effect in terms of oscillations in the electric field of a laser pulse that push and pull at an electron that rebounds toward the atom and frees others. Physicists wrangled for nearly 20 years over an explanation for multiple ionization, until the work of Reinhard Dörner and others put the controversy to rest earlier this year. Researchers monitoring electrons stripped from their atoms by intense laser pulses have strengthened the case for rescattering. The electric field of the pulse is believed to pull an electron from its orbit. The electron recoils, striking another, and the two leave with correlated momenta. Electron momentum The current team, led by Dörner and Harald Giessen from the University of Freiburg and Philipps University in Marburg, Germany, respectively, reported in the June 8 issue of Nature the results of a double-ionization experiment. The physicists illuminated argon with 38-TW/cm2 laser pulses and found that the electrons traveled in roughly the same direction and at the same speed, instead of flying apart by Coulomb forces. The scientists used a compact Ti:sapphire laser from Spectra-Physics Lasers Inc. of Mountain View, Calif. The laser produced linearly polarized, 800-nm light in 220-fs pulses at a 1-kHz repetition rate. A lens focused the light on the target, which was a jet of argon gas cooled to 30 K. Cooling ensured that the atoms initially had low momentum. The researchers monitored the electrons' momenta as they left the atoms. The electrons followed paths that were determined by an applied electrical field to microchannel plate detectors that re-corded their positions and the time of impact. This enabled deductions about the momenta by the field geometry and particle mass. The electron pairs behave like dance partners who know each other's practiced routines, rather than as strangers who happen to be traveling in the same direction. The rescattering theory explains the phenomenon in terms of a billiardlike series of collisions, said Dörner. The researchers next plan to look at ionization with increased laser powers: "Petawatts will allow stripping off 10 electrons together and give large, 'naked' atoms -- interesting with respect to nuclear fusion," Dörner said.