Scientists Resolve Ionization Debate

Daniel S. Burgess

Since 1983, physicists have argued over how intense laser beams can multiply ionized atoms. Three paradigms competed to fill the theoretical void. Now the work of two independent teams in Germany seems to have put the issue to rest and advanced the understanding of the subatomic world.

Both teams, which described their results in the Jan. 17 issue of Physical Review Letters, measured the momentum distributions of laser-ionized atoms with cold target recoil-ion momentum spectroscopy. One focused 220-fs pulses of 800-nm light from a Ti:sapphire laser on a helium target. The other used 30-fs pulses from a 795-nm, mode-locked Ti:sapphire laser and neon.

In each case, the intense pulses -- in the neon study, about 2 PW/cm² -- stripped the atoms of electrons. A homogeneous electric field guided the ions to a detector, and the researchers monitored their time of flight. Together, the measurements indicated that the laser fields had imparted momentum to the ions, which ruled out two of the competing theories.

The remaining theory, known as rescattering, explains that the oscillating electric field of a laser pulse alternately pushes and pulls at electrons in an atom.

"The laser field first liberates one electron," said Reinhard Dörner of the University of Frankfurt, leader of the team that studied doubly ionized helium. "This electron is accelerated by the laser field and then driven back toward its own parent ion. Here it kicks out a second electron in a billiardlike collision."

The experiments demonstrate the importance of the correlated motion of electrons, noted Robert Moshammer of the University of Freiburg, who with his partners investigated Ne^1,2,3+. "The understanding of the correlated motion of many-particle systems under the acting of any time-dependent force is one of the most fundamental (and still open) questions in physics," he said. "In atomic physics ... [they] are still not yet understood sufficiently."

While Dörner suggested that the teams' work might find applications in generating higher harmonics in gases, he said its primary importance is in realizing the promise of quantum mechanics to describe the forces that govern the "structure and evolution of our everyday world."