Precursors to Atomic Stabilization

Although atomic stabilization came as a surprise, it was presaged by two discoveries from 1980s laser experiments. The first is called above-threshold ionization (ATI). Einstein's theory, notes Eberly, is essentially an energy-balance theory: It says that an electron will fly off from an atom as soon as a photon bangs into it with enough energy -- the ionization threshold energy. Experiments showed, however, that an electron could absorb many more photons than required to break free and then fly off with "excess" energy. "This contradicted everybody's preconceptions," says Eberly, "and the key advance that made it possible was the development of lasers with intense pulses short enough to deposit photon energy on an electron very rapidly."

The second discovery is that some of the photons coming out of a laser-zapped atom have much higher energy, therefore shorter wavelength, than they had going in. This happens, says Eberly, because some of the ATI electrons that soak up photons, rather than breaking free, are recaptured by the atom. When this happens, the electron gives up its energy all at once, releasing a photon that may have 100 times the energy of the incoming photons. The short wavelengths produced by this energy multiplier effect have valuable practical applications in studying the dynamics of molecules, surfaces and materials. "It offers a source of coherent short-wavelength light," says Eberly, "that bridges the gap between visible light and x-rays."

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