Electron Movies in Attoseconds

Forget the 3-D glasses. The latest thing in cinematography is super-high-speed frame-stopping electron movies. Scientists at Cornell University have developed a procedure for imaging electron motions on a timescale of a few billionths of a billionth of a second—an attosecond.

The new technique, which also supplies angstrom-scale spatial information, may enable researchers to monitor chemical reactions in unprecedented detail by providing a means for uncovering the most fundamental steps in a reaction—the movements of electrons.

In recent years, researchers working in the area of ultrafast spectroscopy have pushed experimental procedures to provide information on shorter and shorter timescales. Femtosecond laser methods, which have been widely used in “pump-probe” experiments, have given way to the finer time resolution of attosecond (10-18 second) techniques. Thus far, attosecond probes have depended on short laser pulses that lead to even shorter processes. For example, laser ionization of a gas has been used to induce electron-gas collisions that produce attosecond bursts of light.

Now, a group at Cornell has demonstrated that high-energy X-ray-scattering experiments coupled with a new procedure for data analysis can be used to access the attosecond time regime. The team, which includes Peter Abbamonte, Ken D. Finkelstein, Marcus D. Collins, and Sol M. Gruner, used the technique to produce movies of electron motions in water in which the frame-to-frame time interval is just 4 attoseconds [Phys. Rev. Lett., 92, 237401 (2004)].

In the study, water samples were irradiated with intense beams of X-rays from the Cornell High Energy Synchrotron Source and the Advanced Photon Source at Argonne National Laboratory. The inelastically scattered light—meaning the light that lost energy as a result of interactions with the sample—was measured and analyzed using a detector designed and built by the group.

Abbamonte, who is now a staff scientist at Brookhaven National Laboratory, explains that, from the measured X-ray-scattering angles, it should be possible to construct images of the water sample’s electron clouds. And from energy shifts in the scattered light, scientists should be able to deduce the way the pictures evolve over time.

But there’s a hitch: A key piece of information known as the phase, which is related to the X-rays’ electric field, is needed to construct the images but cannot be measured in the experiment. The “phase problem” in inelastic scattering is analogous to the one in X-ray crystallography. Various tricks for side-stepping the phase problem have been devised for crystallography applications but not for inelastic scattering. So the Cornell team developed a mathematical solution to the problem and applied it to the scattering data to determine the way electrons in water respond to various disturbances. Then they created movies showing the effect on the electrons of adding another electron and shooting the sample with a high-energy gold ion.

“It is an excellent piece of work,” says Paul B. Corkum, a research officer at the Canadian National Research Council, Ottawa, Ontario. Corkum, a pioneer in attosecond spectroscopy methods, comments that the novel point of the study comes from the authors realizing that all information needed to determine the position and dynamics of the electrons is written in the scatter angle and frequency shift of the X-ray photons. “I am sure that they are correct,” he says.