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Smile, You’re on Attosecond-Pulse Camera!

Photonics Spectra
Mar 2008
If you want to know the properties of a substance, you have to know about its molecules. If you want to know about the properties of those molecules, you have to know something about the atoms that comprise them. It follows, then, that you must find out as much as you can about electrons to understand atoms. But whereas imaging particles has become increasingly sophisticated over the years, watching individual electrons has been impossible because they move so fast — typically orbiting an atom’s nucleus once every 150 attoseconds.


Using pulses of extreme-UV energy, researchers ionized argon atoms in sync with the cycles of an IR laser field. The resulting strobe effect enabled visualization of electrons by recording changes in momentum distribution. When the electrons are ejected at the maxima of the IR beam’s electric field, the distribution is symmetrical compared with the plane perpendicular to the beam’s polarization (a). When the electrons are ejected at the zero points, the distribution is shifted along the direction of the polarization (b). Shown in (c) are results obtained at four set delays between the beams; from left to right, delays of 0, π/2ω, π/ω and 3π/2ω. Reprinted with permission of Physical Review Letters.

But wait just an attosecond. Researchers at Lund University in Sweden and at Louisiana State University in Baton Rouge now have built a “quantum stroboscope” and used it to watch electrons leave orbit during ionization and to view some of them bouncing off the parent nucleus during their return trip. They reported their findings in the Feb. 22 issue of Physical Review Letters.

To accomplish the feat, the researchers used two beams created with a lab-built Ti:sapphire laser operating at 800 nm and at a repetition rate of 1 kHz. One beam, consisting of 300-attosecond pulses of extreme-ultraviolet energy, passed through a cloud of argon gas. The pulses ionized the argon, creating entities called electron wave packets, which are temporally localized, thus skirting the issue of quantum uncertainty.

The second beam established a background field in which the first beam interacted with the argon. The scientists used the second harmonic of the IR beam to ensure that the UV pulses were spaced exactly one full light cycle apart, permitting full synchronization of the probe and the event — such as with macroscale photography using a strobe light.

They also used the technique on helium to image some of the scattered ions as they were redirected by the infrared laser back into the original atoms.

According to the researchers, controlled scattering and attosecond-scale imaging will enable time-resolved measurements with very high spatial resolution.

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