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Billionth Of A Billionth Second Pulse Now Detected

Attophysics has become the short-timescale frontier of physics, with a report from an international collaboration that the briefest lengths yet have now been detected.

An Austria-Canada-Germany collaboration reports that it has produced and detected, for the first time, isolated x-ray pulses lasting on the scale of attoseconds, where one attosecond is a billionth of a billionth of a second (10^-18 s).

The reported pulses, lasting approximately 650 attoseconds (as) and residing in the soft x-ray part of the electromagnetic spectrum, subsequently provided attosecond-scale measurements of a physical phenomenon (specifically, the detachment of an electron from an atom by an x-ray photon).

With these observations, and several earlier ones by other groups, attophysics becomes the short-timescale frontier of physics. It replaces femtochemistry, the production of light pulses at the 10^-15 s (femtosecond) scale, in this regard.

Just as a strobelight can yield stop-action photographs of a falling water drop, femtosecond pulses can capture the ultrafast steps of a chemical reaction between multiple atoms or molecules. But attosecond pulses are better equipped to capture the even speedier motions of electrons within atoms.

If light can be imagined as a wave of peaks and valleys, a one-second visible light pulse is a train of roughly 600 trillion (6*10^14) peaks and valleys in length.

The researchers report an attosecond pulse just 200 nanometers long, carrying just over a dozen peaks and valleys. The duration of a light pulse can be thought of as the length along its direction of travel. A 1.28-second pulse can stretch from an Earthbound laboratory to the moon; a 650-attosecond pulse would barely span the length of two typical viruses.

Previous experiments have reported evidence of trains of attosecond pulses following each other roughly every 1 fs (Papadogiannis et al, Phys. Rev. Lett., 22 November 1999; Paul et al., Science, 1 June 2001; Bartels et al., Nature, 13 July 2000), but the new experiment, according to the researchers, represents the first detection and measurement of isolated attosecond pulses. Such isolated pulses, Krausz states, are important for taking attosecond-resolution snapshots of electron motion in atoms.

To accomplish their feat, the researchers first prepared an intense fsec pulse and aimed it at neon gas. The interaction between the neon gas and the fsec pulse created an attosecond-scale pulse in the soft x-ray range.

According to a helpful theoretical picture (Corkum, Physical Review Letters, 27 September 1993), the fsec pulse ejects electrons from neon atoms, and the resulting oscillations of the electrons in the bath of fsec light produce an even shorter-duration soft-x-ray pulse.

Producing attosecond light is only half the battle. The researchers then had to measure its duration.

By adjusting the delay between the times at which the x-ray pulse and a fsec visible pulse hit a gas of krypton atoms, the researchers affected the spectrum of energies in the electrons liberated from the atoms. Such modulations in the observed energy spread served as evidence for an x-ray pulse of 650 attoseconds.

Henry Kapteyn of JILA/University of Colorado, a member of a competing group, claims that the evidence is ambiguous as to whether the collaboration detected isolated attosecond pulses or trains of attosecond pulses. Both Kapteyn and Krausz have respected colleagues who back their differing views.

However this debate pans out, attosecond metrology has arrived, and it will doubtless lead to some staggering physics experiments never before possible. (Hentschel et al., Nature, 29 November 2001; some other associated journal articles can be found at this URL.)

(Editor's Note: This story, with editing, is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 567, November 29, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon)

[Contact: Ferenc Krausz, Henry Kapteyn ]






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