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December 3, 2001
Volume 79, Number 49
CENEAR 79 49 p. 11
ISSN 0009-2347
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Extremely short bursts of light probe ultrafast electronic processes


How short is short? When it comes to light pulses, the bar has just been lowered to mere billionths of a billionth of a second. Researchers have demonstrated a new laser method that produces attosecond (10–18 second) bursts of light--the shortest duration measured to date.

FAST! Attosecond researchers Kienberger (left) and Hentschel push the limits of time resolution.
A number of techniques for probing atomic and molecular events on very fast time scales have been developed and put to use in recent years. State-of-the-art femtosecond (10–15 second) lasers and laser procedures provide probes of exceptional fineness for studying molecular vibrations and chemical reaction dynamics.

But now the scale of time-dependent measurements has just passed into the realm of attoseconds. Physicists in Austria have devised a procedure that produces 650-attosecond bursts of soft X-rays [Nature, 414, 509 (2001)]. The group has used the record-breaking laser method to study the dynamics of electronic processes in real time.

To produce attosecond light pulses, physicists at Vienna University of Technology's Photonics Institute, including Reinhard Kienberger, Michael Hentschel, Ferenc Krausz, and their coworkers, irradiate neon with bursts of red light lasting just 7 femtoseconds. The laser light strips electrons from the gas atoms and causes them to interact with Ne ions in a process that gives rise to high-order harmonics--pulses of light (extreme UV and X-ray, in this case) with much higher frequencies than that of the red light. The team filters the harmonic light to allow only select attosecond bursts of X-rays to pass.

Putting the fleeting beams of X-rays to use, the Vienna scientists train the red light and the X-rays onto a krypton target in such a way that they can monitor the dynamics of photoelectrons emitted by the rare gas with attosecond resolution.

"These are the kinds of exciting developments that take us into new territory," comments Ahmed H. Zewail, professor of chemistry and physics at California Institute of Technology. The chemistry Nobel Laureate notes that by stepping into the realm of attosecond time resolution, scientists can look forward to studying electron rearrangement processes in molecules and changes in molecular structures as molecules undergo electron-transfer reactions. Offering an example, Zewail suggests that, with further advances in attosecond techniques, "We may be able to catch a benzene molecule in various electronic configurations."

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