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First light for attophysics

24 Oct 2002

Thanks to an attosecond X-ray source, physicists measure directly the decay of an electron cloud in a core-excited atom for the first time.

Physicists in Germany and Austria have performed the first experiments with an attosecond (10-18s) light source.

The ultrashort pulses allowed Markus Drescher and colleagues at the University of Bielefeld to investigate the rearrangement of an electron cloud around a krypton atom using pump-probe spectroscopy (Nature 419 803).

In the experiment, the X-ray pulse first ejects an electron out of an orbit close to the atom's nucleus. The Bielefeld team measured the time it took this vacancy to refill by carefully controlling the time delay between the X-ray pump and subsequent probe laser pulses.

This has not previously been feasible, as the decay processes in the electron cloud take just a few femtoseconds - and the fastest laser sources have similar pulse durations. However, the attosecond source recently developed by Ferenc Krausz and colleagues at the Vienna University of Technology (see related story) has opened up new possibilities for probing ultrafast atomic phenomena.

"It is essential that the exciting X-ray pulse is much shorter than the decay process being investigated to achieve good temporal resolution," said Drescher.

The pump pulse must be in the X-ray region because the pulse duration is fundamentally limited by its wavelength. "The limit for visible pulses, given by the light oscillation period, is about 2.5 fs. For our [X-ray source] the fundamental limit is pushed to 40 attoseconds," explained Drescher.

He told Optics.org that the next aim is to measure sub-femtosecond phenomena, such as ionization processes including two or more competing reaction pathways.

Commenting on the work in Nature, Brookhaven National Laboratory scientist Louis DiMauro likened the attosecond source to the shutter speed of a camera - but one which is fast enough to take a snapshot of electrons tumbling between energy levels close to the atom's nucleus.

"There are few papers that announce the beginning of a new era, but [this paper] falls into that category," gushed DiMauro. "We are entering a new realm of hyperfast measurement - the age of attophysics has begun."

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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