[cdn-nucl-l] X-ray Flashes Provide Peek Into Atom Core

["Adam McLean" <adam.mclean@utoronto.ca>]

Posted in Science Magazine, Volume 298, Number 5594, Issue of 25 Oct
2002, p. 727. and at:
http://www.sciencemag.org/cgi/content/full/298/5594/727

Adam

-------------------

X-ray Flashes Provide Peek Into Atom Core
Adrian Cho*

Using ultrashort pulses of x-rays, physicists have taken a "movie" of
electrons frenetically rearranging themselves deep inside an atom. The
technique opens the way for a new class of experiments in which
researchers should be able to trace and control changes within atoms
that take place in billionths of a billionth of a second, or
attoseconds.

Researchers have been steadily developing sources that pump out soft
x-rays or ultraviolet light in pulses only a few hundred attoseconds
long (Science, 30 November 2001, p. 1805). But the new result marks an
important milestone in the emerging field of attophysics, says Philip
Bucksbaum, a physicist at the University of Michigan, Ann Arbor: "This
is the first demonstration of a real experiment. It's not just measuring
the pulse itself."

Electrons stacked deep in an atom behave a bit like gumballs piled into
a vending machine: Pop one out from the bottom of the heap, and the
others move to fill the void left behind. But whereas anyone with a keen
eye can see the gumballs shift, the electrons rearrange themselves far
too quickly to be directly observed with even the fastest particle or
radiation detectors. The shuffling reveals itself, however, when the
atoms are prodded with equally speedy bursts of electromagnetic
radiation, Markus Drescher of the University of Bielefeld in Germany,
Ferenc Krausz of the Vienna University of Technology in Austria, and
colleagues report in this week's issue of Nature.

The researchers hit krypton atoms with a one-two punch: a blast of soft
x-rays only 900 attoseconds long, followed by a flash of laser light
roughly seven times longer. The x-rays stripped electrons from the
krypton atoms in a process called photoionization, and some of the atoms
lost an electron from a particular inner shell, creating a core hole. An
electron from an outer shell then fell into the vacancy. In the process,
it gave up some of its energy to yet another electron, which then flew
out of the atom with a specific energy. The core holes decayed through
this complicated Auger process within a few femtoseconds, and the
researchers hoped to track precisely how their numbers dropped.

------------------------------------------------------------------------
--------
Sign of the times. In the electron spectrum, an extra peak (red) next to
the main one lets physicists time changes in the atom. 
CREDIT: ADAPTED FROM M. DRESCHER ET AL., NATURE 419, 803 (2002)
------------------------------------------------------------------------
--------

Through a trick of quantum mechanics, the researchers reduced the
problem of clocking the decay of holes to one of counting electrons. As
the Auger electron emerged, it could absorb a photon from the laser
pulse or even radiate a matching photon into the passing stream of
light. That quantum interaction slightly increased or decreased the
energy of a fraction of the electrons--a fraction determined by the
shape of the laser pulse.

By counting those energy-shifted sideband electrons, the physicists
could deduce how many core holes remained unfilled when the laser pulse
arrived. To see how that number fell with time, they simply varied the
delay between the x-ray flash and the laser pulse. "We basically record
a series of snapshots," Krausz says. "We reconstruct the motion from
them" in the same way a movie recreates a moving image from still ones.

The researchers found that the krypton core holes decayed with a
lifetime of about 8000 attoseconds, or 8 femtoseconds. That relatively
long lifetime matches what other researchers had inferred by indirect
methods. However, the new measurement marks the first time anyone has
used attosecond x-ray sources and lasers to time the flickering changes
within an atom directly.

"I imagine that everybody is going to adopt this technique," says John
Hepburn, a chemist at the University of British Columbia in Vancouver,
Canada. In the meantime, Drescher and Krausz say that their first
priority is to apply the method to a shorter lived core hole, to confirm
that they really can trace attosecond changes.

Volume 298, Number 5594, Issue of 25 Oct 2002, p. 727. 
Copyright C 2002 by The American Association for the Advancement of
Science. All rights reserved.