Physicists at the Max Planck Institute fur Quantum Optics in Munich have previously looked at the
behavior of electrons in atoms in the gas phase over a timescale of a hundred attoseconds (1 as=10^-18 second) or so. Now the
scientists, led by Ferenc Krausz, have, in collaboration with their colleagues from Bielefeld, Hamburg, Vienna and San Sebastian, made a
measurement of electron motion in a solid-state environment over a comparable timescale. The specific measurement-observing the
difference in arrival times of electrons flying out of an atom struck by laser light-represents the sharpest time-resolution ever
achieved in a condensed-matter experiment.
To bring about this feat, a near infrared (NIR) laser pulse consisting of only a few
well-chosen cycles is sent through a column of neon, producing a number of secondary beams of shorter wavelength. One of these
beams, at extreme ultraviolet (XUV) wavelengths, appears in very truncated bursts lasting only 300 as. Next, the XUV pulse is
directed at a tungsten target where atoms lying close to surface can be ionized. Actually the ultraviolet light tends to liberate an
out-lying (delocalized) electron from the atom as well as an inner-lying (localized) electron. These two electrons can proceed
through the crystal and toward a detector where, depending on the time of their arrival, they can be told apart.
This identification process is enhanced in a clever way. Traveling co-linearly with the XUV pulse (and coherently linked to it) is part
of the original NIR laser beam. The NIR intensity was carefully chosen so that it would not do the work of ionization (that task
being assigned to the XUV light) but would be strong enough to accelerate the ionized electrons as they sprang out of the sample
surface. The arrival of the NIR pulse with its well-controlled electric field was staged so that the first of the two electrons to
appear (the faster-moving outer electron) would receive a boost in speed from the electric field of the NIR radiation, while the second
electron (the slower inner-shell electron) received less of a boost. In other words the NIR light acted like a atomic-sized
accelerator, speeding up the electrons, but in differing amounts. This accentuated the difference in the arrival times of the two
electrons, making it easier to tell them apart.
The net result was an ability to measure the time delay of the two electrons coming across the top few layers of the solid-state
sample. The measured interval, 110 attoseconds with an accuracy of 70 attoseconds, constitutes the unprecedented "attosecond"
measurement. One of the researchers, Adrian Cavalieri (adrian.cavalieri@mpq.mpg.de) says that monitoring electron motions in a crystal with this level of precision is the first step in
developing a much faster style of electronics, maybe even at a petahertz (10^15 Hz) rate. First comes measurement at
100-attosecond levels, later comes control of electron activity. (Cavalieri et al., Nature 25 October 2007; http://www.attoworld.de/)
Nuclear Dripline Droops
Several new heavy isotopes have been discovered, at least one of which pushes beyond the neutron
dripline. Driplines are the outer edges defining the zone of observed or expected bound nuclei on a map whose horizontal axis is
the number of neutrons in a nucleus (denoted by the letter N) and whose vertical axis corresponds to the number of protons (Z).
Unlike the Coulomb force which holds atoms together, and where electron behavior and the expected chemical properties of that
element can be predicted pretty accurately, with nuclei it's different.
The nuclear force holding neutrons and protons together
(even as the like-charged protons repel each other electrostatically) is so strong that no theory (not even the so
called nuclear shell model, fashioned in analogy to the atomic model) can confidently predict whether a particular combination of
neutrons and protons will form a bound nucleus. Instead experimenters must help theorists by going out and finding or making
each nuclide in the lab.
In an experiment conducted recently at the National Superconducting Cyclotron Lab (NSCL) at Michigan State University, a beam of calcium
ions was smashed into a tungsten target. A myriad of different nuclides emerged and streamed into a sensitive detector for
identification. Two newly found nuclides-Mg-40 and Al-43-came as no surprise. But another, Al-42, was more unusual since it violated
the provisional prohibition against nuclei of this size having an odd number of protons and neutrons.
The new nuclides are not stable
since they decay within a few milliseconds. But this is pretty long by nuclear standards. Why study such fleeting nuclei? Even though
they might not exist naturally, the new nuclides still might play a role inside stars or novas where heavy elements, including those
that make up our planet and our bodies, are created. Thomas Baumann (baumann@nscl.msu.edu) suggests that even heavier aluminum-isotopes
might exist, and that it is worth exploring any possible islands of stability, not just those at the very edge of the periodic table.
(Baumann et al., Nature 25 October 2007; http://www.nscl.msu.edu/magnesium40)
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