Physicists at Lawrence Livermore Laboratory in California have used
intense light to convert a small solid gold target into a plasma of
electrons and positive ions. In the instant before the sample flies
apart the physicists are able to record some surprising results. The
most important finding is that even at extreme conditions of high
energy density (107 joules per kilogram), the gold metal still retains the band
structure exhibited by all metals -- the allowed electron energies
are not continuous but fall into certain allowed energy bands.
With
light from a femtosecond laser falling on the sample, the Livermore
scientists achieve the highest isochoric (meaning under conditions
of constant density) energy density ever observed for a solid -- 107
joules per kilogram.
According to one of the researchers, Andrew Ng
(ng16@llnl.gov), who is the scientific director of Livermore's
Jupiter Laser Facility, expressing the energy density in terms of
energy per unit mass, rather the customary energy per unit volume,
gives a more direct sense of the excitation energy
being invested in each atom or molecule. (Higher energy densities
have been achieved by imploding a target with laser light or a
nuclear explosion, but the new result is the highest for a sample at
its original volume.)
Furthermore, this experiment achieves a record
for heating rate -- exceeding 1017 degrees Fahrenheit per second --
for the electrons in the
solid; the ions forming the lattice of the solid are heavier and
warm up at a much slower rate.
This work can be considered as part
of an emerging new subject, "warm dense matter," at the crossroads
between condensed matter physics and plasma physics. This research
area, related to another topic called high energy density physics,
is also of interest to workers in disciplines such as high pressure
science, planetary science, geophysics, and shock compression.
Ping et al.,
Physical Review Letters, 30 June 2006
Contact Andrew Ng, ng16@llnl.gov
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