A
team from the CNRS "Laboratoire d'optique appliquée"
(LOA, Applied Optics Laboratory) has observed solid-liquid transformations
on a femtosecond timescale, i.e., 10-15 seconds, which is sufficiently
brief to follow directly the movement of the atoms during the transformation.
They have obtained X-ray diffraction patterns of samples using an
ultra-short X-ray flash produced by a laser. This technique will enable
the dynamics of atomic positions to be explored during elementary
reactions (dissociation, isomerization, charge transfer, and disorder).
It is therefore of interest to many scientific fields: chemistry,
biology, physics, etc.
The
LOA team presented their ultrafast X-ray diffraction technique in
1997 and have now published results regarding the first application
of this technique: non-thermal melting in semiconductors measured
at femtosecond resolution. Non-thermal melting, as opposed to thermal
melting, is not produced by heating a solid material but by rapidly
exciting a large population of electrons initially bonded to atoms
into the conduction band of the materials using a short laser pulse,
immediately producing repulsive effects and destroying the cohesion
of the atomic system. The team observed that the velocity acquired
by the atoms under the action of these forces destroys the solid state
structure within a few femtoseconds - non-thermal melting. Since this
occurs well before the atoms have had time to vibrate to create disorder
in the structure, a picture of the actual structure can be obtained.
Many
international programs have been launched to devise instruments that
can study these non-thermal phenomena; fourth generation synchotrons
should be ready in 5-10 years to produce 100 femtosecond X-ray flashes.
LOA is now applying its technique to biological materials.
The
procedure can be applied to ceramic and metal matrix composites and
will also be capable of analyzing old, corroded and stressed materials.
