Laser physicists have developed a new ultrafast X-ray diffraction technique that can probe extreme and short-lived states of matter. Researchers from Germany and the US have used the new approach to 'watch' the atoms in a solid piece of germanium become disordered as the material melts (Science 286 1340). The technique, which relies on a new generation of table-top X-ray sources, could be used to probe ultrafast processes in a wide range of experiments in physics, chemistry and biology.
The germanium sample was bombarded with powerful laser pulses to excite the electrons and cause thermal melting. In this process, unlike normal melting, the metallic crystal goes from a cold solid to a hot liquid state and never passes through the classical equilibrium stage of a hot solid. “The short laser pulse neutralises the glue that keeps atoms in a crystal and they are instantaneously freed,” said one of the team, Andrea Cavalleri from the University of California at San Diego (UCSD). “The initial and final state are the same as in traditional melting, but the pathway is very different.”
At the same time the laser pulses were hitting the sample, the researchers fired a series of ultra-fast X-rays at the germanium. “This is the first time that we have been able to directly see such non-thermal rearrangement of the atoms and molecules in a material,” said Craig Siders, also of UCSD. “With ultrafast x-rays we could watch this rigorously symmetric crystal lattice of germanium atoms just fall apart in an incredibly short period of time. The X-rays allowed us to see this not just at the surface but also, and importantly, inside the material too.”
