X-Rays Expose Nonthermal Melting

Kevin Robinson

Using time-resolved x-ray spectroscopy, researchers at Laboratoire d'Optique Appliquée have observed nonthermal melting in InSb. The work illustrates the expanding application of time-resolved femtosecond investigations, designed to describe unstable, short-lived chemical reactions.

"What we did here is apply the technique of femtosecond x-ray diffraction to probe an ultrafast process in the field of condensed-matter physics," said Antoine Rousse, the lead author of a paper describing the study in the March 1 issue of Nature. "This is the first time that such an ultrafast [transient structure] was directly observed with x-rays."

The researchers used the pump-and-probe technique to excite the sample with 120-fs pulses of 800-nm laser light and to detect the resulting structural changes. They generated femtosecond x-ray pulses by focusing a second beam from the laser on a silicon target and then directed the pulses at the sample. They detected the x-rays with a cooled CCD camera, which allowed them to collect the low-amplitude signal and to monitor x-ray diffraction through the sample.

A semiconductor crystal diffracts x-rays, Rousse explained, but the excitation pulse causes disorder in the sample, eliminating diffraction. "At the end of the reaction, the solid is completely melted and the x-ray signal drops to zero," he said. He added that the process happens much faster than conventional thermal melting, which is on a timescale of tens of picoseconds.

In conventional melting, the electrons store this energy for a time and then release it to the crystal lattice through optical and acoustic phonons. The atoms in the lattice oscillate near equilibrium, slowly gaining energy.
In nonthermal melting, however, the electrons transfer the energy directly to the lattice. The material therefore becomes a liquid before there is thermal relaxation.

Protein conformation

Rousse's group, in collaboration with researchers at the University of Copenhagen in Denmark and at Jena University in Germany, has begun to use the method to study proteins. "The conformation of a protein can influence its activity," he said, "[so] the study of structural subpicosecond reactions or transitions has become a major task in this field."