|Simulation of what wave mixing could tell us about diamond|
Some of the light is absorbed by the diamond's valence electrons and its energy is then transferred to some of the X-rays scattering from the sample. This allows the team to differentiate between X-rays that have interacted with valence electrons and X-rays that have scattered from the sample's core electrons – something that has never been done before.
X-ray diffraction involves bouncing X-rays off the electron clouds that surround a material's constituent nuclei and studying the interference patterns that are created. While it gives a wealth of information about the structure and composition of materials, the technique reveals little about the sample's chemically active valence electrons. This is because the majority of electrons involved in the scattering are "core" electrons, which do not take part in chemical processes.
More than 40 years ago, Isaac Freund and Barry Levine at Bell Labs proposed a way of getting round this problem. They pointed out that if the sample is exposed to laser light, the valence electrons will respond by oscillating at the laser frequency. Some of the oscillation energy is then transferred to the X-rays as they scatter from the valence electrons in a process called "wave mixing". As a result, X-rays scattered from the valence electrons will emerge at a slightly higher energy that is equal to the sum of the incoming X-ray and laser energies.