For more than 100 years, X-rays have been used in crystallography to determine the structure of molecules. At the heart of the method are the principles of diffraction and superposition, to which all waves are subject. Light waves consisting of photons are deflected by the atoms in the crystal and overlap — like water waves generated by obstacles in a slowly flowing stream. If a sufficient number of these photons can be measured with a detector, a characteristic diffraction pattern or wave pattern is obtained from which the atomic structure of the crystal can be derived. This requires that photons are scattered coherently, meaning that there is a clear phase relationship between incident and reflected photons. This corresponds to water waves that are deflected from the obstacles without vortexes or turbulences. If photon scattering is incoherent, the fixed phase relationship between the scattered photons disperses, making it impossible to determine the arrangement of the atoms — just like in turbulent waters.
With X-ray light, in most cases, incoherent scattering dominates; for example, in the form of fluorescence resulting from photon absorption and subsequent emission, creating a diffuse background that cannot be used for coherent imaging, and reduces the reproduction fidelity of coherent methods. A new method, incoherent diffractive imaging (IDI), could help to image individual atoms in nanocrystals or molecules faster and with much higher resolution.
The seemingly undesirable incoherent radiation is key to the new imaging technique. The incoherently scattered X-ray photons are not recorded over a longer period of time, but in time-resolved short snapshots. When analyzing the snapshots individually, the information about the arrangement of the atoms can be obtained. The light diffraction is still coherent within short sequences; however, this is only possible with extremely short X-ray flashes with durations of no more than a few femtoseconds — a few quadrillionths of a second — which has only been achieved using free-electron lasers.
Since the new method uses fluorescence light, a much stronger signal than before can be obtained, which is also scattered to significantly larger angles, gaining more detailed spatial information. In addition, filters can be used to measure the light of specific atomic species only. This makes it possible to determine the position of individual atoms in molecules and proteins with a significantly higher resolution compared to coherent imaging using X-ray light of the same wavelength.