The new technique paves the way for imaging individual molecules

(Nanowerk News) An international research team has for the first time successfully used X-rays for an imaging technique that exploits a specific quantum property of light. As the researchers explain in their work just published in the journal Physical Review Letter (“Imaging via X-Ray Fluorescence Photon Correlation”), this technique could enable imaging of non-crystalline macromolecules.

The research team, led by Henry Chapman, distinguished scientist at DESY and professor at Universität Hamburg, used highly intense X-ray pulses from the European XFEL free electron laser to generate fluorescence photons that arrived almost simultaneously at the detector – within the window. shorter than a femtosecond (a quarter trillionth of a second). By calculating the photon-photon correlation of the X-ray fluorescence emitted by an irradiated copper atom, an emission image can be obtained. Light emitted from independent light sources, such as the fluorescence of atoms, creates waves at random times and thus with random phases. When measured in coherence time, this light would be distracting and produce the grain pattern as seen in the background image. This pattern is not stationary and the sum of the many patterns will average out into a uniform distribution. However, if pair correlations are calculated from each pattern and then summed, the random phases will be averaged to leave a map of the spatial frequency content (vector q) of the sources. A total of more than 58 million correlated X-ray fluorescence snapshots are shown in the left inset, which were analyzed by a coherent diffraction imaging method to produce a high-resolution image of the source—here are two bright spots in a rotating copper disc. (Image: DESY, Fabian Trost)

The structures of materials and macromolecules are usually determined on the atomic scale using X-ray crystallography. While the technique relies on coherent X-ray scattering, incoherent processes such as fluorescence emission can predominate even though they are not useful for diffraction measurements. Instead, they add fog or a featureless background to the measured data.

But already in the 1950s, two British astronomers showed that it was indeed possible to extract structural information from the light emitted by self-luminous sources – in their case from stars. Robert Hanbury Brown and Richard Twiss’ method – called intensity interferometry – opened new doors to understanding light and started the field of quantum optics.

Recently, scientists from the University of Erlangen, the Max Planck Institute for Structure and Dynamics of Matter and DESY proposed that intensity interferometry could be adapted for atomic resolution imaging using X-ray fluorescence. The challenge in extending this idea to X-rays is that the photon coherence time, which determines the time interval available for photon-photon correlation, is extremely short. This is governed by the radiative decay time of the excited atoms, which for a copper atom is about 0.6 femtoseconds.

Now the group, along with scientists from Uppsala University and Europe’s XFEL have overcome that challenge by using femtosecond duration XFEL pulses from the facility to initiate X-ray fluorescence photons in coherence time. They produced a source consisting of two fluorescence dots in copper foil and measured the fluorescence on a million-pixel detector placed eight meters away. “For this pioneering experiment, we collected more than three petabytes of data, the largest ever for an experiment in a European XFEL,” explains lead author Fabian Trost of the Center for Free Electron Laser Science (CFEL) at DESY. “However, because the signal scales with the square of intensity, we saw that it might be possible to reduce this substantially in our future experiments.”

Only about 5000 photons were detected in each illumination pulse, and a cumulative count of over 58 million shots gives only a featureless uniform distribution. However, when they added up the photon-photon correlations, a fringe pattern emerged, similar to the famous double slit experiment. This fringe pattern is a smoke gun showing the interference of separate X-ray photons. The fringe patterns are then analyzed as if from a coherent wave field to reconstruct an image of the fluorescent source, which consists of two well-separated points.

“Although the idea of ​​independent wave interference in coherence time can be understood classically and can be noticed in radio station interference for example, with X-rays we are dealing with high energy quanta a lot,” said Chapman, who is a researcher in CUI’s Cluster of Excellence: Advanced Imaging of matter. “Each fluorescence photon is born in an atom, and this photon is then placed at a certain pixel of our detector. However, these photons carry hidden information that is only revealed when high-level photon-photon correlations are examined.”

Scientists now hope to combine this new method with diffraction to image single molecules. Fluorescence will provide specific sub-structures for certain atoms and even specific chemical states of those atoms, which can help reveal the function of important enzymes such as those involved in photosynthesis.