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Watching Electrons Move in Real Time

At its most basic level, understanding chemistry means understanding what electrons are doing. Research published in The Journal of Chemical Physics not only maps the movement of electrons in real time but also observes a concerted electron and proton transfer that is quite different from any previously known phase transitions in the model crystal, ammonium sulfate. By extending X-ray powder diffraction into the femtosecond realm, the researchers were able to map the relocation of charges in the ammonium sulfate crystal after they were displaced by photoexcitation.

"Our prototype experiment produces a sort of 'molecular movie' of the atoms in action," says author Michael Woerner of the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie in Germany. "The time and spatial resolution is now at atomic time and length scales, respectively."

Electron positions were mapped by observing the diffraction of X-ray pulses lasting tens of femtoseconds (quadrillionth of a second). Positions of protons and other nuclei were deduced from the locations of regions of high electron density. Within the crystal, the excited electrons transferred from the sulfate groups to a tight channel within crystal matrix. This channel was stabilized by the transfer of protons from adjacent ammonium groups into the channel. This transfer mechanism had not been previously observed or proposed, and the researchers had expected to see much smaller displacements.

According to Woerner, the technique should be applicable to structural studies of materials ranging from biomolecules to high-temperature superconductors. "We expect that the technique will be applied to many interesting material systems." He says. "In principle, femtosecond X-ray powder diffraction can be applied to any crystalline form of matter. Only the complexity of crystals and the presence of heavy elements, which reduces the penetration depth of X-rays, set some constraints."

The Article, "Concerted electron and proton transfer in ionic crystals mapped by femtosecond x-ray powder diffraction" by Michael Woerner, Flavio Zamponi, Zunaira Ansari, Jens Dreyer, Benjamin Freyer, Mirabelle Premont-Schwarz, and Thomas Elsaesser is published in The Journal of Chemical Physics.


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Image Caption: Sectional view of the charge density map Dr ( x , y , z , t ) for a plane which is parallel to the z-axis and includes the line connecting the hydrogen atoms of opposite NH + 4 groups for different time delays.

The Journal of Chemical Physics publishes concise and definitive reports of significant research in methods and applications of chemical physics. Innovative research in traditional areas of chemical physics such as spectroscopy, kinetics, statistical mechanics, and quantum mechanics continue to be areas of interest to readers of JCP. In addition, newer areas such as polymers, materials, surfaces/interfaces, information theory, and systems of biological relevance are of increasing importance. Routine applications of chemical physics techniques may not be appropriate for JCP. Content is published online daily, collected into four monthly online and printed issues (48 issues per year); the journal is published by the American Institute of Physics. See:
The American Institute of Physics is a federation of 10 physical science societies representing more than 135,000 scientists, engineers, and educators and is one of the world's largest publishers of scientific information in the physical sciences. Offering partnership solutions for scientific societies and for similar organizations in science and engineering, AIP is a leader in the field of electronic publishing of scholarly journals. AIP publishes 12 journals (some of which are the most highly cited in their respective fields), two magazines, including its flagship publication Physics Today; and the AIP Conference Proceedings series. Its online publishing platform Scitation hosts nearly two million articles from more than 185 scholarly journals and other publications of 28 learned society publishers.

Jason Socrates Bardi | Newswise Science News
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