Second-harmonic generation with soft X-rays probes buried interfaces | January 15, 2018 Issue - Vol. 96 Issue 3 | Chemical & Engineering News
Volume 96 Issue 3 | p. 9 | Concentrates
Issue Date: January 15, 2018

Second-harmonic generation with soft X-rays probes buried interfaces

Method may enable researchers to use X-ray techniques to probe interfacial chemical reactions with femtosecond resolution
Department: Science & Technology
News Channels: Analytical SCENE, Biological SCENE
Keywords: Spectroscopy, nonlinear spectroscopy, second harmonic, sum frequency, interface, X-ray
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Researchers have devised a spectroscopy method based on soft X-ray second-harmonic generation to probe graphene layers inside graphite, as rendered here.
Credit: R. K. Lam/UC Berkeley
This is an artistic rendering of the way X-rays interact inside graphite and produce a second-harmonic generation signal.
 
Researchers have devised a spectroscopy method based on soft X-ray second-harmonic generation to probe graphene layers inside graphite, as rendered here.
Credit: R. K. Lam/UC Berkeley

Second-harmonic generation (SHG) is a nonlinear optical process in which two photons of a given energy interact with select types of materials and combine to form a single photon with double the original energy. The SHG process, and a closely related one known as sum frequency generation, lie at the heart of a number of spectroscopy methods based on infrared, visible, and ultraviolet laser light. As a result of spectroscopy selection rules, these nonlinear processes are particularly adept at probing interfaces, even ones hidden by many layers of molecules, as is the case for a solid catalyst in contact with high-pressure gas or an electrode in contact with a liquid-electrolyte solution. X-rays with photons in the 100-to-1,000-eV energy range, so-called soft X-rays, can provide valuable information about chemical bonding and structure with elemental specificity. But because of the lack of available light sources with the required intensity and coherence, researchers have been unable to develop an SHG version of soft X-ray interface spectroscopy—until now. In a proof-of-concept study, Richard J. Saykally and a large team of researchers working at the FERMI facility in Trieste, Italy, have demonstrated that the method can selectively probe layers of graphene inside a graphite sample (Phys. Rev. Lett. 2018, DOI: 10.1103/physrevlett.120.023901). The new technique may eventually enable researchers to use X-rays to track chemical reactions at interfaces with femtosecond resolution.

 
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