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Analytical Chemistry

Scanning probe method measures vibrations between a pair of molecules

Intermolecular vibrational spectroscopy technique can provide chemical information on the nanoscale

by Mitch Jacoby
January 30, 2017 | APPEARED IN VOLUME 95, ISSUE 5

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Credit: Wilson Ho & Zhumin Han/UC Irvine

The close proximity of two CO molecules (black and red)—one on an STM tip and one on a surface—induces vibrations. In the mode shown here, the molecules move in opposite directions, with the oxygens moving more than the carbon atoms (arrows).
Credit: Wilson Ho & Zhumin Han/UC Irvine

The close proximity of two CO molecules (black and red)—one on an STM tip and one on a surface—induces vibrations. In the mode shown here, the molecules move in opposite directions, with the oxygens moving more than the carbon atoms (arrows).

Forces between closely spaced molecules can cause them to undergo various types of concerted motions or intermolecular vibrations. This kind of molecular group dance can alter vibrations within individual molecules and influence chemical reactivity. Yet details of these collective molecular motions remain unknown because the coupled vibrations of a single pair of molecules have not been measured—until now, that is. Zhumin Han and Wilson Ho of the University of California, Irvine, and coworkers used a specially designed scanning tunneling microscope to probe the coupled vibrations of two CO molecules—one on the STM tip and one on a silver surface (Phys. Rev. Lett. 2017, DOI: 10.1103/physrevlett.118.036801). The vibrations are induced by short-range CO-CO repulsion. The team tuned the distance between the two molecules while measuring inelastic electron tunneling, which is the basis of a highly sensitive vibrational spectroscopy method, and analyzed the results with quantum calculations. The analysis revealed various molecular subtleties, including an antisymmetric vibrational mode corresponding to a type of hindered translational motion. The team explains that these vibrational features, which can be used to deduce chemical information, result from the complex interplay between tip-sample distance and the tilting and orbital alignment of the pair of CO molecules.

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