Scanning probe method measures vibrations between a pair of molecules

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.