No energy flux through the chemical bond!

January 18, 2019

When adsorbed at a salt crystal surface, the vibrations of excited carbon monoxide (CO) molecules are damped within milliseconds. An international team of researchers led by Alec Wodtke at the Max Planck Institute (MPI) for Biophysical Chemistry has now discovered that this is due to electromagnetic interaction of the CO dipole with the crystal. In contrast, energy loss via the chemical bond is less important than previously thought. (Science, January 11, 2019)

Structure of the CO monolayer on a NaCl(100) crystal surface.The CO molecules bind via their carbon atoms to the Na+ions. At temperatures below 35 K the molecules are tilted with respect to the surface normal and form antiparallel oriented rows — **Structure of the CO monolayer on a NaCl(100) crystal surface.**The CO molecules bind via their carbon atoms to the Na⁺ions. At temperatures below 35 K the molecules are tilted with respect to the surface normal and form antiparallel oriented rows

© Max Planck Institute for Biophysical Chemistry

**Structure of the CO monolayer on a NaCl(100) crystal surface.**The CO molecules bind via their carbon atoms to the Na⁺ions. At temperatures below 35 K the molecules are tilted with respect to the surface normal and form antiparallel oriented rows

© Max Planck Institute for Biophysical Chemistry

The exchange of vibrational energy between molecules is usually a consequence of the anharmonic interatomic forces coupling the motion of one molecule to the other. For molecules bound to a surface, for example, the anharmonic interactions with the substrate are so efficient that the adsorbate’s vibrational motion is damped within a few nanoseconds. Researchers of the MPI for Biophysical Chemistry in Göttingen, in collaboration with Leiden University (Netherlands) and colleagues at the National Institute of Standards and Technology, Boulder, Colorado (United States) have now discovered that for oscillating CO molecules adsorbed in a monolayer on a sodium chloride (NaCl) crystal surface this energy loss mechanism is completely unimportant.

The CO molecules are on the one hand sufficiently strongly bound to the solid to form samples which are stable over long periods of time. On the other hand, the contribution of this bond to draining vibrational energy from the CO to the NaCl lattice vibrations is negligible. In fact, the interactions between neighboring CO molecules are stronger than CO-NaCl interactions, leading to conditions were the vibrational energy hops from molecule to molecule and in this way freely migrates within the monolayer for tens of milliseconds. The vibrational lifetime is limited by a loss mechanism that is similar to a radio transmitter experiencing damping due to electromagnetic interaction with the Earth. The corresponding theory has its origin in Arnold Sommerfeld’s work from 1909. In the case of CO/NaCl, the oscillating CO molecules act like dipole antennas being damped by the electromagnetic interactions with the NaCl crystal.