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The Sommerfeld ground-wave limit for a molecule adsorbed at a surface

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Science  11 Jan 2019:
Vol. 363, Issue 6423, pp. 158-161
DOI: 10.1126/science.aav4278

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Climbing vibrational levels

Vibrational excitation of molecules adsorbed on a surface is usually limited because the vibrational energy is rapidly transferred into phonons, the vibrational modes of the substrate. Chen et al. found that this is not the case for CO molecules adsorbed on a surface of NaCl. The CO molecules efficiently transferred vibrational energy within groups of molecules from one high excitation state to another until they reached the dissociation limit. This process was possible because of the close proximity of the molecules and the limited transfer of energy to just one phonon mode in the salt surface.

Science, this issue p. 158

Abstract

Using a mid-infrared emission spectrometer based on a superconducting nanowire single-photon detector, we observed the dynamics of vibrational energy pooling of carbon monoxide (CO) adsorbed at the surface of a sodium chloride (NaCl) crystal. After exciting a majority of the CO molecules to their first vibrationally excited state (v = 1), we observed infrared emission from states up to v = 27. Kinetic Monte Carlo simulations showed that vibrational energy collects in a few CO molecules at the expense of those up to eight lattice sites away by selective excitation of NaCl’s transverse phonons. The vibrating CO molecules behave like classical oscillating dipoles, losing their energy to NaCl lattice vibrations via the electromagnetic near-field. This is analogous to Sommerfeld’s description of radio transmission along Earth’s surface by ground waves.

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