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Observation of an isomerizing double-well quantum system in the condensed phase

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Science  10 Jan 2020:
Vol. 367, Issue 6474, pp. 175-178
DOI: 10.1126/science.aaz3407

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Upside-down molecules on a salt surface

The quantum states of molecules are usually measured in gas-phase studies to minimize collisions that would blur the spectra. However, for carbon monoxide (CO) molecules adsorbed on a NaCl(100) surface, infrared emission from even high vibrational states can be resolved. In quantum-state resolved spectra, Lau et al. found that infrared-laser excitation of a monolayer of CO adsorbed on NaCl(100) forms an isomer in which CO binds to Na+ through its O atom in an upside-down configuration (see the Perspective by Wu). These results could be understood with a simple vibrationally adiabatic electrostatic theory, making this system convenient for studies of the isomerization chemistry.

Science, this issue p. 175; see also p. 148

Abstract

Molecular isomerization fundamentally involves quantum states bound within a potential energy function with multiple minima. For isolated gas-phase molecules, eigenstates well above the isomerization saddle points have been characterized. However, to observe the quantum nature of isomerization, systems in which transitions between the eigenstates occur—such as condensed-phase systems—must be studied. Efforts to resolve quantum states with spectroscopic tools are typically unsuccessful for such systems. An exception is CO adsorbed on NaCl(100), which is bound with the well-known OC–Na+ structure. We observe an unexpected upside-down isomer (CO–Na+) produced by infrared laser excitation and obtain well-resolved infrared fluorescence spectra from highly energetic vibrational states of both orientational isomers. This distinctive condensed-phase system is ideally suited to spectroscopic investigations of the quantum nature of isomerization.

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