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

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.

“Inverted” CO molecules on NaCl(100): a quantum mechanical study

“…Another interesting finding of Wodtke and coworkers, 15 is the ''flipping'' of adsorbed CO molecules on NaCl(100) using infrared laser excitation at a temperature of 7 K, 15 from a ''C-down'' to an ''O-down'' configuration. This configuration was briefly mentioned for an isolated CO molecule adsorbed on a NaCl(100) slab already in an earlier theoretical work of Meredith et al.…”

Potential Energy Landscape of CO Adsorbates on NaCl(100) and Implications in Isomerization of Vibrationally Excited CO

“…Despite the insights they provide, however, these PESs reported in this work cannot be directly used to model the experiment conducted for a monolayer of CO in the p(2 × 1) structure before vibrational excitation. 20,21 This is due to the enforced periodicity in these PESs, which requires the adjacent adsorbates to have the same position, orientation, and vibrational excitation as the ones in the unit cell. This is obviously not the situation in the experiment, 20,21 where a highly excited CO is most likely surrounded by coadsorbates with potentially different coordinates and orientations, as well as no or low vibrational excitations.…”

“…The energy is −679.4 and 701.9 cm −1 , relative to the dissociation limit and the global C-down minimum, respectively. This scenario is also not relevant to the recent experiments 20,21 because of its lower coverage.…”

“…This indicates that the isomerization needs not proceed via desorption. However, the MEP may not be relevant to the recent experiments 20,21 because all CO adsorbates on the surface are assumed to diffuse in sync, due to the periodic conditions, which is unlikely to happen at the coverage of 1 ML. On the 2 × 2-MBD PES, the transition state locates at the middle point of two adjacent Cl − sites, with a tilt angle of 110°and azimuthal angle of 180°.…”

“…Very recently, high-resolution spectra of the CO on the cold NaCl(100) system have been measured suggesting that the energy pooling can result in a population of CO vibrational states as high as v = 30. 20,21 More interestingly, these highly vibrationally excited CO species are observed to flip from a Cdown geometry to an O-down geometry, thus providing an interesting example of double-well systems coupled to a condensed medium. 21 To better understand the energy pooling and the resulting isomerization processes, one needs to map out the global potential energy surface (PES) for CO adsorption on NaCl with the C−O vibrational coordinate included, which is the main objective of this work.…”