A perturbation theory study of adlayer CO on NaCl(100)

CO Inversion on a NaCl(100) Surface: A Multireference Quantum Embedding Study

We develop a multireference quantum embedding model to investigate a recent experimental observation of the isomerization of vibrationally excited CO molecules on a NaCl(100) surface [Science 2020, 367, 175-178]. To explore this mechanism, we built a reduced potential energy surface of CO interacting with NaCl(100) using a second-order multireference perturbation theory, modeling the adsorbate-surface interaction with our previously developed active space embedding theory (ASET). We considered an isolated CO molecule on NaCl(100) and a high-coverage CO monolayer (1/1), and for both we generated potential energy surfaces parametrized by the CO stretching, adsorption, and inversion coordinates. These surfaces are used to determine stationary points and adsorption energies and to perform a vibrational analysis of the states relevant to the inversion mechanism. We found that for near-equilibrium bond lengths, CO adsorbed in the C-down configuration is lower in energy than in the O-down configuration. Stretching of the C-O bond reverses the energetic order of these configurations, supporting the accepted isomerization mechanism. The vibrational constants obtained from these potential energy surfaces show a small (< 10 cm^-1) blue- and red-shift for the C-down and O-down configurations, respectively, in agreement with experimental assignments and previous theoretical studies. Our vibrational analysis of the monolayer case suggests that the O-down configuration is energetically more stable than the C-down one beyond the 16th vibrational excited state of CO, a value slightly smaller than the one from quasi-classical trajectory simulations (22nd) and consistent with the experiment. Our analysis suggests that CO-CO interactions in the monolayer play an important role in stabilizing highly vibrationally excited states in the O-down configuration and reducing the barrier between the C-down and O-down geometries, therefore playing a crucial role in the inversion mechanism.

Quasiclassical simulations based on cluster models reveal vibration-facilitated roaming in the isomerization of CO adsorbed on NaCl

The desire to better understand the quantum nature of isomerization led to recent experimental observations of the vibrationally induced isomerization of OC-NaCl(100) to CO-NaCl(100). To investigate the mechanism of this isomerization, we performed dynamics calculations using finite (CO-NaCl)_n cluster models. We constructed new potential energy surfaces for CO-NaCl and CO-CO interactions using high-level ab initio data and report key properties of the bare CO-NaCl potential energy surface, which show much in common with the experiment. We investigated the isomerization dynamics using several cluster models and, in all cases, isomerization was seen for highly excited CO vibrational states, in agreement with experiments. A detailed examination of the reaction trajectories indicates that isomerization occurs when the distance between CO and NaCl is larger than the distance at the conventional isomerization saddle point, which is a strong indicator of 'roaming'.

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

Inverted (“O-down”) CO adsorbates on NaCl(100), recently observed experimentally after IR vibrational excitation (Lau et al., Science, 2020, 367, 175–178), are characterized using periodic DFT and a quantum mechanical description of vibrations.

The coverage dependence of the infrared absorption of CO adsorbed to NaCl(100)

CO adsorbed to NaCl(100) exhibits perhaps the weakest possible coupling between the adsorbate and solid. It is, therefore, an ideal system to observe the influence of adsorbate-adsorbate interactions on infrared absorption. In this work, we report polarized FTIR absorption spectra of CO/NaCl(100) as a function of coverage (0.02 ≤ θ ≤ 1 ML), where the coverage has been quantitatively determined by temperature-programmed desorption and molecular beam dosing. We extend a previous semi-empirical model designed to describe the screening of the local electric field due to dipole-dipole interactions in a CO monolayer. The extended model applies to sub-monolayer coverages and describes properly the electric field of the absorbed radiation at the vacuum-substrate interface. Fitting this model to coverage-dependent IR absorption data allows us to derive the vibrational and electronic polarizabilities [χ_v = 0.0435(14) ų, χ_e = 3.30(36) ų] and the integrated absorption cross section of 2.51(8) × 10^-17 cm/molecule for an isolated CO molecule adsorbed at the NaCl (100) surface. The determined integrated absorption cross section is substantially smaller than that of gas phase CO.

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

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.

The structure of carbon monoxide adsorbed on the NaCl(100) surface—a combined LEED and DFT-D/vdW-DF study

The structure of the first layer CO adsorbed on NaCl(100) is investigated experimentally by means of quantitative low-energy electron diffraction at 25 K, and theoretically by means of density functional theory. Consistent with earlier helium atom diffraction results, the monolayer structure has p(2×1) symmetry with a glide-plane along the longer axis of the unit cell. The structure analysis confirms the binding of CO via the carbon end to the NaCl(100) surface. The vertical distance of carbon above Na(+) is 2.58 ± 0.08 Å, in good agreement with geometry optimizations based on dispersion-corrected density functional theory, and 0.15 Å lower than predicted in calculations based on the nonlocal van der Waals density functional.

Physisorption of Small Molecules on Single Crystal Alkali Halide Surfaces: CO and N2 Adsorbed on KCl(100)

The adsorption of CO and N2 on KCl(100) single crystal cleavage planes has been investigated by means of low energy electron diffraction (LEED) at primary currents in the nA range as well as polarization infrared spectroscopy (PIRS) in transmittance geometry. These isoelectronic adsorbates behave very similarly, and three different adsorption phases can be distinguished as a function of surface coverage for both of them. Initially a (1×1) structure is observed and assigned to a commensurate monolayer with one molecule per KCl ion pair. The CO infrared spectrum of this ‘phase I’ is characterized by a doublet absorption of the CO stretching vibration, which is discussed in the context of a correlation field splitting. The lack of superstructure diffraction peaks is attributed to a high degree of orientational disorder in this phase. From LEED adsorption isotherms the isosteric heat of adsorption of N2 has been determined to be 11±3 kJ mol−1. Upon increase in coverage by 50% ‘phase II’ is formed which exhibits a large number of additional diffraction spots. It is assigned to a bilayer in which the second layer is only half filled. Based on the LEED experiments a structure model is proposed in which the second layer is characterized by growth in rows along the [210] direction and a high degree of roughness in the perpendicular direction. This model can explain all experimentally observed main features. Finally upon further exposure formation of three-dimensional solid is observed, which proceeds via Stransky–Krastanov growth of crystalline 3D clusters with the structure of the low-temperature cubic α-phase. A simulation of the CO cluster infrared spectra within the dipole–dipole coupling approach can reproduce all major observed vibrational features.

Spectroscopic link between adsorption site occupation and local surface chemical reactivity

In this Letter we show that sequences of adsorbate-induced shifts of surface core level (SCL) x-ray photoelectron spectra contain profound information on surface changes of electronic structure and reactivity. Energy shifts and intensity changes of time-lapsed spectral components follow simple rules, from which adsorption sites are directly determined. Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the d-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity.