Competition between Coulomb and van der Waals Interactions in $\mathrm{Xe}\text{−}{\mathrm{Cs}}^{+}$ Aggregates on Cu(111) Surfaces

Microscopic insight into interactions is a key for understanding the properties of heterogenous interfaces. We analyze local attraction in noncovalently bonded $\mathrm{Xe}\text{−}{\mathrm{Cs}}^{+}$ aggregates and monolayers on Cu(111) as well as repulsion upon electron transfer. Using two-photon photoemission spectroscopy, scanning tunneling microscopy, and coupled cluster calculations combined with an image-charge model, we explain the intricate impact Xe has on ${\mathrm{Cs}}^{+}/\mathrm{Cu}(111)$. We find that attraction between ${\mathrm{Cs}}^{+}$ and Xe counterbalances the screened Coulomb repulsion between ${\mathrm{Cs}}^{+}$ ions on Cu(111). Furthermore, we observe that the Cs $6s$ electron is repelled from Cu(111) due to xenon’s electron density. Together, this yields a dual, i.e., attractive or repulsive, response of Xe depending on the positive or negative charge of the respective counterparticle, which emphasizes the importance of the Coulomb interaction in these systems.

Figure 1

STM images (a) of ${\mathrm{Cs}}^{+}/\mathrm{Cu}(111)$, $-500\mathrm{mV}$, 10 pA, and (b) of a $\mathrm{Xe}\text{−}{\mathrm{Cs}}^{+}$ aggregate on Cu(111) using a false-color scale, 10 pA, $-25\mathrm{mV}$. Insets in (a),(b) show 1 ML $\mathrm{Xe}/\mathrm{Cu}(111)$, recorded at 1.2 nA, 100 mV on a wetting ML and at an aggregate perimeter, 44 pA, 7 mV, respectively. (c) Laplace-filtered image of (b). The $\mathrm{Xe}:{\mathrm{Cs}}^{+}$ ratio in (b),(c) is $40:1$. (d) 0.16 ML ${\mathrm{Cs}}^{+}/\mathrm{Cu}(111)$, 10 pA, 250 mV; (e) Original (top) and Laplace-filtered image (bottom) of 1 ML Xe on top of 0.16 ML ${\mathrm{Cs}}^{+}/\mathrm{Cu}(111)$, 89 pA, 250 mV, (f) apparent height profiles (top) along the lines marked in panel (c) in identical colors and their Gaussian fits with maxima marked (bottom).

Figure 2

${\mathrm{Cs}}^{+}\text{−}{\mathrm{Cs}}^{+}$ distances in $\mathrm{Xe}\text{−}{\mathrm{Cs}}^{+}$ aggregates: (a) Repulsive interactions between two ${\mathrm{Cs}}^{+}$ without (solid line) and with contact to a metal (red dashed line) as a function of the ${\mathrm{Cs}}^{+}\text{−}{\mathrm{Cs}}^{+}$ distance. Circles represent the $\mathrm{CCSD}(\mathrm{T})/\mathrm{CBS}$ reference. Inset: Attractive interactions between ${\mathrm{Cs}}^{+}$ and one (or two) Xe atoms per Xe obtained from $\mathrm{CCSD}(\mathrm{T})/\mathrm{CBS}$ calculations as a function of ${\mathrm{Cs}}^{+}\text{−}\mathrm{Xe}$ (equi)distance; the resulting attraction of 150 meV per nearest-neighbor Xe atom provides the gray dashed line in the main panel. The blue and red isosurfaces of the charge density difference visualize charge accumulation and depletion, respectively, of $\mathrm{Xe}\text{−}{\mathrm{Cs}}^{+}$-Xe at the optimized equidistance of 0.39 nm. Xe cores are yellow [26]. (b) Nearest-neighbor distance histogram of ${\mathrm{Cs}}^{+}$ on Cu(111) in green and in $\mathrm{Xe}\text{−}{\mathrm{Cs}}^{+}$ aggregates in red normalized to the number of distances $N_{\mathrm{tot}}$ in STM images for ${\mathrm{\Theta }}_{{\mathrm{Cs}}^{+}}=7\times {10}^{-4}\mathrm{ML}$ (low) and 0.16 ML (high).

Figure 3

2PPE intensity (a) of 1 ML $\mathrm{Xe}/0.16\mathrm{ML}$ ${\mathrm{Cs}}^{+}/\mathrm{Cu}(111)$ recorded at $T=30\mathrm{K}$ in false color as a function of intermediate energy $E\text{−}{E}_F$ (left), final state energy (right), and time delay. The time-dependent energy of the Cs $6s$ feature is indicated by circles. (b) Autocorrelation traces of the Cs $6s$ state (circles), obtained by integrating the intensity within 200 and 150 meV for $\mathrm{Cs}/\mathrm{Cu}(111)$ (black) and $\mathrm{Xe}/\mathrm{Cs}/\mathrm{Cu}(111)$ (red), respectively. Relaxation times of ($13\pm 3$) and $(80\pm 10)\mathrm{fs}$ for $\mathrm{Cs}/\mathrm{Cu}(111)$ and $\mathrm{Xe}/\mathrm{Cs}/\mathrm{Cu}(111)$, respectively, are obtained by fits of single exponential decays convolved with the autocorrelation of the laser pulse (solid lines) [44]. (c) 2PPE spectra at the indicated time delays.

Figure 4

Time-dependent change of intermediate state energy $\mathrm{\Delta }E$ (a) plotted for $\mathrm{Xe}/{\mathrm{Cs}}^{+}/\mathrm{Cu}(111)$ from Fig. 3 and for bare ${\mathrm{Cs}}^{+}/\mathrm{Cu}(111)$ using data from [19]. The respective distances $R_{\mathrm{Cs}\dots \mathrm{Cu}\text{−}\mathrm{ip}}$ and the excited state potential energy surface $U(R_{\mathrm{Cs}\dots \mathrm{Cu}\text{−}\mathrm{ip}})$ are determined with (red) and without (black) coadsorbed Xe. Note that the absolute energy difference between the two potentials in (c) is not known.