Weak Intermolecular Interactions in an Ionically Bound Molecular Adsorbate: $\mathrm{\text{Cyclopentadienyl}}/\mathrm{Cu}(111)$

The dissociative adsorption of cyclopentadiene (${\mathrm{C}}_5{\mathrm{H}}_6$) on Cu(111) yields a cyclopentadienyl (Cp) species with strongly anionic characteristics. The Cp potential energy surface and frictional coupling to the substrate are determined from measurements of dynamics of the molecule together with density functional calculations. The molecule is shown to occupy degenerate threefold adsorption sites and molecular motion is characterized by a low diffusional energy barrier of $40\pm 3\mathrm{meV}$ with strong frictional dissipation. Repulsive dipole-dipole interactions are not detected despite charge transfer from substrate to adsorbate.

Figure 1

Experimental ISFs measured along the (a) $[1\overline{1}0]$ and (b) $[11\overline{2}]$ directions at $\Delta \mathbf{K}\sim 1.9{\AA }^{-1}$ at room temperature (points). Along the $[1\overline{1}0]$ azimuth, a single exponential decay is seen (solid blue line), in agreement with the expected form for jump diffusion. However, along $[11\overline{2}]$ the line shape is the sum of two exponential decays, indicating jumps between sites forming a non-Bravais lattice (the solid black line indicates the sum of dashed red and blue components). Data below 0.5 ps are heavily influenced by phonons and are excluded. The inset shows the temperature dependency of the dephasing rate, at $\Delta \mathbf{K}=0.4{\AA }^{-1}$ along $[11\overline{2}]$, as an Arrhenius plot, giving an activation energy of $41\pm 1\mathrm{meV}$ for diffusion.

Figure 2

Momentum transfer dependencies of ISF dephasing rates, along the $[1\overline{1}0]$ (left) and $[11\overline{2}]$ (right) azimuths at 300 K, shown as points. Solid lines indicate dephasing rates from an analytical model of jumps between degenerate fcc and hcp hollow sites (the dashed line shows where the second exponential term has zero intensity in the model).

Figure 3

The main figure shows experimental data from Fig. 2 as points while crosses illustrate the best fit rates extracted from Langevin molecular-dynamics simulation data using an optimized PES and adsorbate-substrate friction (see text), with intermolecular interaction strengths as deduced from DFT calculations. Inset (a) shows surface adsorbed Cp with electron accumulation (red) and depletion (blue) relative to the gas phase neutral species as deduced from DFT calculations.