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The dynamics of thermally activated motion of cyclopentadienyl and pyrrole on Cu(111)

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Atomic-scale measurements of the dynamics of cyclopentadienyl (Cp-), C5H5 -, and pyrrole, C4H4NH , on Cu(111) have been carried out using helium spin-echo spectroscopy (HeSE) which provides a unique means of studying surface-dynamic processes on Ångström length scales and picosecond time scales.

The cyclopentadiene (C5H6, CpH) molecule adsorbs dissociatively on a Cu(111) surface resulting in a predominantly ionically bound aromatic species of high anionicity (Cp-). Our HeSE data shows the characteristic lineshapes predicted for single jumps between different sites within the basis of the underlying lattice, in our case energetically equivalent fcc and hcp hollow sites. Temperature-dependent measurements allow us to identify an activation barrier for translation of 41±1 meV, indicating that Cp- is highly mobile. Using molecular dynamics (MD) simulations we determine an effective potential energy landscape and a remarkably high adsorbate-substrate friction parameter for the system similar to that observed for another molecular adsorbate system, benzene on graphite. Contrary to what one would expect from highly charged adsorbates, no evidence for strong repulsive interactions could be found in the HeSE data.

In contrast to the negatively charged Cp-, pyrrole adsorbs as a neutral species on the Cu(111) surface. Surprisingly, however, the HeSE data shows significant evidence for strong repulsive interactions between the pyrrole molecules. In addition to activated jump diffusion between lattice sites the dynamics measurements show evidence for possible rotational motion of the molecule.

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