Barbara A. J. Lechner

Atomic scale friction of molecular adsorbates during diffusion

Experimental observations suggest that molecular adsorbates exhibit a larger friction coefficient than atomic species of comparable mass, yet the origin of this increased friction is not well understood. We present a study of the microscopic origins of friction experienced by molecular adsorbates during surface diffusion. Helium spin-echo measurements of a range of five-membered aromatic molecules, cyclopentadienyl, pyrrole, and thiophene, on a copper(111) surface are compared with molecular dynamics simulations of the respective systems. The adsorbates have different chemical interactions with the surface and differ in bonding geometry, yet the measurements show that the friction is greater than 2 ps(-1) for all these molecules. We demonstrate that the internal and external degrees of freedom of these adsorbate species are a key factor in the underlying microscopic processes and identify the rotation modes as the ones contributing most to the total measured friction coefficient.

Jumping, rotating, and flapping: the atomic-scale motion of thiophene on Cu(111)

Self-assembled monolayers of sulfur-containing heterocycles and linear oligomers containing thiophene groups have been widely employed in organic electronic applications. Here, we investigate the dynamics of isolated thiophene molecules on Cu(111) by combining helium spin-echo (HeSE) spectroscopy with density functional theory calculations. We show that the thiophene/Cu(111) system displays a rich array of aperiodic dynamical phenomena that include jump diffusion between adjacent atop sites over a 59-62 meV barrier and activated rotation around a sulfur-copper anchor, two processes that have been observed previously for related systems. In addition, we present experimental evidence for a new, weakly activated process, the flapping of the molecular ring. Repulsive inter-adsorbate interactions and an exceptionally high friction coefficient of 5 ± 2 ps(-1) are also observed. These experiments demonstrate the versatility of the HeSE technique, and the quantitative information extracted in a detailed analysis provides an ideal benchmark for state-of-the-art theoretical techniques including nonlocal adsorbate-substrate interactions.

Quantum Influences in the Diffusive Motion of Pyrrole on Cu(111)

Pyrrole diffuses in channels on Cu(111), hopping between adjacent bridge sites over a barrier above hollow sites. Strong lateral interactions alter the lineshapes in helium-3 spin-echo measurements from a predicted double exponential toward an apparent single exponential decay. Molecular dynamics simulations reproduce the centre-of-mass motion of pyrrole and reveal a friction coefficient of 2.0 (pm ) 0.4 (mathrm{ps}^{-1}). Density functional theory calculations reveal that a large contribution to the experimentally determined activation barrier of 53 (pm ) 4 meV arises from the quantum character of internal vibrational modes.