Experimental observation of defect pair separation triggering phase transitions.

First-order phase transitions typically exhibit a significant hysteresis resulting for instance in boiling retardation and supercooling. The hysteresis arises, because nucleation of the new phase is activated. The free-energy change is positive until the nucleus reaches a critical size beyond which further growth is downhill. In practice, the barrier is often circumvented by the presence of heterogeneous nucleation centres, e.g. at vessel walls or seed crystals. Recently, it has been proposed that the homogeneous melting of ice proceeds via separation of defect pairs with a substantially smaller barrier as compared to the mere aggregation of defects. Here we report the observation of an analogous mechanism catalysing a two-dimensional homogeneous phase transition. A similar process is believed to occur in spin systems. This suggests that separation of defect pairs is a common trigger for phase transitions. Partially circumventing the activation barrier it reduces the hysteresis and may promote fluctuations within a temperature range increasing with decreasing dimensionality.

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.

Note: a new design for a low-temperature high-intensity helium beam source.

A high-intensity supersonic beam source is a key component of any atom scattering instrument, affecting the sensitivity and energy resolution of the experiment. We present a new design for a source which can operate at temperatures as low as 11.8 K, corresponding to a beam energy of 2.5 meV. The new source improves the resolution of the Cambridge helium spin-echo spectrometer by a factor of 5.5, thus extending the accessible timescales into the nanosecond range. We describe the design of the new source and discuss experiments characterizing its performance. Spin-echo measurements of benzene/Cu(100) illustrate its merit in the study of a typical slow-moving molecular adsorbate species.

Weak intermolecular interactions in an ionically bound molecular adsorbate: cyclopentadienyl/Cu(111).

The dissociative adsorption of cyclopentadiene (C(5)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±3 meV with strong frictional dissipation. Repulsive dipole-dipole interactions are not detected despite charge transfer from substrate to adsorbate.