arXiv subjects

Emanuel Bahn

Publications and source records attributed to Emanuel Bahn.

3 recordsLinked to original sources

arXiv2022

Structure and dynamics investigations of a partially hydrogenated graphene/Ni(111) surface

Using helium-3 atom scattering, we have studied the adsorption kinetics, the structure and the diffusional dynamics of atomic hydrogen on the surface of a graphene monolayer on Ni(111). Diffraction measurements reveal a 4$^\circ$ rotated rectangular hydrogen overstructure. Hydrogen adsorption and desorption exhibit activation barriers of $E_a=(89\pm7)\,$meV and $E_d=(1.8\pm0.2)\,$eV, respectively. Helium-3 spin-echo measurements showed no decay of the spatial correlation function (or intermediate scattering function) within the time range of the spectrometer. Hence, we are able to set lower limits for a possible hydrogen surface diffusion rate.

cond-mat.mes-hall↗

arXiv2021

Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene

The interfacial behaviour of water remains a central question to fields as diverse as protein folding, friction and ice formation[1,2]. While the structural and dynamical properties of water at interfaces differ strongly from those in the bulk, major gaps in our knowledge at the molecular level still prevent us from understanding these ubiquitous chemical processes. Information concerning the microscopic motion of water comes mostly from computational simulation[3,4] but the dynamics of molecules, on the atomic scale, is largely unexplored by experiment. Here we present experimental results combined with ab initio calculations to provide a detailed insight into the behaviour of water monomers on a graphene surface. We show that motion occurs by activated hopping on the graphene lattice. The dynamics of water diffusion displays remarkably strong signatures of cooperative behaviour due to repulsive forces between the monomers. The repulsive forces enhance the monomer lifetime ($t_m \approx 3$ s at $T_S = 125$ K) in a $\textit{free-gas}$ phase that precedes the nucleation of ice islands and, in turn, provides the opportunity for our experiments to be performed. Our results give a unique molecular perspective of barriers to ice nucleation on material surfaces, providing new routes to understand and potentially control the more general process of ice formation.

cond-mat.mtrl-sci↗

arXiv2020

Inter-adsorbate forces and coherent scattering in helium spin-echo experiments

In studies of dynamical systems, helium atoms scatter coherently from an ensemble of adsorbates as they diffuse on the surface. The results give information on the co-operative behaviour of interacting adsorbates and thus include the effects of both adsorbate-substrate and adsorbate-adsorbate interactions. Here, we discuss a method to disentangle the effects of interactions between adsorbates from those with the substrate. The result gives an approximation to observations that would be obtained if the scattering was incoherent. Information from the experiment can therefore be used to distinguish more clearly between long-range inter-adsorbate forces and the short range effects arising from the local lattice potential and associated thermal excitations. The method is discussed in the context of a system with strong inter-adsorbate interactions, sodium atoms diffusing on a copper (111) surface.

cond-mat.mtrl-sci↗