The realization of efficient organic electronic devices requires the controlled preparation of molecular thin films and heterostructures. As top-down structuring methods such as lithography cannot be applied to van der Waals bound materials, surface diffusion becomes a structure-determining factor that requires microscopic understanding. Scanning probe techniques provide atomic resolution, but are limited to observations of slow movements, and therefore constrained to low temperatures. In contrast, the helium-3 spin-echo (HeSE) technique achieves spatial and time resolution on the nm and ps scale, respectively, thus enabling measurements at elevated temperatures. Here we use HeSE to unveil the intricate motion of pentacene admolecules diffusing on a chemisorbed monolayer of pentacene on Cu(110) that serves as a stable, well-ordered organic model surface. We find that pentacene moves along rails parallel and perpendicular to the surface molecules. The experimental data are explained by admolecule rotation that enables a switching between diffusion directions, which extends our molecular level understanding of diffusion in complex organic systems.The authors acknowledge financial support from the EPSRC (EP/E0049621, B.A.J.L., D.J.W., D.M.C., A.P.J., J.E., W.A.), the Austrian Academy of Sciences (B.A.J.L.), the Royal Society (A.P.J.), the E.U. ERASMUS programme (A.M.) and the Deutsche Forschungsgemeinschaft (GRK 1782, P.R.). Underlying data are available at the University of Cambridge Research data repository (https://www.repository.cam.ac.uk).This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nmat457
