Anisotropic atomic motions in structural analysis by low energy electron diffraction

The structure of the 3 3 R30^ overlayer formed by CO molecules adsorbed on a Ru (0001) was analyzed by low energy electron diffraction. Anisotropic atomic motions under the influence of thermal excitation were taken into account by adopting the concept of split positions. Apart from considerable improvement in the structural refinement this technique provides information about dynamic processes. In particular, the molecular axis of the CO molecules was found to be tilted on the average by (123^ at 150 K, which is attributed to excitation of the bending mode vibration (i. e. , frustrated translation).