Low frequency vibrational modes of adsorbates

The vibrational spectrum of adsorbate systems reveals characteristic low energy modes of typically ⩽15 meV which belong to external modes. Due to their low frequency they are thermally occupied even at low temperatures and play an important role in a variety of surface phenomena such as energy accommodation, diffusion and sliding friction. Whereas these properties have been studied quite extensively for model systems of isolated adsorbate on low indexed surfaces, the situation becomes more complex in case of saturated monolayers. In this brief review some related properties of the low energy vibrational dynamics of more complex systems are discussed including the influence of defects, mode identification in saturated monolayer and the coupling with surface phonons. Finally also the low energy dynamics of large adsorbate molecules is briefly discussed.