Anisotropy of surface plasmons in metals

In this paper we propose a phenomenological model to explain the anisotropy in the dispersion of surface plasmons, which we suggest to be related also to the different effective mass of the electrons involved in the excitation. The model is applied to silver surfaces where a large set of experimental data exists, obtaining a good qualitative agreement with experiment. The damping of the surface plasmons is also discussed. Again in agreement with the experimental results, a linear dependence on temperature and momentum transfer is found for the linewidth. The anisotropy of the latter is then determined by the anisotropy of the effective mass and of the relaxation time of the electrons in the surface region, tau(surf), and is small only if tau(surf) approximate to tau(bulk)