Collective excitations of two layers of K on Ni(111)

The nature and the energy dispersion of the collective excitations of two layers of K adsorbed at 200 K on Ni(111) have been studied by high-resolution electron-energy-loss spectroscopy. At small momentum transfer parallel to the surface, q_∥ , the loss spectrum showed two collective modes: an intense feature close to 2.70 eV, attributed to the surface plasmon, and a broad peak at 3.40 eV, interpreted as the multipole surface plasmon. Contrary to theoretical prediction for two layers of K on Al(111), present results demonstrate that in the limit of small q_∥ the excitation of the surface plasmon for two K layers on Ni(111) is possible.