The influence of crystal orientation on the electron energy loss spectrum of monocrystalline silver films

In electron energy loss measurements (50 keV) on monocrystalline silver films in transmission the intensity of the 3.8 eV volume plasma loss is found to be dependent on the crystal orientation. If the crystal is tilted relatively to the electron beam by an angle α the elastic intensity I_el in the primary direction oscillates by 10%, the intensity of the volume plasmon however varies much stronger similar to I_el; it can become even zero at certain angles α. The intensity of the surface loss is nearly proportional to the elastic intensity. An explanation of this phenomenon is missing. Presumably it must lie in the dynamic theory of electron diffraction.