New theoretical results are presented for the probability of exciting surface oscillations (optical phonons or plasmons) by fast electrons reflected from the surface of a thin crystal film. Both specular and Bragg reflections are considered and the effect of the finite slab thickness is included. The theory explains successfully the energy-loss spectra measured by Powell on metallic surfaces and recent measurements by Ibach on ZnO surfaces.

• Received 10 November 1970

DOI:https://doi.org/10.1103/PhysRevLett.26.229