Abstract
Metals with translational invariance in the XY-directions are studied within the jellium and infinite potential barrier model. Dielectric functions and dispersion relations for plasmas in the small Q limit are obtained by the bubble-diagram and local approximations. It is worthy of note that our surface plasma dispersion, depending on z and having a negative coefficient -3π/16k F for the term linear in Q at surface, is similar to the result of time dependent local density approximation under the description of Lang-Kohn potential and consistent qualitatively with the experimental data. Additionally, due to the z-dependence of the dispersion, it exhibits naturally an intrinsic width for surface plasmons. Our mathematical process is straightforward. It produces correct results for several kinds of systems and helps us comprehending them clearly.