Determination of Surface-Plasmon Dispersion Relation by Ricochet Photoemission

Photoelectrons emitted along a metal surface from atoms located at a small distance outside that surface have a high probability to ricochet with the excitation of a surface plasmon if the angle of emergence $\alpha$ is less than a critical angle ${\alpha }_M$. The line shape of the surface-plasmon satellite $I_1({\varepsilon }_k,\alpha )$ peaks strongly at $\alpha =0\mathrm{}$ and presents another peak at $\alpha ={\alpha }_M$. Experiments are suggested to locate the second peak and thereby determine ${\alpha }_M\left({\varepsilon }_k\right)$, a function which should yield direct information on the form and the parameters of the surface-plasmon dispersion relation.