Origin of the Characteristic Electron Energy Losses in Aluminum

The characteristic electron energy loss spectrum of aluminum has been measured by analyzing the energy distribution of 760-, 1000-, 1520-, and 2020-ev electrons scattered by an evaporated specimen through 90'. Twelve loss peaks were observed, made up of combinations of elementary 10.3- and 15.3-ev losses. The former, the low-lying loss, is identified with the lowered plasma loss proposed by Ritchie, and the latter with the plasma loss proposed by Bohm and Pines and previously observed by many other workers. In measurements made with very thin evaporated targets, it was found that the low-lying loss changed considerably in position, as well as in intensity relative to the 15.3-ev loss. These changes, which are interpreted in terms of Ritchie s theory, definitely indicate that the low-lying loss is inQuenced by the surface layers of the specimen. As targets of high surface and volume purity could be prepared, it is concluded that results obtained by the present reQection technique, when examining loss behavior affected by surface phenomena, are superior to measurements of the characteristic loss spectrum of electrons transmitted through thin 61ms. SUMMARY OF PREVIOUS %'ORE HE 6rst systematic measurements of characteristic electron energy losses were carried out by Rudberg' who analyzed the energy distribution of 50— 400 ev electrons scattered from the surfaces of a number of metals. In each energy distribution he found peaks which occurred at fixed energy displacements from the peak of elastically scattered electrons, irrespective of the primary bombarding energy or the scattering angle. These peaks therefore corresponded to electrons which had lost definite amounts of energy in the material. Rudberg and Slater' proposed that the energy losses were due to excitation of conduction electrons to higher allowed energy levels, and calculated the form of the characteristic loss spectrum of copper, in fair agreement with Rudberg's results. Ruthemann' was the first to extend the measurements to the energy analysis of electrons transmitted through thin films of various materials. Since then, nearly all of the published work has been concerned with the latter type of measurement, and in this paper the two techniques will be referred to as reQection and transmission experiments, respectively. A large number of workers4 " has investigated the * Work supported by the Research Grants Committee of the