Transmission electron energy-loss spectroscopy

Abstract

With 22 Figures High-energy electron energy-loss spectroscopy (EELS) in transmission has been in almost continuous use for about 50 years to probe the electronic structure of solids. The reason for this is, firstly, that high-energy electrons with a narrow energy distribution can be easily produced and, secondly, that EELS in transmission can be performed under normal vacuum conditions since it is not a surface-sensitive technique such as, e.g. photoemission or inverse photoemission spectroscopy which need ultra-high vacuum conditions. The Franck-Hertz phenomenon in gases which has provided information on electron energy levels in gaseous atoms and molecules was first applied to solids in 1929 by Rudberg E7.1] performing EELS measurements in reflection on noble metals. The first high-energy (2-8 keV) EELS measurements in transmission were performed in 1941 by Ruthemann [7.2]. In these measurements, discrete excitations in simple metals were recorded which were later explained by Pines and Bohm [7.3] in terms of collective excitation of conduction electrons, i.e. plasmons. Ruthemann reported also in these early days [7.4], excitations of core-level electrons in collodium by EELS in transmission. Both plasmons (for which EELS is the primary source of information) and core-level absorption edges are still the main subjects of investigations by EELS in transmission. Plasmons are caused by a collective excitation of free electrons (intraband plasmons) or a collective excitation of bound electrons (interband plasmons). The latter are related to interband transitions and therefore to the joint density of states between occupied and unoccupied states. Contrary to optical spectroscopy, the momentum between ground state and excited state can be varied systematically in EELS and therefore information on the momentum dependence of the joint density of states is provided. Those results are related to the dispersion of occupied and unoccupied bands. On the other hand, the momentum dependence of the excitation energy ofplasmons yields important information on the electron-electron interaction in electron liquids. Finally, excitations of core-level electrons yield information similar to X-ray absorption spectroscopy (XAS, Chaps. 5 and 6) on the unoccupied density of states at the excited atoms, provided that the interaction of the conduction electrons with the core hole is small. If the latter point is not fulfilled, information is at least provided on the total amount of unoccupied states at that site and often on the nature of the ground state. At Small momentum transfer, dipole-selection rules apply and therefore only a definite symmetry of unoccupied states or multiplet structure is probed. However , in EELS the momentum transfer can also be increased so that other