Reflected Electron Energy Loss Spectroscopy (REELS) is a surface‑analysis technique used to study the electronic properties of materials by examining how electrons lose energy when they interact with a surface. Although it belongs to the broader family of electron energy loss spectroscopy (EELS) methods, REELS focuses specifically on electrons that are reflected back from the surface after interacting with the material. The process begins by directing a beam of monoenergetic electrons—typically with energies below 2 keV—onto the sample. As these electrons strike the surface, they undergo a variety of interactions. Some electrons scatter elastically, meaning they bounce back without losing any energy. Others scatter inelastically, transferring part of their energy to the material. This energy transfer can excite different electronic processes, such as collective oscillations of electrons (plasmons) or transitions between electronic bands. In REELS, both the elastically and inelastically scattered electrons are collected and analysed using the AXIS electron energy analyser. The resulting spectrum shows how many electrons have lost specific amounts of energy. This pattern of energy losses provides valuable information about the material’s electronic structure. The low-energy electron energy loss spectrum (up to ~50eV in energy loss) contains the elastic, or zero-loss, peak as well as the plasmon peaks. This spectrum can be used to determine information about the band structure and dielectric properties of the sample. The high energy-loss region, generally accepted as >50 eV, contains the ionisation edges due to inner shell ionisations. It is these features which are the exact equivalent of an absorption edge in XAS, arising from the same process. They are characteristic of the elements present in the sample and their local bonding environment, providing the atomic and chemical bonding information. Calculated sampling depth for a 1 keV REELS experiment is ca. 15 nm, although it is noted that this will be dependent on the material being analysed and the energy of the primary electron beam. Information from the REEL spectrum is therefore complementary to other surface sensitive techniques, including XPS. - Probe surface valence and conduction band electronic properties. - Band gap determination for semiconductor materials. - Comparison of relative hydrogen content in polymer materials. - Investigation of conjugation or aromaticity in organic materials. - Determination of sp 2 / sp 3 hybridisation of carbon
