MOVES for Ion- und Electron-Induced Electron Emission

This proposal describes planned research on ion- and electron induced electron emission using the existing MOVES (MOmentum Vector Electron Spectrometer) apparatus. Modifications of the pulsed source of primary particles to enhance the energy resolution of the experiment as well as the use of electrons as projectiles are envisioned within this project. After these modification, the MOVES spectrometer will provide high quality experimental data that is unique in the sense that it not only allows one to record the angular and energy distribution of the emitted electrons as well as the corresponding statistics, but the data also automatically contain all information on correlations between different parts of the angular and energy spectrum. Such data should greatly improve our understanding of secondary electron emission (SEE) and ion induced potential and kinetic electron emission (IIEE), both phenomena of fundamental interest and practical importance. The goals of the planned research comprise: (1) development of general purpose transfer optics for production of picosecond ion pulses; (2) measurements of the excitation and concurrent decay of surface and bulk plasmons in nearly-free-electron (NFE) materials; (3) measurements of excitation and decay of surface and bulk modes in non-NFE materials; (4) Investigation of correlated electron emission using a reflection geometry; (5) Study of near-and sub-threshold ion induced kinetic electron emission; (6) Ga-ion induced electron emission from solid surfaces.

Emission of slow electrons from surfaces and, in general, the interaction of low energy electrons with solid materials is important in many technological applications, such as plasma displays, particle accelerators and storage rings, e.g., the large hadron collider at CERN, etc. Interaction of low energy electrons with solids is also important for health related issues since strand breaking in DNA as a consequence of exposure to ionising radiation is to an important extent mediated by slow electrons which play an essential role in the thermalisation of the energy deposited by the incoming high energy particle. Investigation of low energy electrons travelling through solids is very challenging for several reasons, among the most important ones the fact that while the excitation of the solid state can be studied in a straightforward way by measuring energy losses of electrons incident on a surface, formation of the so-called secondary electron cascade obscures any features of individual interactions in the emitted spectra of slow electrons. Therefore, although electron emission from surfaces is a well known phenomenon for over 100 years, it is still a poorly understood phenomenon.In Project P20891-N20 an efficient methodology for the experimental investigation of emission of slow electrons from solids was further developed. This technique, for which the acronym SE2ELCS (Secondary Electron-Electron Energy Loss Coincidence Spectroscopy) was coined, measures coincidences of electrons that are reflected from a surface after suffering an energy loss with secondary electrons to which the energy is transferred. In this way, it becomes possible to study the correlation between energy losses of energetic particles (e.g. electrons) in solids and the slow electrons which are produced as a result. The results indeed exhibit a wealth of structure as compared to the non-coincident secondary electron spectra and reveal an interesting novel mechanism for electron emission: super-surface electron emission, in which a primary electron in vacuum in the immediate vicinity of the vacuum-solid boundary induces emission of a secondary electron from the very surface of the solid, i.e. with a depth of emission essentially equal to zero (!). This follows from a comparison of double differential coincidence spectra with theoretical calculations which were developed for this purpose. The theoretical work carried out within the project also pointed out two other interesting details concerning the dynamical interaction of electrons with solid surfaces: (1) an asymmetry in the energy loss probability with respect to the direction of surface crossing and (2) the possibility to directly experimentally observe super-surface scattering of electrons near solids in a reflection geometry. Both effects were experimentally confirmed within the project.