Low Energy Electrons for the Investigation of Liquid Surfaces

Abstract

Studies of liquid surfaces with electron spectroscopy are comparetively scarce even though the microscopic understanding of these systems could benefit to the same extent as is the case for surfaces of solids. The short mean free path λ_e of slow electrons in condensed matter ensures that electrons leaving the surface must originate from a site which is separated from the very surface by at most a few times λ_e. This holds for XPS(=x-ray photoelectron spectroscopy), UPS (=ultraviolet photoelectron spectroscopy), AES (=Auger electron spectroscopy) and EELS(=electron energy loss spectroscopy). Indeed, these techniques have been used by a small number of groups for the investigation of liquid surfaces. The first such study was published by H. and K. Siegbahn on formamide using XPS ¹, followed by a large number of works by H. Siegbahn and coworkers on several liquid surfaces, e.g. solutions of surface active salts ^2,3,4. Two groups have made use of the HeI resonance line to perform UPS on liquid surfaces ^5,6. Ballard showed that AES and EELS (with energy losses in the eV range) could reproducibly be carried out on these systems ⁷ even though there remains the need to understand the precise contribution of these methods to the diagnosis of liquid surfaces. The above mentioned spectroscopies have in common that their surface sensitivity is governed by the value of the mean free path λ_e of the emitted electrons. There is, however, one electron spectroscopy that is distinguished in that its observation depth is not related to a mean free electron path. It is MIES (=Metastable Induced Electron Spectroscopy), a technique which monitors exclusively the topmost layer of a surface⁸. The energy needed for electron emission is supplied by the excitation energy (19.819 eV) of metastable helium atoms He *(1s2s,³S). The surface sensitivity of MIES goes so far that even the orientation of the molecules in the topmost layer can be determined ⁸.