Metastable induced electron spectroscopy (MIES)

About this technique

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Metastable induced electron spectroscopy allows surface analysis with excellent sensitivity. It measures the electron density on the very surface of the sample facilitating the analysis of surface composition and electronic structure. It can be applied to a large range of different samples including liquid surfaces generated as surfaces of thin liquid films, and solid surfaces such as polymers and insulators mounted on a manipulator. Examples described in the literature include the analysis of catalysis, corrosion, surface coatings with monolayers, surfactant solutions, self-assembled monolayers and nanoparticles. MIES averages the surface composition over an area of 1 mm² up to a few mm².
In a MIES experiment the valence electrons, those that facilitate all chemical bonds, are excited with metastable helium atoms and their binding energy is determined by measuring the kinetic energy of emitted electrons, in a similar way to UV-photoelectron spectroscopy. The spectra show, however, only features that are related to those electron orbitals in the outermost layer. The same molecule placed in two different orientations on a surface will result in two readily distinguishable spectra. For instance, if alkane molecules are adsorbed onto graphite they lie lengthwise along the surface, exposing both the CH₂ backbone as well as the CH₃ terminal groups. If alkanethiolate molecules are adsorbed onto gold through the thiol group at one end only the terminal CH₃ groups at the other ends of the molecules will be exposed to the environment. MIES spectra generated from these two samples would distinguish the different orientation of the molecules on the surface by detecting either the CH₂ or CH₃ groups constituting the outermost layer.