How to probe structure, kinetics and dynamics at complex interfaces in situ and operando by optical spectroscopy

Techniques using light in a large spectral range are well suited to study interfaces. Photon-based techniques have a special strength when it comes to application in situ and operando. This article introduces some of the most commonly used experimental techniques. Absorption spectroscopy in the far-, mid-, and near-infrared (IR), in the visible (VIS) and ultraviolet (UV) is a simple method to obtain the interface equivalent to the type of information normally obtained by the respective bulk method. Also photoluminescence/fluorescence is used to study interfaces. Polarization, for example, in ellipsometry, or polarization-modulated spectroscopy can increase the interface specificity. The same could be achieved by surface enhancement, for example, in surface-enhanced Raman spectroscopy. IR/VIS sum frequency generation spectroscopy (SFG) has intrinsic surface specificity. Main working principles of these techniques are introduced, with reference to more detailed coverage elsewhere, and examples of application. Overall, the techniques are suitable to yield qualitative and quantitative information on which species are present at interfaces, in which conformation state or which defects develop in solids. Frequently, also information on orientation of species can be obtained.