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Development and application of evanescent wave cavity ring-down spectroscopy for studies of electrochemical and interfacial processes

By Mathias Schnippering


This thesis is concerned with the application of evanescent wave\ud cavity ring-down spectroscopy (EW-CRDS) and evanescent wave\ud broadband cavity enhanced absorption spectroscopy (EW-BB-CEAS) for\ud studies of electrochemical and interfacial processes. These include\ud nanoparticle adsorption/dissolution, polymer nanoparticle formation and\ud surface-bound electrochemical redox reactions. Different experimental\ud setups have been designed to investigate these systems.\ud EW-CRDS is a surface sensitive technique, which allows\ud absorption measurements at solid/liquid and solid/air interfaces. Surface\ud reactions can easily be monitored in real time. A pulsed or modulated laser\ud beam is coupled into an optical cavity which consists of at least one optical\ud element, in which the beam is total internal reflected. At the position of\ud total internal reflection (TIR), an evanescent field is established with the\ud amplitude decaying exponentially with distance from the boundary. The\ud evanescent field can be exploited to investigate the absorbance properties\ud of the liquid phase in the first few hundred nanometres of the solution\ud above the silica surface. These types of instruments have high temporal\ud resolution (up to 2 kHz repetition rate), coupled with high sensitivity\ud (minimum detectable interfacial absorbance per pass: ~80 ppm) which\ud enables the investigation of a variety of processes relating to fundamental\ud questions in the field of physical chemistry and materials science. The\ud aforementioned sensitivity and resolution make EW-CRDS an ideal tool\ud for those investigations, especially if combined with other techniques such\ud as electrochemistry or microfluidic and hydrodynamic techniques. In this\ud thesis, different instrumentational setups will be discussed.\ud EW-BB-CEAS is another example for a TIR based absorption\ud spectroscopic technique and can give additional spectral information about\ud the investigated surface processes by employing broadband light such as\ud supercontinuum radiation. In this case, the amplified light intensity within\ud the optical cavity is measured rather than the light decay.\ud By employing complementary techniques, such as electrochemistry\ud and atomic force microscopy and by fitting experimental data using finite-element\ud modelling, surface processes can not only be described accurately\ud but also kinetic information such as rate constants for the aforementioned\ud systems can be calculated

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