External reflectance, electrochemical systems

In Secs. 5-10 we present a series of selected examples of the use of the external reflectance technique to investigate some electrochemical systems of interest. Results from the electrochemical literature on the adsorption of hydrogen, carbon monoxide and alcohols are discussed and compared with the data from UHV measurements (Secs. 5-7). 

In order to calculate A.I/ I) from the measured PM IRRAS spectra, one has to determine functions J2 and Jq in an independent experiment. A reliable method to measure the PEM response functions was described by Buffeteau et al. [69]. Below we describe a similar method that we adapted with minor changes to use for electrochemical systems [81]. The spectroelectrochemical cell is replaced by the dielectric total external reflection mirror (a Cap2 equilateral prism can be used for this purpose). The second polarizer is inserted just after the PEM and set to admit p-polarized light (identical setting to that of the first polarizer). The PEM is turned off and the reference spectrum is acquired. This spectrum gives the intensity of the p-polarized light Ip (cal), which passes through the whole optical bench. 

Though the internal reflection mode (ATR) encounters many applications, the external reflection mode is much more versatile to study metallic electrode surfaces. The external reflection mode has been introduced to study metallic electrode surfaces by Bewick and coworkers [10]. To minimize the strong attenuation of the radiation by the presence of a solvent in electrochemical systems, the electrode was placed very close to the infrared transparent window. Practically, the electrode is slightly pressed against the flat window surface leaving a very thin electrolyte layer between the electrode and the window. 

Ever since the first reports of optical studies of electrochemical systems, efforts have been made to obtain infrared spectra of reaction intermediates and adsorbates. The earliest studies were based on total internal reflection using an n-type germanium electrode (transparent to IR radiation), and OTTLE systems using gold minigrids sandwiched between NaCl plates. These were not particularly successful, however, and it is only recently that these configurations have again been used, this time for Fourier Transform spectroscopy [29,30]. Undoubtedly the most successful technique has been potential modulated external reflectance IR spectroscopy [31]. 

IR spectroelectrochemistry has been the subject of a sizeable amount of early reviews, where the experimental details and applications have been described [5-7]. Regardless the fact that electrochemistry is an extremely broad field, the following discussion will be restricted to classical electrochemical systems where a solid electrode is in contact with a liquid electrolyte solution which may contain electroactive species. Since the typically used electrolyte solutions (mostly aqueous solutions) are strongly IR absorbing, it is not possible to use a standard laboratory electrochemical cell, but for spectroelectrochemical experiments, special cell designs and beam paths have to be employed. There are two general principles on how the IR beam is directed to the electrode surface called internal reflection and external reflection, respectively. 

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