Optical Spectroscopy in the Visible Range

General information about preparation, purification, etc. of solvents for electrolyte solutions is provided by Mann [11]. Suggestions for further reading are provided only where texts deemed suitable are available. 

Fundamentals. Species involved in electrochemical reactions that show optical absorption caused by electronic transitions in the UV-Vis region of the electromagnetic spectrum can be studied in situ with a variety of spectroelectrochemical techniques. The choice of a suitable technique depends on the type of species to be investigated and upon the exact location of the species (e.g. directly adsorbed on the metallic electrode surface, incorporated in a film attached to the electrode surface, dissolved in the electrolyte solution). Basically, two families of spectroelectrochemical techniques have been established so far (see Fig. 5.1)  

We will describe these techniques in detail. Methods where absorption of light causes emission of species, e.g. photons or electrical currents across the electrochemical interface, are treated in Sects. 5.1.6 and 5.1.9. 

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Determination of the stratum corneum depth by Optical Spectroscopy in the Visible Range... 

H.-J. Weigmann, J. Lademann, H. Meffert, H. Schaefer, and W. Sterry. Determination of the horny layer profile by tape stripping in combination with optical spectroscopy in the visible range as a prerequisite to quantify percutaneous absorption. Skin Pharmacol. Appl. Skin. Physiol. 12 34-45 (1999). 

The presence of this and other materials presents potential problems in industrial processing, and there is great interest in characterization of the chemical nature of the vanadium species present. EPR is most widely used in these studies since the metal is in the oxo-vanadium(IV) state. Optical spectroscopy in the visible region can also be used on extracts since oxo-vanadium(IV) porphyrins, which absorb at around 572 nm and 534 nm 116,119), can readily be detected. However, it has been shown (120-122) that the total amount of vanadium present in crude oils and tar sand bitumen is higher than can be accounted for by the presence of oxo-vanadium(IV) porphyrins. It has therefore been suggested that the vanadyl may be bound to a range of different tetra-dentate ligands in crude oils (120-123). 

In Chap. 6, we discussed low-energy optical excitation states, the singlet and triplet excitons and energy transfer. The primary experimental method applied there was optical spectroscopy in the visible, in the near IR and in the UV spectral ranges. In the present chapter, we treat the structure and the dynamics of localised triplet states, of triplet mini-excitons, and of triplet excitons in molecular crystals. The primary experimental method for the investigation of the lowest-energy triplet level Ti is electron-spin resonance (ESR) (Fig. 7.1). 

Laboratory spectrophotometers and fluorimeters which allow monitoring of reactions that take place within a few seconds are fairly routine now, and time resolutions of tenths of a second are available detailed descriptions are available in texts on spectroscopy [26]. Solution cells may be glass or plastic for light in the visible range, but quartz cells are needed for UV work. In an absorption spectrophotometer, the light source and photodetector are in line. In a fluorimeter, the detector is at 90° to the incident light, so cells must have four optical faces. 

Sampling in surface-enhanced Raman and infrared spectroscopy is intimately linked to the optical enhancement induced by arrays and fractals of hot metal particles, primarily of silver and gold. The key to both techniques is preparation of the metal particles either in a suspension or as architectures on the surface of substrates. We will therefore detail the preparation and self-assembly methods used to obtain films, sols, and arrayed architectures coupled with the methods of adsorbing the species of interest on them to obtain optimal enhancement of the Raman and infrared signatures. Surface-enhanced Raman spectroscopy (SERS) has been more widely used and studied because of the relative ease of the sampling process and the ready availability of lasers in the visible range of the optical spectrum. Surface-enhanced infrared spectroscopy (SEIRA) using attenuated total reflection coupled to Fourier transform infrared spectroscopy, on the other hand, is an attractive alternative to SERS but has yet to be widely applied in analytical chemistry. 

In addition to transfer techniques in quantitative analyses, such as polarography, titrimetry, spectroscopy and other analytical methods used after separation by TLC, the in situ optical measurement is the most widely employed technique for quantitative determinations. In most cases UV-absorption is used, while coloured substances can be determined by absorption measurement in the visible range of the spectrum. Fluorescent substances are preferably determined by fluorescence measurement. Infrared absorption techniques are not used in routine analysis up to this date. 

Unlike Raman or infrared spectroscopy, with their well-defined capability of determining vibrational properties, the application of optical spectroscopy in the UV-visible range is less focused. This is because UV-visible reflectance spectroscopy intrinsically is to yield electronic excitation spectra, which unfortunately for solids are often broad and unstructured and hence not very informative as such. Therefore, this technique has increasingly been used for other tasks also, such as the determination of coverages, the assessment of structural information, and the study of surface roughness. The aim of this chapter was to demonstrate this versatility. 

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