Set-ups in reflectance measurements

In Fig. 4.25 a typical arrangement of the measurement of diffuse reflectance is given. Such a set-up is commercially available and any thin-layer spectrometer can be used which normally scans the thin-layer plates. 

The collimated optics of the set-up demonstrated can be substituted by fibre cities. This type of equipment is shown in Fig. 4.11 using a bifurcated fibre and a diode array. Under these conditions white light has to be used to produce a reflection spectrum monitored by the diode array. However, white light causes undefined photochemical conditions. For this reason a third fibre arm is used, which guides monochromatic radiation to the sample. If the white radiation is blocked by a shutter, it may not disturb the monochromatic photoirradiation. Since modem diode arrays take spectra within a few milliseconds the time of measurement stays short compared to the total photochemical process time. This type of equipment can be used to monitor the photoprocess during irradiation of photoresists [101]. 

The spectroscopic observation of rather concentrated solutions is of broad interest in photometry. Because of the necessary transmission of solutions, the concentrations in normal direct spectroscopy measurements in photokinetics are restricted to approximately 10 molar solutions. In many experiments - especially in the case of preparative photochemistry (whoe often 

If irradiation passes the interface from an optically denser medium to one of smaller refractive index, at angles larger than a certain limiting one (with respect to the perpendicular optical axis) the radiation will no longer leave the medium. It is totally reflected at the interface. Electrodynamics explain the fact that the reflected radiation couples to an evanescent field, which exponentially decreases into the medium of lower optical density. Thus the reflected beam gathers information about the optical properties of this medium across the interface. 

Such a device is demonstrated in Fig. 4.26 for a highly absorbent solution. In a flbre the radiation is guided by multiple total reflection. This analytical finger can measure changes in the solution [98,103]. 

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