Zinc selenide infrared-transparent material

In some applications, such as in the field or in humid environments, KBr is simply too moisture sensitive to be used as a beamsplitter window. An alternative is zinc selenide (ZnSe). This material is infrared transparent and is not hygroscopic, so it is frequently used in FTIRs that are used outdoors, on factory floors, outside the lab, etc. For aU its utility there are two problems with ZnSe beamsplitters. First, ZnSe absorbs strongly starting around 700 cm , and any spectral features below this will be lost. Second, a ZnSe beamsplitter adds several thousand dollars to the cost of an instrument. Another beamsplitter window material in use is cesium iodide (Csl). This material is transparent down to 200 cm in the far infrared and can be used if your samples have important features between 400 and 200 cm. However, Csl is very hygroscopic, and windows made from it have to be replaced frequently due to excessive absorption of moisture. 

In a simple transmission experiment the liquid sample is examined in a cell made of a suitable infrared transparent medium. These include sodium chloride, potassium bromide, zinc selenide, cadmium telluride, and germanium. Materials like sodium chloride should not be used to study solutions in protic solvents like methanol and water. 

In the past ATR crystals were shaped into long, thin rods or parallelograms and were made of high-refractive-index materials that were transparent in the mid-infrared such as zinc selenide (ZnSe), silicon, and germanium. These crystals frequently had multiple hot spots on their surface, as illustrated in Figure 4.49. The total path-length for this type of ATR experiment is simply equal to the depth of penetration of the beam into the sample for each hot spot times the number of hot spots covered by the sample. For example, in Figure 4.49 if the beam penetrated 1 micron at each hot... 

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