The spectral region 3700 cm-1 to 500 cm-1 includes almost all the important absorption bands of gaseous molecules. This is the fundamental infrared region where changes in the rotation and vibration of the molecules give rise to the infrared absorption. It has been fashionable lately to call this region mid-infrared, to distinguish it from near infrared and far infrared. In the context of measuring gases, however, mid-infrared is not an appropriate expression because there is nothing of any great value on either side of the fundamental region. HF is the only important molecule that has a strong band at frequencies higher than 3700 cm-1. At higher frequencies there are only overtones, which have extremely small absorption coefficients. At lower frequencies there may be good strong rotation lines, but because of extremely strong absorption by water vapor they are quite inaccessible. For these reasons we choose to retain the traditional name The Fundamental Infrared Region.

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