Knowledge Bank

The method of measurement and the interpretation of the intensities of infrared absorption bands of simple molecules are described. Ordinary methods of making measurements give poor results because the absorption coefficient varies rapidly over the slit width. The true intensity can be obtained by eliminating the violent fluctuations of intensity by (1) Broadening the rotational lines with a nonabsorbing foreign gas and (2) Eliminating the error due to variations of envelope intensity by extrapolating the integrated absorption coefficient to zero partial $pressure.1$ The main emphasis of the paper will be on the interpretation of the intensities in terms of bond constants. Bond moments, $\mu$, and changes of bond moment with distance $\partial \mu /\partial \gamma$ can be obtained from the intensities if the normal modes of motion are known. As an example of the method, some of these parameters in $CO2,CS2$, and OCS are obtained. Whenever a rotational mode has the same symmetry as the vibrational mode, the true vibrational mode must be carefully determined. The future use of absolute intensities to give information for band assignments and to aid in quantitative analysis will be discussed