The Experimental Determination of the Intensities of Infra-Red Absorption Bands I. Theory of the Method

Abstract

With spectrographs of available resolving power, the apparent integrated absorption coefficients of infra‐red bands usually differ greatly from the true coefficients because the spectrograph yields at each setting a weighted average of the fractional light transmitted in a band of frequencies, whereas what is desired is the unweighted average of the logarithm of the fractional light transmitted. It is shown that true absorption coefficients can be obtained by (a) eliminating the violent fluctuations in intensity with frequency by broadening the rotational lines with a non‐absorbing foreign gas and (b) eliminating the error due to the intensity variation of the envelope by extrapolating the apparent integrated absorption coefficient divided by the partial pressure to zero partial pressure of the absorbing gas. These two steps permit vibrational intensities to be measured to a reasonable accuracy even with a spectrograph of low resolving power.