Infrared absorptions occur over a range of frequencies, and absorption coefficients measured at any single frequency may not express the true intensity of a molecular transition. The integrated absorption coefficient is the sum of the absorption coefficients for all such frequencies in the band <i>A</i> = ∫ &epsi;(<i>v</i>) d<i>v</i>. Integrated absorption coefficients have been used for a variety of applications, for the most part ignoring any inherent differences in molar absorptivity for molecules in different states of aggregation. Eischens and Jacknow have used a system using combined gravimetry and spectroscopy to calculate the &epsi; for adsorbed species; that is, the extinction coefficient for a functional group of a molecule adsorbed on another surface. They also report that the &epsi; for carbon monoxide on a surface vs. the same compound in solution can be assumed to be the same within the experimental accuracy; whether this conclusion can be generalized is not known.

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