9.1: The Magnitude of Molar Absorptivities

Molar absorptivities,$${\epsilon}$$, have units of liters/(mole cm) and range in value from 0 to 105.  The relationship between $${\epsilon}$$ and the capture cross-section for a photon - chemical species interaction and the probability for an energy-absorbing transition was shown by Braude (J. Am Chem Soc, 379 (1950) to be

$${\epsilon}$$ = 8.7 x 1019 PA

where P is the transition probability and A is the section target area in units of cm2.  Typical organic molecules have been shown to have cross-sectional areas on the order of 10-15 cm2 and transition probabilities range from 0  to 1.

Absorption bands with $${\epsilon}$$ ranging from 104 to 105 are considered strong absorbers while absorption bands with $${\epsilon}$$ less than or equal to 103 are considered weak absorbers and are likely the results of quantum mechanically forbidden transitions with P < 0.01.