9.1: The Magnitude of Molar Absorptivities

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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.