absorptivities

Absorptivity is a property of a material that describes how readily it absorbs electromagnetic radiation at a given wavelength. It is inherently wavelength dependent and is used in fields such as chemistry, spectroscopy, and optics. The term is applied in slightly different ways depending on context, sometimes referring to a spectral or wavelength-specific quantity that characterizes absorption.

In solution spectroscopy, a closely related and widely used quantity is molar absorptivity (also called molar extinction coefficient), ε(λ). According to Beer-Lambert law, the absorbance A at a wavelength is A = ε(λ) c l, where c is the solute concentration and l is the path length. Absorbance is dimensionless, while ε has units of L mol^-1 cm^-1, chosen so that ε(λ) c l is unitless. This formulation emphasizes how absorptivity depends on both the species and the wavelength.

In general optical or radiative-transfer contexts, the linear (or spectral) absorption coefficient α(λ) describes the fraction of light absorbed per unit path length in a medium, with the transmitted intensity following I(x) = I0 exp(-α(λ) x). The reciprocal length units (for α) contrast with the mass or molar coefficients used in spectroscopy, such as μa (cm^2/g) or ε (L mol^-1 cm^-1). Absorptivity thus appears as a spectrum describing how a material attenuates incident radiation, influenced by electronic transitions, vibrational modes, and material structure.

Terminology can vary by discipline, and absorptivity is sometimes used loosely to denote any wavelength-specific absorption property. When precise comparison is needed, the definition should specify whether the quantity is ε, α, μa, or another related measure and the experimental conditions involved.