absorptivity

Absorptivity, in optics and spectroscopy, is a property that describes how readily a material absorbs electromagnetic radiation at a given wavelength. It is a material-specific quantity that depends on the electronic structure and vibrational modes, the wavelength of light, temperature, and the physical state of the material. Absorptivity is closely related to, but distinct from, absorbance and transmittance, which are measured quantities.

In chemistry, the term molar absorptivity or molar extinction coefficient, ε, is defined within the Beer-Lambert law: A = ε c l, where A is absorbance (dimensionless), c is the concentration in moles per liter, and l is the path length in centimeters. ε has units of L mol^-1 cm^-1 and is wavelength dependent for a given electronic transition or vibrational band.

In solid-state optics or physics, the absorption coefficient α(λ) (units typically cm^-1) characterizes the fraction of light absorbed per unit distance in a material: dI/dx = -α I. The coefficient is related to the complex refractive index n + iκ by α = 4πκ/λ, with κ the extinction coefficient and λ the wavelength in vacuum.

Measuring absorptivity involves spectrophotometric techniques across UV–visible or infrared regions. Beer's law provides a linear relation between A and c at fixed l and λ, but deviations may occur at high concentrations, with scattering, or when the assumption of a homogeneous medium fails.

Applications include quantitative analysis of solutions, characterization of materials, and optical coating design. Related quantities include transmittance, absorptance, and extinction coefficient.