Spectroscopic Nomenclature

Internal absorbance A_i

Negative logarithm to the base 10 of the internal transmittance:

A_i = –log₁₀(T_i).

Absorptivity α

Internal absorbance divided by the product of sample path length, ℓ, and mass concentration, ρ, of the absorbing material.

α = A_i/ρℓ.

SI unit: m² kg⁻¹.

Common unit: cm² g⁻¹; L g⁻¹ cm⁻¹.

(Not used: absorbancy index, extinction coefficient, or specific extinction.)

Molar absorptivity or Molar (decadic) absorption coefficient ε

Internal absorbance divided by the product of sample path length, ℓ, and mole concentration, c, of the absorbing material.

ε = A_i/cℓ.

SI unit: m² mol⁻¹.

Common unit: L mol⁻¹ cm^-1; cm² mol⁻¹.

(Not used: molar absorbancy index or molar extinction coefficient.)

Naperian absorbance, A_e

The absorbance calculated in base e, i.e., ln (1/T)

Linear absorption coefficient, α

The Naperian absorbance divided by the path length,

α = A_e/ℓ

Beer–Lambert law

Absorptivity of a substance is constant with respect to changes in path length and concentration of the absorber. Often called Beer’s law when only changes in concentration are of interest.

Net absorption cross-section σ_net

The absorptivity per molecule, i.e., the molar absorptivity, ε, divided by Avogadro’s number, usually corrected to base e through σ_net = 2.303ε/N_A.

SI unit: m².