## molar absorption coefficient, $$\varepsilon$$

Also contains definition of: molar decadic absorption coefficient
https://doi.org/10.1351/goldbook.M03972
absorbance divided by the absorption pathlength, $$l$$, and the amount concentration, $$c$$: $\varepsilon(\lambda) = \frac{1}{c\, l}\, \text{lg}\! \left ( \frac{P_{\lambda}^{0}}{P_{\lambda}}\right ) = \frac{A(\lambda)}{c\, l}$ where $$P_{\lambda}^{0}$$ and $$P_{\lambda}$$ are, respectively, the incident and transmitted spectral radiant power.
Notes:
1. The term @M03973@ for molar @A00037@ should be avoided.
2. In common usage for $$l/\text{cm}$$ and $$c/\text{mol dm}^{-3}$$ ($$\text{M}$$), $$\varepsilon(\lambda)$$ results in $$\text{dm}^{3}\, \text{mol}^{-1}\, \text{cm}^{-1}$$ ($$\text{M}^{-1}\ \text{cm}^{-1}$$, the most commonly used unit), which equals $$0.1\ \text{m}^{2}\, \text{mol}^{-1}$$ (coherent SI units).
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 371 (https://doi.org/10.1351/pac200779030293)