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)