Extinction coefficient & quantum yield units?


If the extinction coefficient is the amount of light absorbed at a given wavelength, at a given excitation intensity why isn’t it a percentage?
Idem for the quantum yield?

What are the standard units?


1 Like

Tagging @joachimgoedhart & @talley

1 Like

The extinction coefficient is a parameter that reflects how efficiently light is absorbed (or goes ‘extinct’) at the maximum wavelength. This will also depend on concentration and path length. Hence, the units are ‘per molar’ and ‘per cm’ or: M-1 cm-1
You could express a relative extinction coefficient, which would be the absorbance at a certain wavelength relative to the peak wavelength, but this is uncommon (but perhaps useful).

The quantum yield does not have units, it is a relative measure. Usually it is expressed as a number between 0 and 1, but it can equally well be reported as a percentage.

Hopefully this answers (part of) your questions…


@LPUoO If you are interested in the math behind the extinction coefficient, you can check it out here: https://en.wikipedia.org/wiki/Beer–Lambert_law


If you want to have a little extra fun, you can convert the extinction coefficient into the absorption cross-section of a molecule which is usually around 10^-16 cm^2.