What is the absorption cross section, how is it measured?
How to convert it to the absorption coefficient (measured in cm$^{-1}$)?
Answer
The cross section $\sigma$ is related to the absorption coefficient $\alpha$ by:
$$ \sigma = \frac{\alpha}{N} $$
where $N$ is the number density of the scattering medium i.e. the number of particles per unit volume. This is described in more detail in the Wikipedia article on the absorption cross section.
If you want $\alpha$ in units of cm$^{-1}$ you need to express $\sigma$ in cm$^2$ and the density as the number of particles per cubic cm.
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