Emission cross section ($W_{21}={\sigma }_{21}I/(\hslash \omega )$)

The effective area converting incident photon flux into a stimulated emission rate for a specified transition, frequency, polarization and sublevel distribution. For statistically populated energy levels of statistical weights $g_1,g_2$, the Einstein coefficients imply $g_1{\sigma }_{12}=g_2{\sigma }_{21}$. The related absorption cross section therefore differs when the statistical weights differ.