= Angular-frequency Einstein coefficients
{c}
{title2=$A_{21}/B_{21}=\hbar\omega_0^3/(\pi^2c^3)$}
When $B_{12}$ and $B_{21}$ multiply <radiation energy density> per unit <angular frequency>, they differ by a factor $2\pi$ from coefficients multiplying the density per ordinary frequency. For nondegenerate levels, <Boltzmann distribution> populations and <thermal equilibrium> give $B_{12}=B_{21}$ and the free-space ratio in the title. Specifically $B_\omega=2\pi B_\nu$ while $A$ is unchanged. Polarization restrictions and level degeneracy must be specified separately.
Back to article page
