Einstein Transition Probability
Given two atomic states of energy $E_j$ and $E_k$. Let $E_j > E_k$. Assume the atom is bathed in radiation of energy density $u(w)$. Transitions between these states can take place in three different ways. Spontaneous, induced/stimulated emission, and induced absorption. $A_jk$ represents the Einstein transition probability for spontaneous emission.
Applicable units are those of quantitykind:EinsteinTransitionProbability
http://electron6.phys.utk.edu/qm2/modules/m10/einstein.htm
$\frac{-dN_j}{dt} = A_jkN_j$, where $-dN_j$ is the number of molecules spontaneously leaving the state j for the state k during a time interval of duration $dt$, $N_j$ is the number of molecules in the state j, and $E_j > E_k$.