Collisional quenching of a metastable line
= Collisional quenching of a metastable line
{title2=$P_{\rm rad}=A_{ul}/(A_{ul}+n_eq_{ul})$}
A schematic statistical equilibrium is $n_ln_eq_{lu}=n_u(A_{ul}+n_eq_{ul})$. The radiative yield per excitation is $A_{ul}/(A_{ul}+n_eq_{ul})$. It is near unity below critical density $A_{ul}/q_{ul}$ and decreases above it. Quenching suppresses emission relative to naive low-density excitation scaling; it need not make the absolute <emissivity> decrease when density rises.