kT ∼ E_{ionize} = E_{∞} − E_{1}

Z = g_{1} + ∑_{j=2}^{∞} g_{j}e^{−(E_{j}−E_{1})/kT}

\frac{N_{II}}{N_{I}} = \frac{2Z_{II}}{n_{e}Z_{I}} (\frac{2πm_{e}kT}{h^{2}} )^{3/2}e^{−χ/kT}

\frac{N_{II}}{N_{I}} = \frac{2kT Z_{II}}{P_{e}Z_{I}} (\frac{2πm_{e}kT}{h^{2}} )^{3/2}e^{−χ/kT}