part of Ryan Porter’s thesis. Because the high charge states occur in hot gas and the line energies scale as $Z^2$, these iso-sequences sort themselves into two spectral regions. , , and   produce strong lines in the optical while , , , , , , etc produce lines in the X-ray. Despite these differences, the physics has many similarities. These sequences are treated with a common code base, which results in greater simplicity and reliability.

@Porter2005 and @Bauman2005 describe the model of  emission. @PorterFerland2007 describe ions of the He sequence, which mainly emits in the X-Ray. @LuridianaEtAl09 summarize expansions to the H-like sequence while @PorterFerlandMacAdam2007, @Porter.R09Uncertainties-in-theoretical-HeI-emissivities:-HII-regions, and @Porter.R12Improved-He-I-emissivities-in-the-case-B-approximation discuss uncertainties and more recent improvements in the atomic data for .

A schematic representation of one of the elements of the H-sequence is shown in Figure \[fig:LevelsResolvedCollapsed\]. The He-sequence has similar structure except that it is resolved into singlets and triplets with twice as many levels. Principal quantum number increases upward with the continuum at the top and $nl$ terms are indicated from left to right. Lower $n$ configurations are resolved into $nl$ terms, the “resolved” levels. Above a certain quantum number $l$-changing collisions become fast enough to guarantee that $l$ 