2025ApJ...991..203G

With the launch of the XRISM microcalorimeter mission, space-based X-ray observations will achieve a record spectral resolving power of R ≡ E/ΔE ∼ 1200. With this resolving power, emission features associated with fine-structure energy levels of some species will be resolved, sometimes for the first time. The plasma code, CLOUDY, was not originally designed for high-resolution X-ray spectroscopy and throughout its history did not resolve fine-structure components of Lyman lines. Here, we expand CLOUDY to resolve these fine-structure energy levels and obtain predicted X-ray spectra that match the resolution of new microcalorimeter observations. We show how the Lyman lines can be used as column density indicators in the hot X-ray emitting gas in a cluster of galaxies such as Perseus, and examine their sensitivity to external radiation fields and turbulence.