The non-equilibrium cooling efficiencies are suppressed,
compared to CIE cooling, because
the gas remains "over-ionized" as it cools, and the
densities of the coolants that are most effective at each temperature are
reduced. The more highly ionized species that remain present at each
temperature are less efficiently excited by the cooler thermal electrons.
In addition, the distinct peaks and features that
appear for CIE are smeared out, due to the broader ion distributions
that occur for non-equilibrium cooling.
