modeling with state-of-the-art DR data. The stronger suppression tails-off at $x\gtrsim 11$ as three-body recombination starts to become relevant and which, at even higher densities (not shown), causes the suppression factor to rise (since it is a ratio of effective recombination rate coefficients, i.e. includes three-body recombination.)

Suppression Formula at Low Temperatures. {#NSFSLT}
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The preceding formulation was based on the suppression factor found by @summersRAL for electron collisionally ionized plasmas, i.e., at higher temperatures, where DR is dominated by high-$n$ resonances attached to a dipole-allowed core excited state. In photoionization equilibrium, however, the temperature at which a given ion forms is substantially smaller than that found in the electron collisional case. Due to the lower kinetic temperatures, DR occurs mainly through energetically low-lying autoionizing states, often via non-dipole core-excitations for which radiative stabilization is by the (outer) Rydberg electron. These states are not, in general, as susceptible to density suppression as their high-$n$ counterparts, and so it may be necessary to modify the preceding suppression formulation.

We first consider sequences with partially-occupied $p$-subshells in the ground state, which includes the B-like $2p(^2P_{1/2,3/2})$, C-like $2p^2(^3P_{0,1,2})$, O-like $2p^4(^3P_{0,1,2})$, F-like $2p^5(^2P_{3/2,1/2})$, Al-like $3p(^2P_{1/2,3/2})$, Si-like $3p^2(^3P_{0,1,2})$, S-like $3p^4(^3P_{0,1,2})$, and Cl-like $3p^5(^2P_{3/2,1/2})$ systems. For 