pub fn saha_he_ii(z: f64, cosmo: &Cosmology) -> f64Expand description
He II → He I Saha ionization fraction (54.4 eV).
Returns the fraction of helium that is doubly ionized (He²⁺). Statistical weight ratio: g(He²⁺)g(e)/g(He⁺) = 1×2/2 = 1. He²⁺ is a bare alpha particle (spin-0 nucleus), g=1. He⁺ ground state (1s, hydrogen-like), g=2 (electron spin). Free electron, g=2 (spin).
Uses the standard (RECFAST/Seager+1999) total free-electron Saha form y / (1 − y) = K(T) / n_e, where n_e ≈ n_H + (1 + y_II)·n_He is dominated by H⁺ at z ≳ 1500 (H is fully ionized throughout He recombination since χ_I(H) = 13.6 eV ≪ χ_II(He) = 54.4 eV). Using n_e = n_H + 2·n_He (y_II = 1 limit) introduces ≲7% error in n_e vs the fully self-consistent y_II = 0 limit — negligible compared to the ~factor-of-29 error from the old He-only quadratic form that assumed n_e = y·n_He.