pub fn saha_he_i(z: f64, cosmo: &Cosmology) -> f64Expand description
He I → He Saha ionization fraction (24.6 eV).
Returns the fraction of helium that is at least singly ionized (He⁺ or He²⁺). Statistical weight ratio: g(He⁺)g(e)/g(He) = 2×2/1 = 4. He⁺ ground state (1s, hydrogen-like), g=2 (electron spin). Free electron, g=2 (spin). He ground state (1s², singlet), g=1.
Uses the standard total free-electron Saha form y / (1 − y) = K(T) / n_e. At z ~ 2000 where He⁺ recombines, H is still fully ionized (Saha X_H ≈ 1 down to z ~ 1500) and He²⁺ has already recombined to He⁺, so n_e ≈ n_H + y_I·n_He; we approximate y_I = 1 to keep the equation linear, which introduces ≲4% error in n_e.