Notes. Columns are as follows: (1) gas metallicity normalized to the Solar value; (2) log of the ionization parameter U; (3) ionizing SED used (including a black-body SED with T = 45 000 K); (4) average electron temperature in the H⁺ zone; (5) hydrogen neutral fraction; (6) ratio of the column density of ionized hydrogen to the column density of fully-ionized helium; (7) Balmer decrement; (8) Lyα to HeII λ1640 flux ratio; (9) η_Lyα, the ratio of Lyα photons emitted to incident ionizing photons. The rows have been sorted using η_Lyα. As discussed in the main text, reducing the gas metallicity raises the electron temperature, leading to increased collisional excitation of Lyα (and of Hα relative to Hβ), thus increasing η_Lyα. In addition, adopting a filtered ionizing SED (here we show the F_esc = 0.28 case) has the dual effect of reducing the abundance of He⁺⁺ relative to H⁺ and increasing the collisional excitation of Lyα, with the latter effect resulting in increased η_Lyα.