Ionizing radiation from hydrogen recombination strongly suppresses the lithium scattering signature in the CMB

It has been suggested that secondary CMB anisotropies generated by neutral lithium could open a new observational window into the universe around the redshift z ∼ 400, and permit a determination of the primordial lithium abundance. The effect is due to resonant scattering in the allowed Li i doublet (2s 2S1/2 − 2p 2P1/2,3/2), so its observability depends on the formation history of neutral lithium. Here we show that the ultraviolet photons produced during hydrogen recombination are sufficient to keep lithium in the Li ii ionization stage in the relevant redshift range and suppress the neutral fraction by ∼ 3 orders of magnitude from previous calculations, making the lithium signature unobservable.