  During recombination, the cosmic background radiation is disturbed, in
particular, by Lyman-alpha emissions from neutral hydrogen. It is proposed to
account for the subsequent time-dependent partial thermalization of the
Lyman-alpha energy content in an analytically solvable nonlinear diffusion
model. The amplitude of the partially thermalized and redshifted Ly-$\alpha$
line is found to be too low to be visible in the cosmic microwave spectrum, in
accordance with previous numerical models and Planck observations.
