He II absorption and the sawtooth spectrum of the cosmic far-UV   background
  Cosmic ultraviolet background radiation between 3 and 4 Ryd is reprocessed by resonant line absorption in the Lyman series of intergalactic He II. This process results in a sawtooth modulation of the radiation spectrum from the He II Lya frequency to the Lyman limit. The size of this modulation is a sensitive probe of the epoch of helium reionization and of the sources that keep the intergalactic medium (IGM) highly ionized. For large absorption opacities, the background intensity will peak at frequencies just above each resonance, go to zero at resonance, and fluctuate greatly just below resonance. The He II sawtooth modulation may be one of the missing ingredients needed in the modelling of the abundances of metal ions like C III and Si IV observed in the IGM at redshift 3. 
Piero Madau, Francesco Haardt