Freeze-out Effects in Hydrogen and Helium Lines of SN 1987A at the Early   Photospheric Epoch
  We have developed a time-dependent model of ionization, excitation and energy balance of SN 1987A atmosphere at the photospheric epoch to study early behavior of hydrogen and helium lines. The ionization freeze-out effects play a key role in producing both the strong H-alpha during nearly the first month and the He I 5876 A scattering line on day 1.76. Using an extended reaction network between hydrogen molecules and their ions demonstrates that ion-molecular processes are likely responsible for the blue peak in H-alpha profile at the Bochum event epoch. 
V. P. Utrobin and N. N. Chugai