Radiative transfer in hydrogen lines in supernova remnant ( SNR ) shock waves is studied taking into account the population of the hydrogen atom 2s-state .
Measurements of Balmer line emission , especially of H \alpha , are often relied upon to derive physical conditions in the SNR shock .
On the other hand , Lyman series photons , especially Ly \beta , are mostly absorbed by upstream hydrogen atoms .
As a result , atoms are excited to the 3p state , and then emit H \alpha by the spontaneous transition from 3p to 2s .
Thus , the nature of H \alpha depends on how many Ly \beta photons are converted to H \alpha photons .
Moreover , the Balmer lines can be scattered by the 2s-state hydrogen atoms , which are excited not only by collisional excitation but also by the Lyman-Balmer conversion .
It is shown for example that the H \alpha photons are scattered if the shock propagates into an H _ { I } cloud with a density of \sim 30 ~ { } { cm ^ { -3 } } and a size of \sim 1 pc .
We find that the line profile of H \alpha becomes asymmetric resulting from the difference between line centre frequencies among the transitions from 3s to 2p , from 3p to 2s and from 3d to 2p .
We also find that the broad-to-narrow ratio of H \alpha , which is often used to estimate the ion-electron temperature equilibrium , varies at most \simeq 10 per cent depending on the ionization degree of the upstream medium because of incomplete conversion of Lyman lines to Balmer lines .