Hydrogenic impurity absorption ($h{\nu }_{1\to n}=R^{\ast }(1-n^{-2})$)

A sequence of bound-to-bound optical transitions of a hydrogenic donor state or hydrogenic acceptor state, converging on a photoionization threshold $R^{\ast }$. For an initially occupied $1s$ state, ordinary electric dipole transitions reach $np$ states; above the threshold, the final carrier can enter a band continuum.