Hydrogenic impurity absorption
= Hydrogenic impurity absorption
{title2=$h\nu_{1\to n}=R^*(1-n^{-2})$}
A sequence of bound-to-bound optical transitions of a <hydrogenic donor state> or <hydrogenic acceptor state>, converging on a photoionization threshold $R^*$. For an initially occupied $1s$ state, ordinary <electric dipole transitions> reach $np$ states; above the threshold, the final carrier can enter a band continuum.