  Bismuth donors ion-implanted in $^{28}$Si and $^\text{nat}$Si are studied
using magnetic resonance spectroscopy based on spin dependent recombination.
The hyperfine clock transition, at which the linewidth is significantly
narrowed, is observed for the bismuth donors. The experimental results are
modeled quantitatively by molecular orbital theory for a coupled pair
consisting of a bismuth donor and a spin dependent recombination readout
center, including the effect of hyperfine and Zeeman interactions.
