  The electron paramagnetic resonance (EPR) linewidths of B acceptors in Si are
found to reduce dramatically in isotopically purified 28Si single crystals.
Moreover, extremely narrow substructures in the EPR spectra are visible
corresponding to either an enhancement or a reduction of the absorbed microwave
on resonance. The origin of the substructures is attributed to a combination of
simultaneous double excitation and spin relaxation in the four level spin
system of the acceptors. A spin population model is developed which
qualitatively describes the experimental results.
