  We have quantified substitutional impurity concentrations in synthetic
diamond crystals down to sub parts-per-billion levels. The capture lifetimes of
electrons and excitons injected by photoexcitation were compared for several
samples with different impurity concentrations. Based on the assessed impurity
concentrations, we have determined the capture cross section of electrons to
boron impurity, sA=1.3x10^-14 cm2, and that of excitons to nitrogen impurity,
sD^ex=3.1x10^-14 cm2. The general tendency of the mobility values for different
carrier species is successfully reproduced by including carrier scattering by
impurities and by excitons.
