  We investigate the spin species relevant for the spin-dependent recombination
used for the electrical readout of coherent spin manipulation in
phosphorus-doped silicon. Via a multi-frequency pump-probe experiment in pulsed
electrically detected magnetic resonance, we demonstrate that the dominant
spin-dependent recombination transition occurs between phosphorus donors and
Si/SiO2 interface states. Combining pulses at different microwave frequencies
allows us to selectively address the two spin subsystems participating in the
recombination process and to coherently manipulate and detect the relative spin
orientation of the two recombination partners.
