  Dopants in silicon have been studied for many decades using optical and
electron spin resonance (ESR) spectroscopy. Recently, new features have been
observed in the spectra of dopants in isotopically enriched 28Si since the
reduced inhomogeneous linewidth in this material improves spectral resolution.
With this in mind, we measured ESR on exchange coupled phosphorus dimers in
28Si and report two results. First, a new fine structure is observed in the ESR
spectrum arising from state mixing by the hyperfine coupling to the 31P nuclei,
which is enhanced when the exchange energy is comparable to the Zeeman energy.
This fine structure enables us to spectroscopically address two separate dimer
sub-ensembles, the first with exchange (J) coupling ranging from 2 to 7 GHz and
the second with J ranging from 6 to 60 GHz. Next, the average spin relaxation
times, T1 and T2 of both dimer sub-ensembles were measured using pulsed ESR at
0.35 T. Both T1 and T2 for transitions between triplet states of the dimers
were found to be identical to the relaxation times of isolated phosphorus
donors in 28Si, with T2 = 4 ms at 1.7 K limited by spectral diffusion due to
dipolar interactions with neighboring donor electron spins. This result,
consistent with theoretical predictions, implies that an exchange coupling of 2
- 60 GHz does not limit the dimer T1 and T2 in bulk Si at the 10 ms timescale.
