  We characterize the phase memory time of phosphorus donor electron spins in
lightly-doped natural silicon at high magnetic field (8.58 T) in the dark and
under low-power optical excitation. The spin echo decays are dominated by
spectral diffusion due to the presence of the 4.7% abundant spin-1/2 silicon-29
nuclei. At 4.2 K, the spectral diffusion time (T$_{SD}$) measured in the dark
is $124 \pm 7$ $\mu$s, a factor of 2 smaller than that measured at low magnetic
fields (0.35 T). Using a tunable laser we also measured the echo decay as the
wavelength of the optical excitation is swept across the band edge from 1050 nm
to 1090 nm. Above-bandgap optical excitation is seen to increase the spectral
diffusion time of the donor electron spin to $201 \pm 11$ $\mu$s. The physical
mechanism underlying both the decrease of T$_{SD}$ at high field and the
subsequent increase under optical excitation remains unclear.
