  We present time-of-flight inelastic neutron scattering measurements at low
temperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 and
Tb2Sn2O7. These two materials possess related, but different ground states,
with Tb2Sn2O7 displaying "soft" spin ice order below Tn~0.87 K, while Tb2Ti2O7
enters a hybrid, glassy spin ice state below Tg~0.2 K. Our neutron
measurements, performed at T=1.5 K and 30 K, probe the crystal field states
associated with the J=6 states of Tb3+ within the appropriate Fd\bar{3}m
pyrochlore environment. These crystal field states determine the size and
anisotropy of the Tb3+ magnetic moment in each material's ground state,
information that is an essential starting point for any description of the
low-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7.
While these two materials have much in common, the cubic stanate lattice is
expanded compared to the cubic titanate lattice. As our measurements show, this
translates into a factor of ~2 increase in the crystal field bandwidth of the
2J+1=13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent
with previous measurements on crystal field states in Tb2Sn2O7, wherein the
ground state doublet corresponds primarily to m_J=|\pm 5> and the first excited
state doublet to mJ=|\pm 4>. In contrast, our results on Tb2Ti2O7 differ
markedly from earlier studies, showing that the ground state doublet
corresponds to a significant mixture of mJ=|\pm 5>, |\mp 4>, and |\pm 2>, while
the first excited state doublet corresponds to a mixture of mJ=|\pm 4>, |\mp
5>, and |\pm 1>. We discuss these results in the context of proposed mechanisms
for the failure of Tb2Ti2O7 to develop conventional long-range order down to 50
mK.
