  We report direct experimental evidence of interspecies ion separation in
direct-drive, inertial-confinement-fusion experiments on the OMEGA laser
facility. These experiments, which used plastic capsules with D$_2$/Ar gas fill
(1% Ar by atom), were designed specifically to reveal interspecies ion
separation by exploiting the predicted, strong ion thermo-diffusion between ion
species of large mass and charge difference. Via detailed analyses of imaging
x-ray-spectroscopy data, we extract Ar-atom-fraction radial profiles at
different times, and observe both enhancement and depletion compared to the
initial 1%-Ar gas fill. The experimental results are interpreted with
radiation-hydrodynamic simulations that include recently implemented,
first-principles models of interspecies ion diffusion. The experimentally
inferred Ar-atom-fraction profiles agree reasonably, but not exactly, with
calculated profiles associated with the incoming and rebounding first shock.
