Positrons at keV energies, incident on Mo(111) are observed to escape the sample prior to thermalization. Analysis of positron back-diffusion data shows that an excess contribution of ``hot'' positronium (Ps) seriously affects positron mobility measurements. Suppressing the ``hot'' Ps fraction the positron diffusion coefficient is shown to vary as ${T}^{\mathrm{\ensuremath{-}}0.50(4)}$. The positron implantation profile possesses a shape of a derivative of a Gaussian function. The implications of these observations to related experiments are discussed.