  Sykes {\it et al.} [Proc. Natl. Acad. Sci. {\bf 102}, 17907 (2005)] have
reported how electrons injected from a scanning tunneling microscope modify the
diffusion rates of H buried beneath Pd(111). A key point in that experiment is
the symmetry between positive and negative voltages for H extraction, which is
difficult to explain in view of the large asymmetry in Pd between the electron
and hole densities of states. Combining concepts from the theory of ballistic
electron microscopy and electron-phonon scattering we show that H diffusion is
driven by the $s$-band electrons only, which explains the observed symmetry.
