Strong Low-Temperature Photoemission Peak in CeH2.7

The growth of a strong peak at $E_{\mathrm{F}}$ is observed in photoelectron spectra of Ce${\mathrm{H}}_{2.7}$ below 70 K in addition to the hydrogen-induced band at 5 eV and the $4f$ emission at 2 eV. From the photonenergy dependence and from the absence of $4f-4d$ resonance enhancement we conclude that the peak is neither $4f$ derived nor related to screening or Kondo resonance, but is due to a surface semiconductor-metal transition. It can be explained by the removal of hydrogen from surface sites which pushes $d$ states from the hydrogen band back to $E_{\mathrm{F}}$ and results in a metallic surface dihydride.