Photoemission Studies of Energy Levels in the Palladium-Hydrogen System

Photoemission studies of the Pd/H system show a band of hydrogen-induced energy states centered at 1 eV below the bottom of Pd-derived d bands of width 4.4 eV. These results show that the widely accepted simple "proton model" of Pd/H is misleading. Our augmented-plane-wave calculations of PdH, ${\mathrm{Pd}}_4$${\mathrm{H}}_3$, and ${\mathrm{Pd}}_4$H exhibit these lowlying states and also explain why the added number of electrons (0.6) in $\beta$-phase PdH exceeds the number of $d$-band holes (0.36) in Pd.