Hydrogen segregation on a palladium hydride surface

Surface phenomena occurring in the process of palladium hydride formation during the interaction of thin Pd film with molecular hydrogen were studied by means of simultaneous measurements of surface potential and H₂ pressure. This allows to differentiate between various states of the adsorbate, and to correlate their behaviour with hydrogen concentration on the surface and in the bulk. Two distinct states of the adsorbate were determined: (i) the negatively polarized, atomic adspecies, stable on the surface, arising at the beginning of the adsorption, referred to as β^-, and (ii) the induced, positively polarized, atomic adspecies, incorporating quickly from the surface into the bulk, referred to as β⁺. The β⁺ adspecies form a precursor surface state for PdH_x creation. It has been found that at low temperature (78K) the β⁺ adspecies are placed above the surface image plane (SIP). Under these conditions, the maximal hydrogen concentration on the palladium hydride surface approaches 2, while in the bulk the (H/Pd) ratio does not exceed 1. At higher temperatures (120K, 160K), when the β⁺ adspecies are located below the SIP, hydrogen concentration on the surface and in the bulk is uniform, approaching (H/Pd) ˜ 1.