Depth-resolved analysis of subsurface hydrogen absorbed by Pd(1 0 0)

The adsorption and initial absorption of hydrogen (H2) by Pd(1 0 0) at T=100–160 K are investigated by thermal desorption spectroscopy (TDS) and nuclear reaction analysis, which enables a depth-resolved determination of the hydrogen concentration at and below the Pd surface. Under 3×10−8 mbar H2 and T<130 K hydrogen is first chemisorbed at the surface and the coverage saturates at 1.0 ML. A small fraction of further dosed hydrogen penetrates the surface and accumulates in a region of up to ≈4 nm depth, contrasting previous assignments of a corresponding ‘α1’-TDS peak at 180 K to exclusive occupation of first-layer subsurface sites. At T>130 K H-migration from such a near-surface hydride phase to the deeper bulk of the metal occurs and a solution of H in Pd is formed by means of facile diffusion. This bulk dissolved hydrogen emerges as a broad structure in TDS at T=200–260 K exhibiting a tailing at high temperatures characteristic of diffusion-controlled desorption kinetics.