The adsorption and initial absorption of hydrogen (H-2) 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 x 10(8) mba
r H-2 and T < 130 K hydrogen is first chemisorbed at the surface and the co
verage saturates at 1.0 ML. A small fraction of further dosed hydrogen pene
trates the surface and accumulates in a region of up to approximate to4 nm
depth, contrasting previous assignments of a corresponding 'alpha (1)'-TDS
peak at 180 K to exclusive occupation of first-layer subsurface sites. At T
> 130 K H-migration from such a nearsurface hydride phase to the deeper bu
lk of the metal occurs and a solution of H in Pd is formed by means of faci
le diffusion. This bulk dissolved hydrogen emerges as a broad structure in
TDS at T = 200-260 K exhibiting a tailing at high temperatures characterist
ic of diffusion-controlled desorption kinetics. (C) 2001 Elsevier Science B
.V. All rights reserved.