Paper SS-MoA8
H Absorption Depth Profiling Measurement at Ultra-thin Pd(111) Film by Thermal Desorption Spectroscopy

Monday, October 29, 2012, 4:20 pm, Room 21

Palladium is a peculiar metal with non-activated dissociation and bulk incorporation of hydrogen. Previous studies of H thermal desorption spectroscopy (TDS) at Pd(111) indicated that large H₂ exposures at ~100K induce a non-saturating desorption signal which is so called as α peak at 180 K [1]. The absorption origin of the α peak was inferred as the hydrogen located in subsurface sites just below the top surface Pd atoms layer, where it is stabilized at low temperatures by an energy barrier with respect to bulk sites. However, a hydrogen depth profiling measurement by nuclear reaction analysis (NRA) at the Pd(001) surface indicated that the α₁ peak (which regards as α peak at the Pd(111)) has to be assigned to the deeper located H in the 0-50 monolayer [ML] subsurface region [2]. The actual H depth distribution however was difficult to assess precisely with NRA, since the beam induced local heating was suspected to cause a partial escape of H from detection by diffusion into the bulk. To investigate the α-H origin without any heating ambiguity, TDS has been observed at well-defined ultra-thin Pd(111) films. Controlling the Pd film thickness enables us to seek the α-H depth profiling.

Pd film was deposited at the 50 ML of Ag(111) coated Si(111) surface by a Knudsen cell at room temperature. The Pd surface structure was characterized by a reflection high-energy electron diffracion (RHEED) and an atomic force microscopy (AFM). Pd(111) film grew layer-by-layer at the Ag(111) surface when grown at room temperature without any interdiffusion. The lattice constant mismatch between Pd and Ag of 4.9 % expanded the Pd lattice constant when the Pd thickness (θ_Pd) was below 80 ML. It recovered the bulk lattice constant at θ_Pd > 80 ML. H₂ was exposed to the Pd(111) surface at 102 K with an exposure pressure of 1x10^-4 Pa. By integrating α-TDS signal, α-H absorption amount of H₂ exposure dependence (<4x10⁴ L) and of θ_Pd dependence (<560 ML) were observed.

H absorption on the Pd film (θ_Pd=420 ML) saturated as ~8 ML at the maximum exposure of 4x10⁴ L. The averaged α-H concentration corresponded to 1.9 %. The θ_Pd dependence of α-H at the exposure of 4050 L showed that the α-H absorption increased with θ_Pd (maximum θ_Pd was 560 ML). H concentration at θ_Pd=80 ML was 1.8 % which was close to the saturated concentration, it decreased to 1.2 % at θ_Pd=560 ML. Our result indicated that the α-H absorption progress from near the surface region, but it possibly to absorb at the deeper bulk region than that of the previous NRA measurement at the high exposures.

References;

[1]. G. E. Gdowski et al. Surf. Sci. 181, L147 (1987).

[2]. M. Wilde et al. Surf. Sci. 482, 346 (2001).