Surface phases of Ni(110) induced by adsorption of deuterium

Abstract

The surface phases induced by D 2 adsorption on Ni(110) have been investigated by LEED, thermal desorption spectroscopy (TDS), Rutherford backscattering (RBS) and nuclear reaction analysis (NRA). The data were correlated via accurate work function (Δφ) measurements. The (2 × 1) and (1 × 2) phases saturate at coverages (θ) of 1.0 and 1.5 monolayer respectively. Nickel atoms are not displaced laterally from their bulk like positions for 0 < θ < 1.0. Over the range 1.0 < θ < 1.5 the surface reconstructs by a first order transition to the (1 × 2) phase in which, at saturation, an entire monolayer of Ni atoms are displaced laterally by > 0.01 nm. The fraction of the surface in the (1 × 2) phase increases linearly with D-coverage from 0% at θ = 1.0 to 100% at θ = 1.5. The streaked (1 × 2) phase formed by cooling in D 2 or by heating the (1 × 2) phase above ~ 220 K is not simply a disordered (1 × 2) phase. Only 20% of the Ni atoms have displacements large enough to be in the (1 × 2) phase. The sticking coefficient (S) is ~ 0.35 at 175 K atθ → 0 and after decreasing to ~ 0.1 by θ ≈ 0.95 either levels out or increases around θ = 1.0 at which coverage, the (1 × 2) phase nucleates. The desorption of D 2 occurs in 3 states with increasing temperature: α state, associated with the (1 × 2) → streaked (1 × 2); β 1, associated with desorption from the (1 × 1) surface and β 2 associated with desorption from the streaked (1 × 2) phase. The fraction of the adsorbed deuterium desorbing in the various states depends on the kinetics of the (1 × 2) → (1×1) and (1 × 2) → streaked (1 × 2) phase transitions and can be altered by trace amounts of impurities such as adsorbed oxygen.