Phase Transitions in Adsorption Layers on bcc(110) Surfaces: S and O on Mo(110)

The adsorption of sulfur on Mo(110) was studied by a combination of low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function change (ΔΦ) measurements using very fine coverage increments. The evolution of the structures with coverage θ was found to be quite different from that formerly reported for S/Mo(110): discrete commensurate structures exist only for θ ≤ 1/2, whereas for θ > 1/2 a continuous series of incommensurate structures is interrupted by narrow regions of commensurate structures similar to those reported for S/W(110). Thus the “high symmetry hypothesis” is no longer applicable for the interpretation of the structures of S/Mo(110). High resolution LEED diffractometry was used for detailed studies of the S and O/Mo(110)‐p(2 × 2)→(1 × 1) transition. For O this is a single continuous order‐disorder transition contrary to S for which the transition occurs in two steps: a continuous order‐order transition from the p(2 × 2) to a two domain (4 × 1) structure which is followed by an order‐disorder transition. For both adsorbates critical exponents β(θ) and phase diagrams were derived from the data. Finally the kinetics of the domain growth were investigated for S/Mo(110)—p(2 × 2). The time dependence of the average domain size L holds the predicted power law L ( t ) ∼ t x ; x depends on θ. For the ideal p(2 × 2)‐structure x (θ = 1/4) = 0.36 ± 0.03, which is close to the predicted value for Kawasaki dynamics x = 1/3.