UNUSUAL DISORDERING PROCESSES OF OXYGEN OVERLAYERS ON RH(111) - A COMBINED DIFFRACTION STUDY USING THERMAL HE ATOMS AND LOW-ENERGY ELECTRONS

The temperature-dependent behavior of the Rh(111)-(2X2)-10 phase was i nvestigated by He-atom scattering (HAS) and low-energy electron diffra ction. The adsorption system undergoes an order-disorder phase transit ion at T-c=280+/-5 K, with critical exponents found to be consistent w ith the four-state Potts model. Beyond the phase transition the HAS sp ecular peak intensity exhibits a strong and reversible increase. This finding points toward a reduction of the surface charge-density corrug ation induced by the phase transition itself. Around 160 K, hydrogen a dsorbed on the Rh(111)-(2X2)-10 surface reacts with oxygen to form wat er, and drives the overlayer in an out-of-equilibrium condition which is characterized by a dramatic domain-wall proliferation.