Temperature behavior of the (sqrt3 x sqrt3)R30-1CO and the (2 x 2)-3CO overlayers on Rh(111): A combined HAS and LEED investigation

The order-disorder phase transition of the Rh(111)-(√3 x √3)R30°-CO phase was investigated by He scattering. The critical temperature turned out to be 330±5 K, while the critical exponents are consistent with a phase transition belonging to the 3-state Potts universality class. The temperature dependence below the critical temperature of both CO phases was studied using elastic He scattering and LEED. These measurements demonstrate that a simple Debye-Waller evaluation of the diffraction data is not appropriate to provide information on the vibrational properties of the CO molecules. From LEED measurements at low temperatures (<70 K) and various CO partial pressures there is evidence of the formation of a disordered second CO layer in equilibrium with the gas phase. The isosteric heat of CO adsorption in the second layer is about 6 kJ mol-1