STABILITY OF ADSORBED STATES AND SITE-CONVERSION KINETICS - CO ON NI(100)

The site conversion of adsorbed CO between the terminal site and the b ridged site on Ni(100) was studied by means of infrared reflection abs orption spectroscopy (IRAS). The temperature dependence of the relativ e occupation for two sites was measured from 80 to 266 K in detail, wh ere the binding-energy difference was determined to be 11 meV. The dri ving force for the predominant occupation of the terminal site at high er temperature is ascribed to the vibrational entropy of the low-energ y degenerate-hindered translational mode of the terminal CO. The kinet ics of approaching the equilibrium was studied by time-resolved IRAS c ombined with a pulsed gas dose. Following a rapid dose, CO molecules a re initially adsorbed at the terminal site and the bridged site with t he a priori ratio of 1:2, indicating that gas-phase CO molecules are d irectly trapped by the potential minima initially, are thermalized, an d migrate on the surface to approach the equilibrium occupation ratio. The microscopic hopping rate from the terminal site to the bridged si te was estimated to be 0.02 s-1 and that from the bridged site to the terminal site was estimated to be 0.005 s-1 at 83 K. A random-walk mod el assuming the microscopic hopping rates gives a self-diffusion coeff icient of 3.1 X 10(-19) cm2 s-1 at 83 K, which is in good agreement wi th the previously reported macroscopic results. Thus, the elementary s tep of surface diffusion is ascribed to the hopping between the termin al site and the bridged site. The difference between the estimated bar rier by assuming a harmonic potential and the activation energy for di ffusion suggests the presence of anharmonicity in the potential betwee n the terminal site and the bridged site.