THE NONLINEAR KINETICS OF GAS-SURFACE EXCHANGE BETWEEN CO/CO-ASTERISK ON RE(0001)

The isotopic switch technique has been used to study the desorption of (CO)-O-16/Re(001) in the presence of (CO)-O-18 gas or a (CO)-O-18 + (CO)-O-16 gas mixture. The desorption rate curve at a constant pressure of ((CO)-O-18 + (CO)-O-16) declines nonlinearly with the surface coverage-theta of (CO)-O-16 which implies that the distribution of different isotope molecules on the surface is non-uniform even when the isotope exchange between gas and surface approaches a steady state. A concerted adsorption/desorption mechanism involving a dynamic feature of mutual screening between the isotope species on the surface is postulated for interpretation of the observed nonlinear desorption kinetics. It is emphasized in accordance with this mechanism that due to the specific screening effect between coadsorbed isotope species, the experimental desorption curves are best fitted by combined 1st- and 2nd-order desorption kinetics rather than the 1st-order kinetics commonly used in the literature.