Temporal and Local Reduction of a Potential Energy Well under Dynamic Equilibrium: CO on Ni(100)

Direct observation of dynamic equilibrium between preadsorbed ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ and gas phase ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ by means of time-resolved infrared reflection absorption spectroscopy at several temperatures from 350 to 400 K reveals that the desorption process under gas phase CO is enhanced due to the repulsive interaction between preadsorbed ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ and incident ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ molecules from the gas phase arriving at nearby sites of adsorbed CO. As a result, the activation energy is reduced from 127 kJ ${\mathrm{mol}}^{-1\mathrm{}}$, the value for desorption into vacuum, to 25 kJ ${\mathrm{mol}}^{-1\mathrm{}}$ even for $\theta =0.1-0.3\mathrm{}$ monolayer (ML) and a CO flux of 0.001 ML${\mathrm{s}}^{-1\mathrm{}}$.