Precursors and trapping in the molecular chemisorption of CO on Ni(100)

Abstract

The dynamics of molecular chemisorption of CO on Ni(100) has been investigated using supersonic molecular beam techniques. In order to probe the role of energy accommodation and of so-called precursor states, we have determined the dependence of the molecular sticking probability and the angular scattering distribution on the translational energy and incident angle of the incoming molecules, on surface temperature, and on surface coverage. At low translational energy CO adsorption proceeds in a fashion indicative of classical precursor kinetics. However, both the temperature independence of the initial sticking probability and the form of the angular scattering distribution of non-chemisorbing molecules (obtained using modulated beam techniques) indicate that molecules that fail to chemisorb on the clean surface are directly scattered and do not become fully accommodated in a precursor state. As the translational energy of the CO molecules at normal incidence is increased from 2 to 30 kcal mol , S 0 decreases smoothly from 0.91 to 0.5. Further, for translational energies greater than 10 kcal mol S 0 decreases with increasing angle of incidence. This surprising result indicates that the chemisorption probability decreases as the component of incident velocity parallel to the surface increases, just as it does for the normal component, but at a larger rate. The dynamics of the accommodation process are discussed in terms of a “hot”, not thermalized, precursor. Increasing the translational energy of the molecule affects not only the value of the sticking probability but also the form of the variation of sticking probability with coverage and with surface temperature. We report for the first time a transition between precursor adsorption kinetics at low translational energy (2 kcal mol ) to direct (Langmuirian) adsorption at high incident energy (21 kcal mol ). Scattering experiments performed on a surface with saturation coverage confirm this result.