Dynamics of the displacement of CO from Cu(111) by H atoms incident from the gas phase

Abstract

We find that CO is displaced from a ∼90 K Cu(111) surface by an incident H atom beam with a cross section of ∼10-16 cm2/H atom. As for a previous study of the ejection of O2 from Pt(111), our results indicate that part of the heat of adsorption of the incident species is carried away by the ejected molecule in a "dynamic displacement" process. We have determined the internal-state distribution of the ejected CO using quantum-state-specific laser ionization detection. We have also determined its angular and velocity distribution using a rotatable quadrupole mass spectrometer. The rotational distribution of molecules displaced in the ν=0 and ν=1 vibrational states are close to Boltzmann distributions at 390 K and 940 K, respectively. While the ν=1 population is approximately proportional to the CO coverage, that for ν=0 has a more complex coverage dependence, approximately following the presence of the CO α state, which gives a distinct temperature-programmed desorption peak for coverages above 1/3 ML. The equivalent vibration temperature ranges from 1500 K at low coverage to 800 K for a saturated surface. The velocity distribution of the ejected molecules is close to a Boltzmann distribution at 1300 K, corresponding to a translational energy of ∼0.22 eV. The angular distribution is symmetric about the normal and is close to a cos5 θf at small angles, desorption angles, θf, approximately following a cosine distribution for θf>40°. We discuss the results in terms of the dynamic displacement model, where desorption of CO(ν=0) is driven by a sudden switch from the chemisorption to physisorption wells. In the case of CO(ν=1), we suggest that desorption may follow the formation of a temporary HCO intermediate. © 1996 American Institute of Physics.