DYNAMICS OF THE DISPLACEMENT OF CO FROM CU(111) BY H-ATOMS INCIDENT FROM THE GAS-PHASE

We find that CO is displaced from a similar to 90 K Cu(111) surface by an incident H atom beam with a cross section of similar to 10(-16) cm (2)/H atom. As for a previous study of the ejection of O-2 from Pt(111 ), our results indicate that part of the heat of adsorption of the inc ident species is carried away by the ejected molecule in a ''dynamic d isplacement'' process. We have determined the internal-state distribut ion of the ejected CO using quantum-state-specific laser ionization de tection. We have also determined its angular and velocity distribution using a rotatable quadrupole mass spectrometer. The rotational distri bution of molecules displaced in the v=0 and v=1 vibrational states ar e close to Boltzmann distributions at 390 K and 940 K, respectively. W hile the v=1 population is approximately proportional to the CO covera ge, that for v=0 has a more complex coverage dependence, approximately following the presence of the CO alpha 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 t o 800 K for a saturated surface. The velocity distribution of the ejec ted molecules is close to a Boltzmann distribution at 1300 K, correspo nding to a translational energy of similar to 0.22 eV. The angular dis tribution is symmetric about the normal and is close to a cos(5) theta (f) at small angles, desorption angles, theta(f), approximately follow ing a cosine distribution for theta(f)>40 degrees. We discuss the resu lts in terms of the dynamic displacement model, where desorption of CO (v=0) is driven by a sudden switch from the chemisorption to physisor ption wells. In the case of CO (v=1), we suggest that desorption may f ollow the formation of a temporary HCO intermediate. (C) 1996 American Institute of Physics.