AVS 46th International Symposium
    Applied Surface Science Division Wednesday Sessions
       Session AS-WeA

Paper AS-WeA4
Dynamics of CO Adsorption on O-ZnO: a Molecular Beam Study

Wednesday, October 27, 1999, 3:00 pm, Room 6A

Session: Oxides and Insulators
Presenter: U. Burghaus, Ruhr-Universität Bochum, Germany
Authors: Th. Becker, Ruhr-Universität Bochum, Germany
Ch. Boas, Ruhr-Universität Bochum, Germany
U. Burghaus, Ruhr-Universität Bochum, Germany
Ch. Woell, Ruhr-Universität Bochum, Germany
Correspondent: Click to Email
We present measurements of the initial adsorption probability, S@sub 0@, as well as its coverage dependence, S(@THETA@@sub CO@), of CO on oxygen terminated ZnO(0001) surfaces. Two different substrates with significantly different density of defects, as determined by He atom scattering, were investigated. Using a well defined molecular beam of CO seeded in He the He atom reflectivity could be monitored simultaneously with the flux of backscattered CO molecules. These results allow to separately determine the total sticking coefficient averaged over the whole surface (using the King and Wells method) and the sticking coefficient on flat terraces. Measurements were carried out for different impact energies (48 meV < E@sub i@ < 0.74 eV) and surface temperatures (77 K < T@sub s@ < 800 K). The crystallographic structure and chemical composition of the surfaces have been characterized by LEED, He atom scattering, and XPS. The shape of the S(@THETA@@sub CO@,T@sub s@) curves points to a precursor mediated adsorption, since, for medium (100 - 180 K) surface temperatures S(@THETA@@sub CO@) remains approximately constant up to saturation coverage, @THETA@@sub sat@. For low adsorption temperatures (70 - 90 K) a rather unique shape of S(@THETA@@sub CO@) has been observed, namely an increase of S(@THETA@@sub CO@) with @THETA@@sub CO@ followed by an abrupt decrease when approaching @THETA@@sub sat@. S@sub 0@ is essentially independent of T@sub s@ and decreases linearly with E@sub i@, consistent with a precursor-mediated adsorption and a decreasing probability of adsorption into the precursor state, respectively. The results indicate that the presence of defects does affect the CO sticking probability.