Elementary Surface Processes on Graphite and Aluminium
Doctoral thesis, 1997
Molecular processes on solid surfaces are of fundamental and technological importance. This work focuses on various reactive processes on a carbon (graphite(0001)) surface, and on the initial oxygen interaction with an aluminium (Al(111)) surface. In the former case, the interaction of pure K and H2O overlayers, and coadsorbed K+H2O overlayers have been addressed, while in the latter case the energy dependent dissociation probability of O2 has been investigated. The methods employed in these studies include high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), work function mea-surements, and molecular beam techniques.
At low cover-ages and low temperatures (T~83 K), potassium is found to adsorb in a two-dimensional overlayer that consists of mobile, ionic K adatoms. At higher coverages, K is forced to condense into 2x2 islands with metallic-like character at .THÄTA.#126;0.3. The bind-ing energy is estimated to be between 1-1.3 eV for the different K submonolayer phases. The data analysis is complicated by intercalation, even at low temperatures.
Photon irradiation (hv>3 eV) of the K/graphite(0001) system leads to desorption and intercalation of K atoms. The wavelength, photon power and polarization dependencies indicate a substrate mediated des-orption process. The coverage dependence of the photodesorption shows that only the ionic (but not the metallic) K adatoms are photoactive. A model has been developed, which explains the observed K photodes-orption quantitatively. The key ingredients include photoexcitation of electrons in the graphite bulk by a pi.-pi.* interband transition. The primary photoexcited electrons are elastically or weakly inelastically scattered to-wards the surface and attach to the K 4s-derived resonance, which results in a neutralization of the K ions. The neutral K atoms are thereby repelled from the surface and may, during the excited state motion escape from the surface as neutrals, before they decay back to the ionic ground state. Recaptured K atoms may intercalate before the excitation energy is dissipated.
Water adsorbs as low coordinated, two-dimensional clusters on graphite at low temperatures and low cov-erages, but transforms into three-dimensional structures by mild annealing. The bind-ing energy of the H2O molecules is 0.45 eV per molecule, close to the sublimation energy of ice.
Coadsorption of H2O and K on graphite(0001) at low temperatures (T~83K) and low K coverages leads to disruption of the two-dimensional H2O clusters, and to formation of hydrated-ion like complexes. Water dissociates at a K cov-erage threshold at .THÄTA.#126;0.3, which corresponds to the critical coverage for K condensation to a metallic phase on the bare graphite surface. At elevated temperatures, H2O and K react to yield KH, KOH, and K-O com-plexes on the sur-face. H2 and H2O is released into vacuum, and successively more oxygen-rich surface complexes are formed, as identified by HREELS. The graphite surface itself gasifies at T?750 K, to yield carbon dioxide.
The initial steps of aluminium oxidation has been studied in a newly constructed molecular beam apparatus. The O2 sticking on Al(111) is found to be an activated event, which does not depend on the surface tem-perature. By preparing O2 molecules with high translational energy, or in vibrationally excited states, an enhanced sticking is ob-served. The sticking coefficient shows a maximum at an incident angle of #126;25degree;. The results are in-terpreted as a direct sticking mechanism (no precursor), which depends sensitively on molecule-surface impact parameters and surface corrugation.