46394-AC5
Quantum Mechanical Control of Surface Chemical Reactivity
This work focuses on the role of quantum confinement of conduction electrons in ultrathin metal films for the kinetics and thermodynamics of hydrogen adsorption, dissociation, absorption, diffusion, and recombination. Fundamental understanding of these quantum phenomena, including the quantum stabilization of thermodynamically immiscible alloy species and their potential for tuning surface catalytic processes, are directly relevant to the use of light-metal hydrides as storage media for hydrogen. During the first year of this contract, we focused our efforts on growing atomically smooth ultrathin magnesium films on Mo(110) and Si(111) substrates in ultrahigh vacuum. Low temperature growth of Mg on either substrate results in the formation of atomically smooth thin films with precisely controlled film thickness, as indicated by scanning tunneling microscopy and Auger electron spectroscopy data. Depending on the growth temperature, Mg growth on Si(111) is preceded by the formation of an ultrathin silicide layer. The Mg films grow layer-by-layer in the hexagonal close packed (0001) orientation and the existence of discrete quantum well states was inferred from scanning tunneling spectroscopy data as a function of the film thickness. We furthermore employed x-ray photoelectron spectroscopy to monitor the evolution of a sharp shake-up satellite in the Mg 1s core level as a function of the film thickness. For films with thicknesses between five and twenty five atomic layers the energy position of this peak is inversely proportional to the square of the film thickness. These results are consistent with the existence of quantized plasmons, which we interpret on the basis of theoretical (hydrodynamics and random-phase approximation) descriptions of the density-response function (in collaboration with Prof. A.G. Eguiluz). We find that the observed loss feature corresponds to the n = 1 anti-symmetric normal mode of the thin film, consistently with the fact that in the ultrathin film limit the symmetric plasmons have vanishing spectral weight —a striking manifestation of the role of size quantization on plasmon resonances in precisely controlled nanostructures. In the second year of this project, we will use these ultrathin Mg films as substrates for deuterium chemisorption experiments. To this end, we constructed a new system for temperature-programmed desorption (TPS) and succeeded in producing highly linear temperature ramps between 150 K and 700 K. Preliminary investigations of the Mg desorption suggest rather complex line shapes for multilayer desorption, which are currently being analyzed. Hydrogen (or deuterium) chemisorption will be studied using TPD, nuclear reaction analysis, low energy electron diffraction, and electron energy loss spectroscopy. The grant provided full support for a graduate research assistant, Mr. Madhusudan Ojha.
