Professor Theodore E. Madey 
    Research interest and co-workers

Professor Madey's research is in experimental surface physics. Particular emphasis is on the use of ultrahigh vacuum methods to characterize the physics and chemistry of processes that occur on atomically-clean single-crystal surfaces, including physical adsorption, chemisorption, the nucleation and growth of ultrathin metal films, catalysis, photoelectron emission, ion scattering, and electron/solid interactions. 

The present research activities of my group are focused in three areas:

Current members of Madey's group include:
Some specific recent activities of our group include the following:

I) "Faceting of Surfaces Induced by Ultrathin Films: Atomically-resolved Structure, Reactivity, and Electronic Properties"
(work of C.-H. Nien, K. Pelhos, I. Abdelrehim, H.-S. Tao)

One of our projects deals with ultrathin films of metals on metals; these are model systems for bimetallic catalysts, and we are searching for relations between the surface structure and the surface chemical reactivity. We find that the atomically rough, open bcc(111) surfaces are morphologically unstable when covered by films one monolayer (ML) thick of certain metals, i.e., they form faceted structures. To characterize these phenomena, we have studied ultrathin films of transition and noble metals on Mo(111) and W(111) using Auger Spectroscopy, LEED, thermal desorption spectroscopy (TDS), soft x-ray photoemission spectroscopy (SXPS), field emission microscopy (FEM), and scanning tunneling microscopy (STM). For example, using a UHV STM to study bimetallic Pd/W(111), we find that the Pd-covered W(111) surface becomes completely faceted to three-sided {211} pyramids upon annealing, for Pd coverages greater than a critical coverage ~ 1 ML. Formation of pyramidal facets also occurs when W(111) or Mo(111) surfaces are dosed with Pt, Au, Ir, Rh, or oxygen. In contrast, monolayer films of Ti, Co, Ni, Cu, Ag and Gd do not induce massive reconstruction or faceting on W(111) and Mo(111) surfaces. The faceting appears to be thermodynamically driven but kinetically limited: faceting is caused by an increased anisotropy in surface free energy that occurs for the film-covered surfaces.

As part of this program to study structure-reactivity relationships in the morphologically-unstable W(111) system, we are examining known structure sensitive reactions, acetylene tricyclization to benzene and n-butane hydrogenolysis. Work in progress focuses on the reactivity of planar and faceted Pt/W(111) and Pd/W(111). Finally, high resolution SXPS using synchrotron radiation provides insights into electronic factors that influence faceting at the bimetallic interface. Recent theoretical descriptions of morphological instability provide new insights into these processes.

II) "Far-out surface science: role of surface phenomena in the formation of tenuous planetary atmospheres"
(work of B. V. Yakshinskiy)

Another of our projects deals with surface processes that affect the formation of tenuous planetary atmospheres, i.e., smaller bodies in the solar system (Mercury, the Moon, other planetary satellites) whose atmospheres are essentially high vacuum. For example, neutral sodium vapor with a column density of ~10E11 - 10E12 Na atoms/cm^{2} has been observed on the solar facing side of the planet Mercury; a somewhat lower concentration of potassium has also been found. Na vapor is also a component of the lunar atmosphere and its density varies with latitude. Both the Moon and Mercury have tenuous atmospheres (p=BE10^{-10} Torr) and neither can retain gaseous species for a considerable time; for example, neutral Na atoms are lost by adsorption, by escape, or by photoionization. Thus, the atmospheric constituents must be continuously resupplied. Various mechanisms have been proposed as the source of supply of atmospheric sodium and potassium: these include charged particle sputtering by the solar wind, impact vaporization from meteoroid collision (with Na and K coming from either the rocky surface or the micrometeoroids), and photon stimulated desorption. In addition, readsorbed Na or K may be thermally desorbed, an atmospheric recycling process. Evidence for 'hot' non-thermal Na has been reported.

Our work deals with the adsorption and desorption of alkalis from oxide surfaces: the focus is on the relevance of surface science studies to the origins of alkali vapors in tenuous planetary atmospheres. Emphasis is on desorption phenomena: thermal desorption, electron- and photon-stimulated desorption to produce non-thermal alkalis, and ion-induced desorption (sputtering). Recent measurements are on a model mineral system, Na on SiO2 thin films. We find strong evidence that non-thermal processes can play an important role in desorption of Na from mineral surfaces.

III) Giant Cl- and F- Enhancements in Electron-Stimulated Desorption of CF2Cl2 Coadsorbed with Water or Ammonia Ices: Implications for Atmospheric Ozone Depletion
(work of Qing-Bin Lu)

Dissociative electron attachment to form Cl- and F- can be an important process for the destruction of ozone-depleting chlorofluorocarbons (CFCs) in the upper atmosphere, because of their extremely large electron attachment cross sections. We have observed giant Cl- and F- enhancements by several orders of magnitude in electron-stimulated desorption of a fractional monolayer of CF2Cl2 coadsorbed with water ice and ammonia ice on a Ru(0001) surface at ~25 K, respectively. The yields of negative ions are measured by an electron-stimulated-desorption ion angular distribution (ESDIAD) detector with time-of-flight capability. The enhancement of Cl- is much stronger than that of F-, and the enhancements for both ion species by NH3 coadsorbate are stronger than by H2O. Moreover, all magnitudes of enhancement increase strongly with decreasing CF2Cl2 concentration; for 0.3 ML CF2Cl2, the largest Cl- enhancements are ~3x10e4 for NH3 and ~10e2 for H2O. In contrast, the enhancements are much smaller for CF2Cl2 coadsorption with rare-gas atoms or nonpolar molecules. Whereas the primary electron beam energy is 250 eV, the giant negative-ion enhancements are attributed to dissociation of CF2Cl2 by capture of low-energy secondary electrons self-trapped (solvated) in polar water or ammonia clusters. This process may be an important sink for chlorofluorocarbons (CFCs) in the atmosphere, where low-energy electrons created by cosmic ray ionization can be trapped in clouds. Cl- ions produced may be directly or indirectly converted to Cl atoms, which then destroy ozone.