AVS 45th International Symposium
    Surface Science Division Friday Sessions
       Session SS2-FrM

Paper SS2-FrM5
Low Energy Dynamics through the Verwey Transition : Water Adsorbed on Fe@sub 3@O@sub 4@(100)/MgO(100)

Friday, November 6, 1998, 9:40 am, Room 309

Session: Water and Ice Interfaces
Presenter: C.J. Hirschmugl, University of Wisconsin, Milwaukee
Authors: C.J. Hirschmugl, University of Wisconsin, Milwaukee
M. Takasaki, University of Wisconsin, Milwaukee
M. Collins, University of Wisconsin, Milwaukee
C.H.F. Peden, Pacific Northwest National Laboratory
S.A. Chambers, Pacific Northwest National Laboratory
Correspondent: Click to Email
The far- and mid-infrared broadband absorptions and discrete vibrations have been studied for water adsorbed on an epitaxial 2000 Å Fe@sub 3@O@sub 4@(100) film using infrared synchrotron radiation. Water on Fe@sub 3@O@sub 4@ represents an ideal example both practically and fundamentally, as Fe@sub 3@O@sub 4@ is a prominent subsurface mineral (magnetite), and the material exhibits an interesting electronic transition. Specifically, the substrate undergoes a metal-semiconductor (Verwey) transition at 120 K (ten-thousand-fold change in conductivity from approximately 10@super 2@ [ohm-cm]@super -1@ to approximately .01 [ohm-cm]@super -1@), while water adsorbed on the surface is stable in vacuum until above 350 K. The frequency-dependent conductivity in normal-incidence reflectance measurements follows a Drude behavior above the transition, and is non-Drude below the transition. Hence, the infrared response of this system, both above and below the transition, provides a comparison of the water-substrate interaction in the metallic and semiconducting states. The present infrared studies employ synchrotron radiation, which can extend traditional IRAS measurements to below 400 cm@super -1@ with noise levels of approximately .01% attainable in 100 seconds measuring time. In addition, these measurements are complemented by TPD and concurrent resistivity measurements. Notably, three distinct cation adsorption sites are available on the reconstructed Fe@sub 3@O@sub 4@(100) surface: a tetrahedrally bonded Fe@super 2+@; a tetrahedrally bonded Fe@super 3+@; and an octahedrally bonded Fe@super 3+@. Molecularly adsorbed water is shown to sequentially fill these sites. In addition, adsorbed multilayers of water reveal large anti-absorption resonances in the infrared spectra for the molecular vibrations and the substrate phonons.