Graduate Thesis Or Dissertation
 

Submonolayer adsorption on a metal surface, investigated with Rydberg atoms as electric-field-sensitive probes

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  • Electric fields due to adsorbates on a metal surface are studied experimentally with Rydberg atoms as probes. A beam of lithium Rydberg atoms, produced by an oven in a high-vacuum system, passes within 3μm of the surface of a metal mesh having 6μm square openings. Rydberg states with principal quantum numbers in the range 20 < n < 80 are produced by electron impact. Detected changes in the Rydberg atom flux through the mesh allow adsorption to be studied. The adsorption information obtained includes sticking probability, electric dipole moment, and desorption energy. Methods are presented for the calculation of surface electric fields produced by adsorbed atoms or molecules, which are modeled as individual dipoles. Coverages of 10⁻² monolayer can produce electric field magnitudes E > 120 V/cm at distances < 3μm from the surface. Fields of this magnitude can ionize Rydberg atoms present in the atomic beam, reducing the flux through the mesh. A relationship is derived connecting the adsorbate dipole-moment density on a metal surface to the electric field produced by adsorbates. Desorption energies were measured at surface temperatures near 300 K for H₂O (1.05±0.05 eV), CO (0.70±0.05 eV), and 0₂ (0.86±0.06 eV) on a polycrystalline Ni surface and for H₂O (0.85±0.05 eV) and SO₂ (0.93±0.06 eV) on a polycrystalline Au surface. The product of the sticking probability and dipole moment, ps, was measured at surface temperatures near 300 K for CO, NO, 0₂ , H₂0, and SO₂ on the Ni surface and for H₂0 and SO₂ on the Au surface. The orientations of the dipole moments for CO, NO, and 0₂ were inferred from the experimental data and the dipole moment for SO₂ (0.086±0.004 ea[subscript o] ) on Ni was measured. The measured desorption energies are comparable, within approximately 30%, to values reported in the literature for adsorbates on specific crystal faces. Other measured quantities are compared to values reported in the literature. The limitations of the technique are discussed.
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