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要旨:
The interaction of potassium atoms on top of Cr2O3/Cr(110) has been studied using work-function (ΔΦ), temperature programmed desorption (TPD), and optical second-harmonic generation (SHG) measurements. Potassium grows via the completion of a first layer, followed by a second layer in the form of two-dimensional (2D) islands, and at higher coverage 3D clusters are formed. This growth model is supported by and consistent with the results obtained from all three methods. Work-function data suggest that annealing at temperatures above 350 K results in the formation of a surface potassium oxide compound, provided the potassium coverage is higher than 0.5 monolayers (ML). Diffusion of alkali-metal atoms on an oxide surface is reported here over distances of several micrometers. This was measured using optical SH diffraction from coverage gratings that were generated by laser-induced thermal desorption. The activation energy for surface diffusion of potassium on Cr2O3(0001)/Cr(110) has been determined to be 11±0.5kcal/mol with a preexponential factor D0=105cm2/sec in the coverage range of 1.5–2.5 ML, dropping to 9 kcal/mol and D0=3×103cm2/sec at a coverage of 3.0 ML. These results are consistent with the diffusion of atoms in the third layer, on top of two-dimensional potassium islands in the second layer, the activation energy represent the barrier for descending from the 2D islands.
