Interaction and diffusion of potassium on Cr2O3(0001)/Cr(110)

The interaction of potassium atoms on top of Cr2O3(0001)/Cr(110) has been s tudied using work-function (Delta phi), 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 th e form of two-dimensional (2D) islands, and at higher coverage 3D clusters are formed. This growth model is supported by and consistent with the resul ts obtained from all three methods. Work-function data suggest that anneali ng at temperatures above 350 K results in the formation of a surface potass ium oxide compound, provided the potassium coverage is higher than 0.5 mono layers (ML). Diffusion of alkali-metal atoms on an oxide surface is reporte d here over distances of several micrometers. This was measured using optic al SH diffraction from coverage gratings that were generated by laser-induc ed thermal desorption. The activation energy for surface diffusion of potas sium on Cr2O3(0001)/Cr(110) has been determined to be 11+/-0.5 kcal/mol wit h a preexponential factor D-0=10(5) cm(2)/sec in the coverage range of 1.5- 2.5 ML, dropping to 9 kcal/mol and D-0=3 X 10(3) cm(2)/sec at a coverage of 3.0 ML. These results are consistent with the diffusion of atoms in the th ird layer, on top of two-dimensional potassium islands in the second layer, the activation energy represent the barrier for descending from the 2D isl ands.