Adsorption of potassium on Cr2O3(0001) at ionic and metallic coverages anduv-laser-induced desorption

Translational energy distributions of neutral potassium atoms are reported as a function of potassium coverage after uv-laser-induced desorption from well-characterized adsorption sites on an epitaxial film of Cr2O3(0001)/Cr( 110). Measurements using x-ray photoelectron spectroscopy, low-energy elect ron diffraction, and work-function measurements revealed that potassium ads orbs in a nonmetallic phase for deposition temperatures around 280-300 K al lowing only a maximal saturation coverage to be grown for moderate growth r ates. Aggregates are observed after deposition at 90 K; at this temperature any layer thickness is obtainable. The uv-laser-induced desorption for the se two different phases was studied using excitation energies of 3.5 eV, 5. 0 eV, and 6.4 eV and (1 + 1)-resonantly enhanced multiphonon ionization via the 6p P-2 State for detection. Desorption of potassium atoms from the non metallic phase proves to be ten times [sigma(6.4 eV) = (2 +/- 1) x 10(-19) cm(2)] more efficient than desorption from metallic potassium aggregates. T he mechanism of desorption from the nonmetallic phase appears to be the inv erse harpooning process starting with an ion pair followed by a transfer of hot electrons from the substrate to unoccupied potassium states to neutral ize the initially positively charged potassium. The maximum of the translat ional energy distribution (starting at 0.65 eV for low coverages) decreases with increasing potassium coverage and is by a factor of approximately 4 s maller for desorption from large potassium aggregates (0.16 eV). The decrea se of the translational energy with increasing coverages for isolated atoms is ascribed to an increasing lateral interaction between the adsorbates an d a concomittant smooth change of the ionicity of the atoms from partially ionic to neutral. [S0163-1829(99)01619-7].