PNNL

Abstract

The ability to grow thin MgO(100) films of quality approaching that of vacuum cleaved MgO(100) is demonstrated using low energy electron diffraction and temperature programmed desorption of CO. The highly ordered MgO(100) surfaces are used to study adsorption and desorption of CO. A linearly increasing sticking coefficient from 0.47 ? 0.03 to 0.90 is observed for relative CO coverages less than 0.8 ML. This is a consequence of constant but different sticking coefficient on bare (0.47 ? 0.03) and CO covered (0.90) MgO(100) surface. In TPD, the desorption of CO is dominated at very low coverages by desorption from the defects. At intermediate coverages (0.25 - 0.8 ML) the CO desorbs via first order desorption with the desorption energy of 17 ? 2 kJ/mol and preexponential factor of 1x10(superscript)15?2 s(superscript)-1 at 0.8 ML when the peak saturates. The desorption energy increases linearly as coverage decreases due to repulsive interactions between adsorbed CO molecules. Above Theta = 0.8 ML the adsorption occurs on fully CO covered MgO(100) surfaces and further increase in Theta is achieved by compression of the CO layer. This results in a sharp decrease in desorption energy which upon saturation of the first CO layer (Theta = 1 ML) and formation of c(4x2) ordered phase has value of ~9 kJ/mol.