Physisorption of CO on the MgO(100) Surface

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 (TPD) of CO. Highly ordered MgO(100) surfaces are used to study the adsorption and desorption of CO. A linearly increasing sticking coefficient from 0.47 ± 0.03 to 0.90 is observed for relative CO coverages, θ, less than 0.8 monolayer (ML). For this coverage range, the total sticking coefficient is given by SMgO(1 − PCO) + SCOPCO, where SMgO (SCO) is the sticking on the bare (CO-covered) MgO and PCO is the probability of striking the CO-covered surface. In TPD, the desorption of CO is dominated at very low coverages by desorption from sites influenced by defects. At intermediate coverages (0.25−0.8 ML) the CO desorbs via first-order desorption. At 0.8 ML where the monolayer peak saturates, the desorption energy is 17 ± 2 kJ/mol and the preexponential factor is 1 × 1015±2 s-1. The desorption energy increases linearly as coverage decreases due to repulsive interactions between adsorbed CO molecules. Above θ = 0.8 ML the adsorption occurs on fully-CO-covered MgO(100) surfaces, and further increases in θ are achieved by compression of the CO layer. The compression results in a sharp decrease in desorption energy, which, upon saturation of the first CO layer (θ = 1 ML) and the formation of a c(4×2) ordered phase, has a value of ∼9 kJ/mol.