Physisorption of CO on the MgO(100) surface

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, theta, less than 0.8 monolayer (ML). For this coverage range, the total sticking coefficient is given by S-MgO(1 - P-CO) + SCOPCO, where S-MgO (Sco) is the sticking on the bare (CO-covered) MgO and P-CO is the probability of striking the GO-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 x 10(15 +/-2) s(-1). 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-GO-covered MgO(100) surfaces, and further increases in theta 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 (theta = 1 ML) and the formation of a c(4x2) 'ordered phase, has a value of similar to9 kJ/mol.