THERMAL-DESORPTION FROM HYDROGENATED AND OXYGENATED DIAMOND (100) SURFACES

Low energy electron diffraction (LEED) has been used to study the effects of atomic and molecular species of hydrogen and oxygen on the reconstructed C(100)-(2 X 1) surface. Thermal desorption spectroscopy was also used to study desorption products and kinetics from hydrogenated and oxygenated surfaces. Atomic hydrogen appears relatively inefficient at breaking C-C dimer bonds on the (100)-(2 X 1) surface. Atomic oxygen, in contrast, readily converts the surface from the 2 X 1 state to the 1 X 1 state. This process is reversible for a limited number of cycles before degradation of the surface obscures the 2 X 1 LEED pattern. Oxygen is thought to adsorb in one of two configurations, bridging carbon atoms on the surface, or double bonded to a single carbon atom on the surface. Thermal desorption of molecular hydrogen from hydrogenated C(100)-(2 X 1):H surfaces occurs at approximately 900-degrees-C for a heating rate of 20-degrees-C/s. Molecular hydrogen is the major desorption product and the desorption temperature appears to be coverage independent. Thus the desorption kinetics are most likely first order. Thermal desorption of carbon monoxide from oxygenated C(100)-(1 X 1):O surfaces occurs at approximately 600-degrees-C for a heating rate of 20-degrees-C/s. Carbon monoxide is the major product seen, with small quantities of carbon dioxide also observed. For increasing oxygen coverages, the desorption peak is observed to shift to lower temperatures. A peak shift to lower temperatures can be interpreted as either first order kinetics with a coverage dependent activation energy or second order kinetics. The reaction order is not known in this case, but from analysis of the peak shapes and from the fact that CO can desorb without prepairing, the data suggest that the reaction is first order.