Structural characterization of the hydrogen-covered C(100) surface by density functional theory calculations

We present the results of a density functional theory (generalized gradient approximation) study of the hydrogenated C(100) surfaces. We have analyzed the formation energy of phases with different hydrogen coverages (theta=1.0, 1.2, 1.33, 1.5, and 2.0) as a function of the hydrogen chemical potential. As the hydrogen chemical potential increases, the stable phase changes from the bare surface through all the hydrides considered, in order of increasing coverage. The value of the hydrogen chemical potential beyond which dihydride units are stabilized on the surface nearly coincides with the potential at which the the formation energy of methane is zero. However, since the desorption of hydrocarbons is an activated process, dihydride units can appear in metastable surface phases. To investigate this possibility, we have calculated vibrational frequencies for the (2x1):H, (5x1):1.2H, and (3x1):1.33H phases of the H/C(100) surface. The presence of a dihydride unit on the surface leads to a H-C-H bending mode with a frequency near 1475 cm(-1). Because there is no vibrational mode with a frequency in this region of the spectrum for the monohydride surface, this peak may be viewed as evidence that dihydride units are present on the surface.