EXTERNAL INFRARED REFLECTION ABSORPTION-SPECTROSCOPY OF METHANOL ON AN EPITAXIALLY GROWN SI(100)2X1 SURFACE

Infrared spectra of thin layers of hydrocarbons adsorbed on a Si single-crystal surface (monolayer or less) using a single reflection are difficult to obtain as this material is highly transparent in the infrared region, resulting in a relatively poor signal-to-noise ratio of the spectra. To overcome this problem, in a first step a highly reflecting CoSi2 layer was imbedded into a Si single crystal using ion bombardment and subsequent annealing. In a second step, a well-ordered and impurity-free Si layer of 600 angstrom thickness was grown onto the surface, using molecular beam epitaxy. This peculiarly prepared sample was then used to study the adsorption of methanol at 100 K and its subsequent decomposition upon heating. At 100 K, the initial adsorption in the first layer at low methanol exposure and the following multilayer formation after high methanol exposure could be clearly distinguished by the observation of specific bands. The condensed phase is completely desorbed after heating the sample to 150 K, leaving a spectrum which is identical to the spectrum obtained at 100 K for low methanol exposures and which is attributed to a surface methoxy species adsorbed on Si(100). No decomposition of this surface intermediate is observed below 500 K, indicating a relatively high stability of the methoxy species.