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Dimer preparation that mimics the transition state for the adsorption of H2 on the Si(100)-2 x 1 surface.

A chemically induced dimer configuration was prepared on the silicon (Si) (100) surface and was characterized by scanning tunneling microscopy (STM) and spectroscopy (STS). These prepared dimers, which are essentially untilted and differ both electronically and structurally from the dynamically tilting dimers normally found on this surface, are more reactive than normal dimers. For molecular hydrogen (H2) adsorption, the enhancement is about 10(9) at room temperature. There is no appreciable barrier for the H2 reaction at prepared sites, indicating the prepared configuration closely approximates the actual dimer structure in the transition state. This previously unknown ability to prepare specific surface configurations has important implications for understanding and controlling reaction dynamics on semiconductor surfaces.