Diels–Alder reactions of butadienes with the Si(100)-2×1 surface as a dienophile: Vibrational spectroscopy, thermal desorption and near edge x-ray absorption fine structure studies

The mechanism and intermediates of a Diels–Alder-type cycloaddition reaction between dienes and the silicon dimers of a Si(100)-$\text{2×1}$ surface, which was theoretically predicted by Doren and Konecny, have been investigated. The reactions of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene were studied using multiple internal reflection infrared spectroscopy, thermal desorption spectrometry, and near edge x-ray absorption fine structure (NEXAFS) measurements. The results show that the compounds physisorb on Si(100)-$\text{2×1}$ at cryogenic temperature. Infrared studies of the room temperature adsorption of both dienes indicate that reaction leads to the formation of stable, chemisorbed Diels–Alder adducts. By NEXAFS measurements on 2,3-dimethyl-1,3-butadiene, we determine that the angle between the π orbitals of the reaction product and the Si(100)-$\text{2×1}$ surface is near 40°. Upon heating, the chemisorbed butadienes primarily decompose to form adsorbed carbon and hydrogen at the surface. Hydrogenation of chemisorbed butadienes by atomic hydrogen was also investigated, and conversion from the Diels–Alder geometry to $\text{[2+2]}$ bonding is observed. This effect is attributed to cleavage of the Si–Si dimer bond upon atomic hydrogen exposure. Temperature dependent studies suggest the presence of an activation barrier for chemisorption at low temperature. The parallels between our experimental results and the theoretical studies of these reactions are discussed.