While the hydrogen elimination reaction is frequently invoked when describing the dissociative desorption mechanisms of molecules from surfaces, its kinetics are less understood. This desorption mechanism occurs on Si(100)-2×1, an environment with highly directional, singly-occupied surface bonds in rows of dimers. The antiparallel nature of the dangling surface bonds gives rise to a low-lying electronic excited state following a spin flip. Using a combination of experimental and theoretical techniques, we demonstrate that the production of alkenes from halogenated alkanes on the Si(100) surface is driven by nonadiabatic β-hydrogen elimination, during which the electronic spin multiplicity of the system changes from singlet to triplet. Rate constants calculated with nonadiabatic transition state theory with the Landau-Zener probability of transition indicate that nonadiabatic reaction channels are kinetically favored, despite the reaction being nominally spin-forbidden. Experimental desorption rates obtained using temperature programmed desorption yielded activation energies in strong agreement with the theoretical energy barrier to the minimum energy crossing point between the two spin states. The relatively low values for the probability of transition and rate constants through the crossing point are consistent with the spin-forbidden, nonadiabatic step being the kinetic bottleneck.

Alkyl desorption from Si(100)-2×1 by nonadiabatic hydrogen elimination

253rd National Meeting (2017)

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