The role of tunneling in precursor mediated dissociation: Alkanes on metal surfaces

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Abstract
While all experimental results for methane dissociation on metals are compatible with a direct on‐impact tunneling mechanism, some larger alkane‐metal systems display both direct and indirect dissociation channels, the latter involving a molecularly adsorbed intermediate. To date, decay of such intermediates via dissociation or desorption has only been treated within transition‐state theory, i.e., in terms of classical ‘‘over the barrier’’ processes. We show here that an alternative mode of dissociation, tunneling, can compete effectively with desorption even when barriers to dissociation are relatively high. We discuss several experiments involving alkane dissociation on metals in light of this and show that they can be understood solely in terms of tunneling, both for the direct on‐impact process and for precursor mediated processes. This interpretation resolves an apparent contradiction that arises when these experiments are interpreted strictly within transition‐state theory.