Ab initio electronic structure calculations are performed on ethyl adsorbed on Pt(100) to investigate whether photoexcitation and the resulting electron attachment to ethyl will induce dissociation by H transfer to the surface. Calculations are carried out using an embedded cluster description of the adsorbate-metal system, and excited electronic states are resolved by configuration interaction theory. The results show there is no barrier to H transfer in the excited state, thereby predicting that ethylene will be formed if the excited state has sufficient lifetime.