Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 5×10(-5) electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions.

The chemicurrent response of Au−TaOx−Ta metal−insulator−metal junctions exposed to a flux of atomic hydrogen has been studied in detail for device temperatures between 110 and 300 K. Currents of some 100 pA are observed at an H atom flux of 4 × 10 14 atoms cm −2 s −1 . The steady-state current closely tracks the rate expected for the Langmuir−Hinschelwood recombination reaction as a function of temperature. The current trace reflects the reaction kinetics when the H-atom flux is modulated. The rate constant is directly determined from the individual trace at each temperature. Moreover, it is observed that the Fermi level of the 20 nm thick top metal film shifts with varying H coverage, giving rise to an additional charging/discharging current. For device temperatures below 200 K, subsurface hydrogen becomes significant.

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The creation of electronic excitations during the reaction of atomic hydrogen on and with coinage and noble metals has been studied using metal-insulator-metal heterostructures. A characteristic current trace is observed when the outer metal surface of the structure is exposed to a 20 s pulse of H atoms. Comparison to the chemical kinetics allows to disentangle the contributions from the different chemical processes to this current. In the case of the coinage metals studied the observation is interpreted to suggest that predominantly electrons excited in connection with the hydrogen recombination reaction are the source of the current. For Pt such phenomenon is not observed. This difference allows insights into the role of the transition state for the non-adiabaticity in this simple chemical reaction.