The energy released in low-energy chemisorption or physisorption of molecules on metal surfaces is usually expected to be dissipated by surface vibrations (phonons). Theoretical descriptions of competing electronic excitations are incomplete, and experimental observation of excited charge carriers has been difficult except at energies high enough to eject electrons from the surface. We observed reaction-induced electron excitations during gas interactions with polycrystalline silver for a variety of species with adsorption energies between 0.2 and 3.5 electron volts. The probability of exciting a detectable electron increases with increasing adsorption energy, and the measured time dependence of the electron current can be understood in terms of the strength and mechanism of adsorption.

Hot electrons and holes created at Ag and Cu surfaces by adsorption of thermal hydrogen and deuterium atoms have been measured directly with ultrathin metal film Schottky diode detectors on Si(111). When the metal surface is exposed to these atoms, charge carriers are excited at the surface, travel ballistically toward the interface, and have been detected as a chemicurrent in the diode. The current decreases with increasing exposure and eventually reaches a constant value at the steady-state coverage. This is the first direct evidence of nonadiabatic energy dissipation during adsorption at transition metal surfaces. [S0031-9007(98) PACS numbers: 82.20. Rp, 34.50.Dy, 79.20.Rf, 82.65.My The pathways of energy dissipation in exothermic reactions at metal surfaces are of fundamental interest but still poorly understood. Bond formation energies of up to several eV are transferred into the substrate. Since bulk phonon energies are typically 2 orders of magnitude smaller, nonadiabatic excitations of electron-hole (e-h) pairs may be a significant alternative to multiple phonon creation [1][2][3][4]. With surface reactions at thermal collision energies, there are a few examples of energy transfer into the electronic system accompanied by light emission (chemiluminescence) [5] and exoelectron ejection [6]. These effects are observed only with the exothermic adsorption of electronegative molecules on reactive metal surfaces. In addition, exoelectron emission requires that the metal has a low work function. Heretofore, there has been no direct experimental evidence for adsorptioninduced e-h pair excitations at transition metal surfaces. Although the maximum energy of any hot charge carriers is smaller than the metal work function, precluding exoelectron emission, the energy may be sufficiently large to enable the charge carriers to be collected by crossing a smaller potential barrier.We demonstrate in the present study that the direct detection of chemisorption-induced e-h pairs is feasible using the Schottky barrier of a transition metal-semiconductor diode detector. We chose the adsorption of atomic hydrogen on Ag and Cu film surfaces as model systems. These metals exhibit high reactivity to atomic hydrogen but negligible dissociative adsorption of H 2 [7]. The formation energy of the H-metal bond is large, about 2.5 eV in both cases [7]. To detect the hot charge carriers, we designed a sensor consisting of a large-area metal-semiconductor contact with an ultrathin metal film. The proposed mechanism of current production in the sensor is illustrated in Fig. 1(a) for the case of hot electrons. The transition metal film is evaporated on n-type Si forming a diode with a Schottky barrier, F. Figure 1(a) shows the Fermi level, E F , the conduction band minimum (CBM) and the valence band maximum (VBM). If the exothermic chemisorption of H atoms creates e-h pairs, hot electrons may travel ballistically through the film and cross the barrier. They can be detected as a current which we hereafter call a chemicurrent. Similarly...

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