Observation of excited electrons from nonadiabatic molecular reactions of NO and O2 on polycrystalline Ag

Abstract

Adsorption of ground state nitric oxide and molecular oxygen on Ag at 130 K were observed to produce excited electrons which were detectable as a "chemicurrent" in a large area ultrathin film Ag/Si(1 1 1) Schottky diode sensor. The charge carriers produced at the surface had sufficient lifetimes and energies to reach the Ag/Si interface (6-8 nm) and surmount the Schottky barrier (≤0.5 eV). The detected current from nitric oxide exposure decreased with increasing coverage from an initial peak intensity of 1 × 10-4 e-/incident molecule. A secondary peak (5 × 10-5 e-/incident molecule) was observed at an exposure of approximately 22 ML. The signal decayed to the noise floor (∼100 fA) at longer exposures (>40 ML). With molecular oxygen exposure a smaller peak intensity of 1 × 10-5 e-/incident molecule was observed, followed by a decay in signal to the noise floor at longer exposures (10 ML). The signal from nitric oxide is attributed to the superposition of charge carriers produced by nonadiabatic adsorption of NO at 130 K on the Ag surface with carriers generated from the coverage dependent chemisorption of atomic oxygen produced during the formation and decomposition of (NO2) dimers into atomic nitrous oxide. The latter is the cause of the observed secondary peak. The detected current from molecular oxygen exposure on the Ag surface at 130 K is consistent with molecular adsorption generating the detected current. The detection of this "chemicurrent" is direct experimental evidence of nonadiabatic energy transfer during molecular adsorption. © 2001 Published by Elsevier Science B.V.