Observation of excited electrons from nonadiabatic molecular reactions of NO and O-2 on polycrystalline Ag

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 "che micurrent" in a large area ultrathin film Ag/Si(1 1 1) Schottky diode senso r. 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 b arrier ( less than or equal to 0.5 eV). The detected current from nitric ox ide exposure decreased with increasing coverage from an initial peak intens ity of 1 x 10(-4) e(-)/incident molecule. A secondary peak (5 x 10(-5) e(-) /incident molecule) was observed at an exposure of approximately 22 ML. The signal decayed to the noise floor (similar to 100 fA) at longer exposures (> 40 ML). With molecular oxygen exposure a smaller peak intensity of 1 x 1 0(-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 oxid e is attributed to the superposition of charge carriers produced by nonadia batic adsorption of NO at 130 K on the Ag surface with carriers generated f rom the coverage dependent chemisorption of atomic oxygen produced during t he formation and decomposition of (NO2) dimers into atomic nitrous oxide. T he 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 wi th molecular adsorption generating the detected current. The detection of t his "chemicurrent" is direct experimental evidence of nonadiabatic energy t ransfer during molecular adsorption, (C) 2001 Published by Elsevier Science B.V.