Photon and electron emission as indicators of intermediate states in surface reactions

Abstract

We have further developed a conceptual model for the photon emission observed during chemisorption on metals proposed by Nørskov, Newns and Lundqvist, and extended it to electron emission. With estimated molecular velocities and electronic structure in the region slightly outside the surface, the probability of an electronically excited intermediate state is calculated. In the deexcitation of this state, valence band Auger transitions are much more probable than radiative decay, but the photon emission probability is substantial enough to give experimental spectra in several cases. Of the Auger electrons, those in the tail above the work-function barrier can be detected as so called exoelectrons. The photon and electron emission is direct evidence that chemisorption processes may take place via non-adiabatic processes. The model has been used to discuss new experimental data for the reaction of oxygen with Al. The emitted light for O 2/Al has been spectrally analyzed with optical filters. The spectrum consists of a broad emission band with a peak at about 5700 Å (2.2 eV) in the chemisorption region. With increasing surface coverage the spectrum shifts towards lower energy.