Book/Report

FZJ-2018-02472

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Elektronenstimulierte Desorption an Stickoxiden auf Pt(100) Flächen

Schwalke, U. (Corresponding author)FZJ*

1984
Kernforschungsanlage Jülich, Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/18142

Report No.: Juel-1963

Abstract: Electron stimulated desorption (ESD) of NO and NO$_{2}$ adsorbed on Pt(100) surfaces has been investigated. The ESD process as well as chemical reaction of nitrogen oxides on Pt(100) have been studied in combination with AES (Auger-electron spectroscopy), LEED (low energy electron diffraction) and TPD (temperature programmed desorption). Different ESD mechanisms are found to be responsible for generating positive ions from molecular and dissociated NO on Pt(100). In case of completely dissociated NO O$^{+}$ desorption occurs at low primary electron energies (< 50 eV) due to valence-band excitations. In contrast, ions from molecularly adsorbed NO on Pt(100) are desorbed by the core-level assisted ESD process exclusively: O$^{+}$ and O$^{2+}$ desorption occurs only upon bombardment with electron energies above 565 eV and 590 eV, respectively, i.e. with energies larger than the binding energy of the O$_{ls}$ adsorbate core-level. The fact that the threshold energies are 34 eV and 60 eV above the corresponding core-level binding energy is attributed to multielectron excitations. The O$^{+}$ ESD signal of this core-level assisted ESD process is found to be proportional to NO coverage up to $\theta_{NO}$ ~ 0.8. Accordingly this ESD signal can be used quantitatively to a good approximation to detect NO on Pt(100). It is shown that due to the large differences between the threshold energies for O$^{+}$ desorption from molecular and dissociated NO ESD can be applied to discriminate between these two species. For molecularly adsorbed NO$_{2}$ on Pt(100) again only the core-level assisted ESD process is operative for ion desorption. It should be noted that only N$^{+}$ but neither O$^{+}$ nor NO$^{+}_{2}$ desorption occurs. The threshold energy of 460 eV lies 55 eV above the N$_{ls}$ adsorbate binding energy. Considering the proposed bonding geometry of NO$_{2}$ on Pt(100), the lack of O$^{+}$ emission is explained by the recapture of O$^{+}$ ions due to the attractive image force. This suggestion is confirmed by the experimental findings of the electron induced dissociation of NO$_{2}$. On the basis of the detailed knowledge of the characteristic ESD signals from NO$_{2}$(ad), NO$_{(ad)}$ and O$_{(ad)}$, the ESD technique was used to investigate the adsorption and decomposition of NO$_{2}$ on Pt(100). The two modifications of the Pt(100) surface (nonreconstructed lxI and reconstructed 5x20 phase) were found to play an important role in the adsorption and decomposition behavior of NO$_{2}$ on Pt(100). By means of ESO as well as LEEO and TPO measurements it is shown that the decomposition of molecularly adsorbed NO$_{2}$ on the reconstructed surface does not follow simple dissociation kinetics and that it is accompanied by a 5x20 $\rightarrow$ 1x1 surface phase transition. Isothermal measurements give evidence for an autocatalytic reaction which occurs only above a critical coverage; the autocatalytic reaction is strongly related to the structure transformation of the Pt(100) surface.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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