Comparison of photon stimulated dissociation of gas phase and chemisorbed CO

Abstract

Theoretical interpretation of electron–electron and electron–ion coincidence data for CO in the gas phase indicates one hole, two hole, and two hole–one electron excited states (e.g., the 3σ⁻¹, 5σ⁻², and 5 σ⁻² 6σ states) contribute to molecular dissociation. Detailed comparison with O⁺ ion yield data for photon stimulated desorption from CO/Ru (001) at both valence and core level excitation energies reveals many similarities but also significant adsorbate‐substrate interaction effects; the most important are due to charge transfer from the substrate which screens the core or valence holes in the adsorbate. Appreciation of the competitive decay rates of the excited states is critical to an understanding of which excited states of either CO gas or the CO/metal system may be expected to lead to dissociation. Resonant decay to the continuum and Auger decay are important decay mechanisms in both systems. Adsorbate–adsorbate interactions on the surface are seen to provide an additional decay mechanism. The excited states responsible for O⁺, CO⁺, and CO desorption from the surface are assigned for each of the threshold energies. The two hole–one electron states are the most important for desorption from the surface.