Recombinant desorption reaction dynamics

Experimental probes of Born-Oppenheimer breakdown under conditions where large amplitude vibrational motion can occur are now becoming available. One approach to this problem is to compare theoretical predictions and experimental observations for reactive properties that are sensitive to the Born-Oppenheimer potential energy surface. Particularly useful for this endeavor are recombinative desorption and Eley-Rideal reactions. In both cases, gas-phase reaction products may be probed by modern state-specific detection methods, providing detailed characterization of the product reaction dynamics. Theoretical predictions based on Born-Oppenheimer potential energy surfaces should be capable of reproducing experiment. Observed deviations between experiment and theory may be attributed to Born-Oppenheimer breakdown. 

Product state analysis offers a flexible way to obtain detailed state resolved information on simple surface reactions and to explore how their dynamics differ from the behaviour observed for H2 desorption [7]. In this chapter, we will discuss some simple surface reactions for which detailed product state distributions are available. We will concentrate on N2 formation in systems where the product desorbs back into the gas phase promptly carrying information about the dynamics of reaction. Different experimental techniques are discussed, emphasising those which give fully quantum state resolved translational energy distributions. The use of detailed balance to relate recombinative desorption measurements to the reverse, dissociation process is outlined and the influence of the surface temperature on the product state distributions discussed. Simple low dimensional models which provide a reference point for discussing the product energy disposal are described and then results for some surface reactions which form N2 are discussed in detail, emphasising differences with the behaviour of H2. 

In the 1970s and 1980s both the clean and H-covered Si surfaces were characterized by diffraction and spectroscopic methods, but only in the last decade have there been reproducible studies of chemical kinetics and dynamics on well-characterized silicon surfaces. Despite the conceptual simplicity of hydrogen as an adsorbate, this system has turned out to be rich and complex, revealing new principles of surface chemistry that are not typical of reactions on metal surfaces. For example, the desorption of hydrogen, in which two adsorbed H atoms recombine to form H2, is approximately first order in H coverage on the Si(lOO) surface. This result is unexpected for an elementary reaction between two atoms, and recombi-native desorption on metals is typically second order. The fact that first-order desorption kinetics has now been observed on a number of covalent surfaces demonstrates its broader significance. 

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