Dynamical studies of molecular dissociation on Mo(100)

Abstract

The dynamics of the dissociative chemisorption of H2 on clean, hydrogen covered and nitrogen covered Mo(100), and of N2 on clean and nitrogen covered Mo(100), has been studied under UHV conditions using a supersonic molecular beam. H2 and N2 dissociative adsorption on the clean Mo(100) surface are found to proceed via both a direct and classical accommodated indirect channel. Additionally a dynamic channel is identified specific to the H2/Mo(100) adsorption system. The dynamic channel allows dissociative adsorption to take place at incident energies greater than can be accounted for in terms of a fully accommodated molecular precursor, the channel extending to incident energies of around 70meV, and has very little, or no, surface temperature dependence (dSo/dTs<-1.4xlO"4 K"1). Two possible mechanisms are suggested to account for the dynamic channel, both consistent with the experimental observations. One mechanism is that of "dynamic steering", where incident molecules' are very strongly steered into particularly favourable dissociation geometries on particular surface sites. The other suggested mechanism is that of a "dynamic precursor", where, rather than trapping taking place via accommodation of the molecules' incident energy to the surface (as is the case for a typical accommodated precursor), steering forces instead allow the transfer of energy from momentum normal to the surface to other molecular degrees of freedom. The creation of an Mo(100)-c(2x2)N surface causes a considerable increase in the barrier to direct dissociative adsorption encountered by the H2 molecule, the minimum barrier being shifted to >70meV. This change is also accompanied by the loss of the dynamic channel, although a concurrent increase in the contribution of the fully . accommodated precursor channel is thought to somewhat mask this loss. No evidence of a dynamic channel is found when examining the dynamics of dissociative adsorption of N2 on the Mo(100) surface, all data pertaining to the system being accounted for within the confines of a combination of direct and classical accommodated indirect channels. The apparent lack of a dynamic channel is explained by a combination of the larger mass of the N2 molecule with respect to the lighter H2 (the steering forces having a greater impact on lighter molecules), and their differing electronic structures (this difference defining the depth of the dynamic well into which the molecule might trap). A relatively large amount of research has, in the past, been directed at comparing the surface structure and adsorption kinetics of hydrogen adsorption upon Mo(100) and W(100). This thesis adds to this a comparison of the adsorption dynamics. In addition a comparison is drawn between N2 adsorption on Mo(100) and W(100) with the molybdenum surface seen to mirror the tungsten surface in many ways.

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