The indirect channel un-accommodated precursor

The results for Pt(5 3 3) (Figs. 11 and 26) are consistent with the increased sticking probability observed for a Maxwellian source of H2 on Pt(9 9 7) over Pt(l 1 1) [81]. It also provides direct evidence for an additional channel to dissociative adsorption through step sites which was invoked to explain the enhanced rate of H2 + D2 exchange reaction at Pt(3 3 2) over that observed on Pt(l 1 1) surfaces investigated using a Maxwellian beam source [82]. 

In order to reveal the effect of blocking the dissociation at the step sites by oxygen, curves were fitted to the energy dependent sticking probability on the step-decorated and clean Pt(5 3 3) surfaces (Fig. 28), and subtracted the former from the latter. The result is shown in the heavy curve in Fig. 28. This proportion of S0 decreases with increasing energy over the range 0 Ej(meV) 150 to a constant 

Hydrogen dissociation on tungsten is facile, with significant probabilities of dissociation at very low 

W(1 0 0) surface [164]. A similar very small dependence of So on Ts was also observed elsewhere [165]. This lead to the suggestion [164, 168] that the precursor responsible for dissociation may not be fully accommodated at Ts, but had sufficient lifetime at the surface to undergo dissociation if it encountered a defect (or step) site. It also leads to the series of experiments in which H2 and N2 dissociation was investigated on the W(1 0 0)-c(2 x 2)Cu alloy surface in order to establish the effect of changing the activation barrier to direct dissociation in the surface unit cell, and concomitant effects on the indirect dissociation channel. 

Subtraction of the indirect channel contribution to the sticking data on W(1 0 0) and W(1 0 0)-c(2 x 2)Cu (Fig. 30) allows a comparison of the barriers to dissociation via the direct channel on these surfaces 

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