Precursor mediated channel

Often, at least for systems where the activation barrier is not too large, both of these channels can co-exist, though each tend to dominate in different temperature regimes. In general the precursor mediated channel will dominate at low substrate and gas temperatures, while the direct channel will be dominant at high gas temperatures, examples of this being O2 adsorption on Cu(llO) (Pudney and Bowker, 1990 Hodgson et ah, 1993) and N2 on Fe(lll) (Ertl, 1991 Rettner and Stein, 1987), discussed below (sect. 3.3). 

The rapid decrease in So(E ) observed below 0.15 eV on Pt(5 3 3) (Fig. 18) has also been observed on the Pt(l 1 1) surface [134] and is consistent with a trapping mechanism where the need to dissipate energy limits the probability of adsorption, and subsequent dissociation, via the physisorbed precursor. In order to assess the contribution of the physisorption mediated channel, the contribution to sticking directly via the chemisorbed channel must be subtracted from the measured So. The proportion of So derived from the direct chemisorption channel on Pt(5 3 3) at Ex = 0.05 eV is significantly higher than on Pt(l 1 1) (ca. 10%) [137]. Once this direct contribution is subtracted, the dependence S0(Ts)can be used to obtain kinetic parameters relating to the partition of the physisorbed precursor. This is achieved... 

Further evidence that it is a step mediated channel responsible for the indirect dissociation on the W(1 0 0) surfaces comes from a comparison with the results for the step mediated dissociation on Pt(5 3 3) (Section 3.2) which exhibit very similar dynamical characteristics. A similar mechanism is likely to be responsible for the indirect dissociation channel on Ni(9 9 7) [89] (Fig. 25)., Y0 decays with Ei over the range 0 < Tq(meV) < 150 on all of the metal surfaces studied where steps are suggested to be responsible for indirect dissociation. S0 is also rather insensitive to 7 s in all cases, and, S (0n) exhibits precursor type dependencies. 

It can be the case that both adsorption channels are important for a particular system. Examples of this are given here for O2 adsorption on Ag and Cu and for N2 dissociation on Fe. In these cases we can generalise and say that the precursor mediated route tends to dominate at low substrate and gas temperatures, while direct activated adsorption dominates at high gas temperatures. Furthermore, in all these cases, molecular chemisorbed states of adsorption can exist which complicate the pathway of adsorption. A one dimensional potential energy profile is shown in fig. 8 for the case of O2 adsorption on Ag taken from the work of Dean and Bowker (1988/89, 1989) and of Campbell (1985), although this is likely to be a general representation for this type of adsorption system with other adsorbate/metal combinations. 

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