Adsorption-trapping and sticking

For molecular dissociation, the qualitative behavior of S(Et, 0 , Ts) depends greatly on the mechanism of dissociation, i.e., whether it is direct or precursor-mediated. When the dissociation is direct, absolute values of S are generally only weakly dependent on Ts. On the other hand, 5 is a very strong function of Ts for precursor-mediated dissociation (eq. (2.7)), decreasing nearly exponentially with Ts when Ec Ed and increasing nearly exponentially when Ec Ed. Generally, the entrance to the precursor state is non-activated and S decreases with Ej due to a 

High translational energies are available by seeding a small mole fraction of a heavy gas in a light carrier gas (He or H2) and by raising the nozzle temperature (b) 

2 and a static V 1.0eV. However, as discussed earlier, the only experimental evidence for the presence of lattice coupling in direct dissociation is a 

Ts dependence of S. This seems well-described as a linear broadening of the S with Ts [113,114] 

One issue that is particularly interesting for activated dissociation is the importance of translational vs. vibrational activation since this relates to the topology of the barrier location on the PES (see Section 2.3.1). In analyzing experiments, it has been traditional to define the vibrational efficacy rjv as in eq. (2.5). This analysis, however, assumes that A(v) is the same for all v and this may not be universally true. In this case, describing vibrational efficacy is more complicated. Very recently, experiments for CH4 dissociation on transition metals even combine supersonic nozzle molecular beams with laser state preparation techniques to probe the reactivity of specifically prepared vibration rotation states [115-118] (see Section 4.3.1.3). 

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