Precursor states in reactive gas—solid interactions

Even when the result of a gas—solid collision is the formation of a stable chemisorbed species, weakly bound precursor states can play a major role in the kinetic process. Evidence for such precursor states has recently been reviewed by Cassuto and King, [21] who draw a distinction between intrinsic precursor states, which exist at empty surface sites, and extrinsic precursor states, which exist over sites filled with chemisorbed species. The ability of colliding species to be trapped in these states and to be efficiently transported across the surface is an important mechanistic feature in adsorption. A confusion in nomenclature can arise when a metastable, or virgin , chemisorbed state can be formed on the surface as an intermediate between physisorbed and stable chemisorbed states for example, at low temperatures, a virgin, non-dissociatively chemisorbed state of CO is formed on tungsten which can be converted to a dissociatively bound state on heating [102]. In the few cases that have been investigated, 

The striking kinetic consequence of the mobile extrinsic precursor — adsorption rates which may be effectively coverage-independent over a wide range of coverage — in fact constituted the first experimental evidence for its existence [2, 10]. It is not, however, the only evidence, as has recently been suggested [297]. 

Experimental evidence for the existence of intrinsic precursor states is rather more difficult to come by. The common observation that the initial sticking probability, s0, often decreases with increaing substrate temperature is consistent with the existence of such a state, as discussed here. Indirect evidence is also provided by molecular beam studies, for example, Hayward and Walters [401] (for H2 on W 001 ) and Engel [402] (for 02 on Pd 111 ) have observed scattered particle intensity distributions which, even at a fractional coverage in the chemisorbed layer close to zero, exhibit a strong directional lobe in the specular direction superimposed on a cosine law distribution. The specular lobe clearly contains molecules scattered at the first collision, while the cosine law component is most readily attributed to the particles which are trapped in the precursor state and then scattered back into the gas phase. Of 

Finally, in systems where trapping into the precursor state is efficient, relative to the process of direct transfer on impact into the chemisorbed state, it would be possible to freeze the oncoming molecule into the precursor state by cooling to very low temperatures ( 30 K) spectroscopic techniques could then be employed to examine the state of the precursor and its conversion on heating. 

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