Sticking coefficient surface coverage

To determine the surface coverage at steady state we balance the adsorption rate with the desorption rate. Setting equations 10.5 and 10.6 equal  

Beeause As is supphed as a tetramer when a standard evaporation souree is used it has been found to be neeessary to look closely at the actual site where the tetramer arrives to determine whether it will stick. One might imagine that an As tetramer would require as many as four open sites on the sample surface sufficiently close to eaeh other to obtain a high stieking coefficient. In practice the number of sites required is eloser to two but is signifieantly greater than one. A theory of sticking of 

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Surface plane Adsorbate Adsorbed species Initial sticking coefficient Saturation coverage, lO sites cm LEED pattern at 25 C FD peak temperatures, °C Activation energies for desorption of original adsorbate, kJ mole Ref... 

The probability for sticking is known as the sticking coefficient, S. Usually,. S decreases with coverage. Thus, the sticking coefficient at zero coverage, the so-called initial sticking coefficient,. S q, reflects the interaction of a molecule with the bare surface. 

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

Measuring the uptake of a gas by a surface as a function of the dose to which the surface is exposed is the most straightforward way to determine a sticking coefficient. In such experiments, great care should be taken to ensure that gas and surface are in thermal equilibrium. In addition, we need to determine the coverage, either by surface sensitive methods (XPS, AES, IR) or by thermal desorption and ensure that adsorption is not accompanied by desorption. 

Frequently, adsorption proceeds via a mobile precursor, in which the adsorbate diffuses over the surface in a physisorbed state before finding a free site. In such cases the rate of adsorption and the sticking coefficient are constant until a relatively high coverage is reached, after which the sticking probability declines rapidly. If the precursor resides only on empty surface sites it is called an intrinsic precursor, while if it exits on already occupied sites it is called extrinsic. Here we simply note such effects, without further discussion. 

Sticking coefficient is the ratio of the rate of adsorption to the rate at which the adsorptive strikes the total surface, i.e., covered and uncovered. It is usually a function of surface coverage, of temperature and of the details of the surface structure of the adsorbent. 

Equations similar to eqns. (5), (6) and (8) were obtained by Zhdanov [104] to describe the monomolecular adsorption and associative desorption and Eley-Rideal s bimolecular reaction. He examined the dependence of the rate constants of these processes on the surface coverages and discussed various approximations applied previously to describe the effect of lateral interaction of adsorbed molecules on the desorption rate constant. He also considered the effect of the lateral interaction on the pre-exponential factor of the rate constants for various processes, and in terms of the "precursor state model, the effect of ordering the adsorbed molecules on the sticking coefficient and the rate constant of monomolecular desorption. 

As usual, the rate of dissociative adsorption (e.g. of 02 on various metals [92, 95, 99, 100]) rapidly decreases with increasing surface coverage. As a rule, this is attributed to the fact that dissociative adsorption requires two unoccupied cells, i.e. the sticking coefficient must be S(9) = S(60) Po (0). If a solid surface adsorbs only molecules A, in the quasi-chemical approximation we will have the set of equations... 

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