Sticking coefficient, measurement

This is our principal result for the rate of desorption from an adsorbate that remains in quasi-equihbrium throughout desorption. Noteworthy is the clear separation into a dynamic factor, the sticking coefficient S 6, T), and a thermodynamic factor involving single-particle partition functions and the chemical potential of the adsorbate. The sticking coefficient is a measure of the efficiency of energy transfer in adsorption. Since energy supply from the... 

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. 

Contamination is another pitfall that can influence the measurement of a sticking coefficient, particularly when its value is small. For example, if the sticking coefficient of the gas of interest is of the order of 10 , an impurity gas present at the ppm... 

Figure 7.5. Sticking coefficients along with differential heats of adsorption as measured by microcalorimetry for ethylene and acetylene on Rh(lOO). [Adapted from R. Kose, W.A. Brown and D.A. King, Chem. Rhys. Lett. 311 (1999) 109.]...

Suppose we successfully measured the sticking coefficient and the activation energy for adsorption of a certain molecule, as well as the rate of desorption. Is it then possible to estimate the equilibrium constant for adsorp-tion/desorption ... 

Fig. 8. Comparison of dissociative sticking coefficients of methane on Ni( 100) and Ni(ll 1) measured at 1.0 torr with sticking coefScients predicted from molecular beam data. (From Ref. 99.)...

The possibility that free radicals, particularly hydroperoxy, have significant effects on biologic surfaces exposed to the irradiated atmosphere should be investigated. Sticking coefficients are needed. In experiments in which the observed biologic effects cannot be attributed to the measured ozone and PAN concentrations, the possibility of damage by the steady-state concentrations of free radicals in the atmosphere should be considered. 

This section introduces the principal experimental methods used to study the dynamics of bond making/breaking at surfaces. The aim is to measure atomic/molecular adsorption, dissociation, scattering or desorption probabilities with as much experimental resolution as possible. For example, the most detailed description of dissociation of a diatomic molecule at a surface would involve measurements of the dependence of the dissociation probability (sticking coefficient) S on various experimentally controllable variables, e.g., S 0 , v, J, M, Ts). In a similar manner, detailed measurements of the associative desorption flux Df may yield Df (Ef, 6f, v, 7, M, Ts) where Ef is the produced molecular translational energy, 6f is the angle of desorption from the surface and v, J and M are the quantum numbers for the associatively desorbed molecule. Since dissociative adsorption and... 

Figure 4.19. Arrhenius plot of measured thermal sticking coefficients of N2 on a clean Ru(0001) surface and the same surface covered with 0.01-0.02 ML of gold. The point taken from Dietrich et al. [68] is the result from a similar measurement at room temperature. Adapted from Ref. [62].

---