Molecules, sticking coefficient

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. 

In order to calibrate the sticking coefficient, one needs to detemiine the exposure, i.e. how many molecules... 

L exposure would produce 1 ML of adsorbates if the sticking coefficient were unity. Note that a quantitative calculation of the exposure per surface atom depends on the molecular weight of the gas molecules and on the actual density of surface atoms, but the approximations inlierent in the definition of tire Langmuir are often inconsequential. 

C2 molecules. Thus the respective rate of arrival and subsequent sticking coefficient are Fa k e, and Yq, which are normalized so that Fa + Tb + Fc = 1, and consequently the model has two parameters, namely Fa and Yq-... 

Hence, according to the transition state theory, adsorption becomes more likely if the molecule in the mobile physisorbed precursor state retains its freedom to rotate and vibrate as it did in the gas phase. Of course, this situation corresponds to minimal entropy loss in the adsorption process. In general, the transition from the gas phase into confinement in two dimensions will always be associated with a loss in entropy and the sticking coefficient is normally smaller than unity. 

Clearly, the sticking coefficient for the direct adsorption process is small since a considerable amount of entropy is lost when the molecule is frozen in on an adsorption site. In fact, adsorption of most molecules occurs via a mobile precursor state. Nevertheless, direct adsorption does occur, but it is usually coupled with the activated dissociation of a highly stable molecule. An example is the dissociative adsorption of CH4, with sticking coefScients of the order 10 -10 . In this case the sticking coefficient not only contains the partition functions but also an exponential... 

If we now assume that this surface at temperature T is in equilibrium with a gas then the adsorption rate equals the desorption rate. Since the atoms/molecules are physisorbed in a weak adsorption potential there are no barriers and the sticking coefficient (the probability that a molecule adsorbs) is unity. This is not entirely consistent since there is an entropic barrier to direct adsorption on a specific site from the gas phase. Nevertheless, a lower sticking probability does not change the overall characteristics of the model. Hence, at equilibrium we have... 

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 ... 

Etchant species (for example, fluorine atoms) diffuse to the surface of the material and adsorb onto a surface site. It has been suggested (20) that free radicals have fairly large sticking coefficients compared with relatively inert molecules such as CF4, so adsorption occurs easily. In addition, it is generally assumed (20) that a free radical will chemisorb and react with a solid surface. Further, surface diffusion of the adsorbed species or of the product molecule can occur. 

By careful optimization of the MAPLE deposition conditions (laser wavelength, repetition rate, solvent type, concentration, temperature, background gas and gas pressure), this process can occur without any significant chemical decomposition. When a substrate is positioned directly in the path of the plume, a coating starts to form from the evaporated organic molecules, while the volatile solvent molecules, which have very low sticking coefficients, are evacuated by the pump in the deposition chamber. 

Eor inert SAMs such as n-aUcanethiolates/Au, alkaline earth and alkali metal deposition on inert SAMs tends to exhibit low sticking coefficients of the nascent metal atoms due to quite weak interactions with the -CH3 terminus sometimes <10 of the impinging metal atoms stick to the surface while the rest scatter off the smface [23, 58]. Bammel and co-workers observed quite slow penetration of Na through this inert SAM [59]. In the case of Mg and Ca depositions on n-aUcanethiolate SAMs it was observed that while Mg does not react it does undergo continuous penetration thorough the SAM. In contrast, Ca does react to some extent resulting in calcium carbide species formation [56, 57]. In the case of K on an n-aUcanethiolate SAM the results are more complicated. For example, at 10 K atoms per SAM molecule, it has been reported that half of the deposited metal penetrates to the SAM/Au interface while the remainder is claimed to remain embedded within the SAM matrix [60], though such space is not available theoretically in a dense SAM. 

Sticking is a process where a gas molecule collides with a surface and ends in an adsorbed state. There are two interesting aspects of this process, the probability that the molecule is adsorbed and the state of the adsorbed molecule. The probability that an atom or molecule hitting the surface will adsorp is known as the sticking coefficient. If the sticking coefficient is sigma, the adsorption rate is o. ... 

This is the probability that a molecule that strikes the surface will cross the interface into the condensed phase. It does not represent the net uptake i.e., it does not include the reverse effect of reevaporation from the condensed phase into the gas phase, a is also sometimes referred to as a sticking coefficient for uptake on solid surfaces. 

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... 

In [4] we have introduced a CA model for the NH3 formation which accounts only for a few aspects of the reaction system. In our simulations the surface was represented as a two-dimensional square lattice with periodic boundary conditions. A gas phase containing N2 and H2 with the mole fraction of t/N and j/h = 1 — j/n, respectively, is above this surface. Because the adsorption of H2 is dissociative an H2 molecule requires two adjacent vacant sites. The adsorption rule for the N2 molecule is more difficult to be described because experiments show that the sticking coefficient of N2 is unusually small (10-7). The adsorption probability can be increased by high energy impact of N2 on the surface. This process is interpreted as tunnelling through the barrier to dissociation [32]. 

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