Hydrogen sticking coefficient

Figure 7.3. Uptake curves of hydrogen on Cu(lOO). Here the dosage has been converted into the equivalent number of monolayers (ML). Note that the sticking coefficient is very low and that 1.8 bar of H2 was required. The insert shows Arrhenius plots ofthe extracted...

The sticking coefficient of H2 on a metal has been determined through an adsorption experiment. The metal surface is assumed to have No = 1.5 x 10 sites m and each adsorption site is assumed to be occupied by one hydrogen atom when the surface is saturated. The experiment was performed by exposing the surface to a known pressure of hydrogen over a well-defined period of time (dosis) and then sequentially determining how much was adsorbed by, for example, TPD. All adsorption experiments where performed at such low temperatures that desorption could be neglected. 

For the SiH2 radicals the surface reaction coefficients have been taken as 5 = P = 0.8 [192]. This sticking coefficient is large because there is no barrier for insertion of this species into the a-Si H surface. Kae-Nune et al. [217] specify a surface recombination probability of about 1 for atomic hydrogen on an a-Si H surface during deposition that results mainly in recombination of H with an H-atom bounded to the surface. 

Fig. 1.28. The change in sticking coefficient of atoms of hydrogen during doping of film of ZnO by atoms of Ag (/), Zn (2) and applying the transverse electric field to the film O) [198, 199]...

The activation energy for adsorption of hydrogen on copper was set at 30 kJ mol-1, in agreement with the literature (80). A sticking coefficient of unity was assumed for this step. Furthermore, the entropy of the adsorbed surface hydrogen was adjusted in the analysis. 

It is a well-studied system [2, 61], but still it is discussed very controversially, as far as experiment [62-67] as well as theory is concerned [14, 36, 37, 68-73], This debate was fueled by the so-called barrier puzle While the sticking coefficient of molecular hydrogen on Si surfaces is very small [67, 74]... 

Fig. 18. Variation of the initial sticking coefficient of hydrogen on polycrystalline nickel as a function of sulfur coverage (adapted from Ref. 163).

Another set of experiments was carried out in an IJHV-based system working with well-defined, quantified particle beams. This system was employed to measure the sticking coefficient of methyl radicals (CH3), the simultaneous interaction of CH3 radicals and atomic hydrogen or low energy ions leading to chemical sputtering and ion-induced deposition, respectively, and the simultaneous interaction of all tree species (CH3, H, and ions). 

The sticking coefficient of methyl radicals on a hydrocarbon surface at 340 K is of the order of 10-5 to 10 4. The temperature dependence of this process was determined in the range from 340 to 800 K. Simultaneous exposure of the surface to atomic hydrogen and CH3 leads to an increase of the sticking coefficient up to 1(P2 depending on the H flux. Simultaneous interaction of CH3 and low-energy ions (E < 1 keV) also causes an enhancement of CH3 sticking to about HP2. 

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