Helium, accommodation coefficient

SO2 uptake was measured at total system pressures in the range of 20 to 50 Torr, consisting of 17.5 Torr H2O vapor with the balance either helium or argon. The observed mass accommodation coefficients, 74, are plotted in Figure 2 as a function of the inverse of the calculated diffusion coefficient of SO2 in each H20-He and l O-Ar mixture. The diffusion coefficients are calculated as a sum of the diffusion coefficients of SO in each component. The diffusion coefficients for SO in He and in Ar are estimated from the diffusion coefficient of SO2 in H 0 (Dg p = 0.124 (101) by multiplying this value by the quantity (mH-/mH Q)V2, anti (mAr/m 2o) 2> respectively. The curves in Figure 2 are plots ofEquation 7 with three assumed values for 7 0.08,0.11 and 0.14. The best fit to the experimental values of is provided by 7 = 0.11. Since gas uptake could be further limited by liquid phase phenomena as discussed in the following section, 7502 = 0.11 is a lower limit to the true mass accommodation coefficient for SO2 on water. 

Results. For O3, experiments were made with both nitrogen and helium as carrier gas in the pressure range of 29 to 85 torr, covering an effective diffusivity range of 1.46 to 5.61 cm /sec. Data were taken at three different temperatures, namely, 0, 10 and 19°C. The effects of added chemical reagent on the apparent accommodation coefficient,... 

More reeently, [26] has eonfirmed the need to include the second order slip condition at higher Kn number values. Their work was both theoretical and experimental using nitrogen and helium in a silicon channels. They used the second order slip approximation to obtain the equation for the volumetric flow rate and related it to the ratio of inlet to outlet pressure. It was shown that when using the Navier-Stokes equation, the boundary conditions must be modified to include second order slip terms as the Knudsen number increases. They also studied in depth the accommodation coefficient Fv and verified the need for further study. It was shown that as the Knudsen number increases, the momentum accommodation value deviates further and further from unity for instance Kn -0.5 yields Fv 0.8 for helium. The values found for nitrogen were quite similar. The measurements agreed with past studies such as [11] for lower Kn. 

Impurity adatoms, such as H, N and O, have a dramatic influence on measured accommodation coefficients for example, a monolayer of oxygen adatoms on polycrystalline tungsten raises the ac for helium from 0.02 for a clean surface to 0.6. Roberts [388] and co-workers made use of this sensitivity to the presence of adsorbates to determine sticking probabilities for reactive gases on tungsten. West and Somorjai [389] have used the extent of He elastic scattering as a sensitive measure of surface cleanliness. 

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