Knudsen cells design

In a separate set of experiments designed to follow the gas phase reactions of CHj-radicals with NO, CHj- radicals were generated by the thermal decomposition of azomethane, CHjN NCHj, at 980 °C. The CH3- radicals were subsequently allowed to react with themselves and with NO in a Knudsen cell that has been described previously [12]. Analysis of intermediates and products was again done by mass spectrometry, using the VIEMS. Calibration of the mass spectrometer with respect to CH,- radicals was carried out by introducing the products of azomethane decomposition directly into the high vacuum region of the instrument. 

For this purpose a special Knudsen cell with exchangeable orifice plates has been designed. When the equilibrium pressure, which corresponds to the saturation pressure ps is reached, effusion of a part of the vaporizing substance takes place through the orifice. The amount depends strongly on the shape and length of the orifice (Fig. 68). From the substance which vaporizes in all directions, only a partial beam can pass freely through the orifice. 

Caloz, F F. F. Fenter, K. D. Tabor, and M. J. Rossi, I Design and Construction of a Knudsen-Cell Reactor for the Study of Heterogeneous Reactions over the Temperature Range 130-750 K, Performances and Limitations, Rev. Sci. Instrum., 68, 3172-3179 (1997). 

The model molecules which are solid at room temperature (except for benzene), were evaporated from a source consisted of a borosilicate glass container with a small opening, like the design of a Knudsen cell. The source was mounted inside a heatable and coolable copper shaft, that could be inserted to the vaccum system through a load-lock arrangement. Upon heating, the pressure was... 

Here Pe is the equilibrium pressure inside the Knudsen cell,pm is the measured vapor pressure, Wc is the Clausing factor for the orifice, C is the surface area of the orifice, D is the surface area of the bottom of the cell cavity, Ov is the vaporization coefficient, and Wd is the Clausing factor for the cell itself. A number of assumptions have been made in this derivation, most notably that the condensation coefficient equals the vaporization coefficient. However, the important points about the design of a Knudsen cell are captured in the basic Whitman-Motzfeld equation. Our cells have a cavity with a diameter of 10 mm and height of 7.6 mm, and a typical orifice has a diameter of 1.5 mm and a length of 4.0 mm. If tabulated Clausing factors [27] and a vaporization coefficient of 1 are assumed, the ratio Pe. Pm has a value of 1.005. 

This paper has explored some Knudsen cell experiments designed to elucidate... 

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