Problem 2:a UHV system with a doser tube When a sample in a UHV system has to be exposed to molecules that are hard to p

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Problem 2 A Uhv System With A Doser Tube When A Sample In A Uhv System Has To Be Exposed To Molecules That Are Hard To P 1 (77.46 KiB) Viewed 46 times

Problem 2:a UHV system with a doser tube When a sample in a UHV system has to be exposed to molecules that are hard to pump down, such as for instance water or alcohols, it is usually not desirable to introduce these molecules to the vacuum chamber by backdosing (filling the entire chamber with the gas to be dosed). A way for avoiding this is the use of a directional tube doser. As sketched in the figure, a tube with inner diameter d, about the same as the sample size, is brought very close to the sample with the aid of a translation mechanism. The tube is connected via a circular pinhole with diameter D and a dosing valve to a smaller dosing chamber with separate pump For dosing the molecules, in this example water, the throttle valve in the picture is almost closed, such that the needle valve can be used to admit a relatively high pressure of water in the dosing chamber. For dosing the molecules, the dosing valve is opened for a certain time, such that molecules can enter the dosing tube through the pinhole and adsorb on the sample. Because the molecules will collide with the walls of the tube doser, they will homogeneously deposit on the sample. After closing the dosing valve, the dead volume between the dose valve and the pinhole is evacuated by opening the "stop valve" for a few seconds. In this way the dosing directly stops. We now want to calculate the time required for exposing the sample to a certain dose of water molecules, for a given pressure in the dosing system and dimensions of the dosertube and pinhole. Our doser tube has an inner diameter d of 10 millimetre and a pinhole diameter of D-10 um. The water pressure in the dosing chamber is 5x10mbar. The system is used at room temperature (T-298 K). I atomic mass unit is 1.67x10 kg. a) Calculate the time needed for exposing the sample to a dose of 5 Langmuir (L) of water. A dose of Lis equal to 1x10 Tors- 1.3310*mbar's. b) Suppose the sample is cooled with liquid nitrogen, such that all impinging molecules stick to the sample. What is the surface density of adsorbed water leg. in units of molecules/cm) after a dose of 5 L? The diameter of the sample is exactly equal to the inner diameter of the tube. Hint: for calculating the flux of molecules that make it to the sample, you can use the Hertz- Knudsen equation for the flux through the area of the pinhole. Since the pressure in the dosing chamber is much high than in the UHV system, you may assume that the flux of molecules going in reverse direction (from the dosing tube back to dosing chamber) is negligible. Furthermore, you can assume that all molecules coming in the doser tube will hit the sample, because the desertube is brought very close to the sample. sample holder with sample pinhole, diameter D dose valve needle valve water reservoir throttle valve dosing tube, inner diameterd stop valve pump dosing chamber pump main chamber Schematic picture of a UHV system with a dosing chamber (small circle on the left side) and tube doser with pinhole.