Differential pumping

This term refers simply to situations in which different regions within the vacuum chamber are to be operated at different pressures while still permitting free passage of ions from one region to the other. The most common examples involve ion sources that operate at appreciably higher pressures (up to atmospheric) that must be coupled with analyzers operating at high vacuum ( 10 5 torr). 

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Figure A3.10.12 Side view of die high-pressure eell showing die eomieedons to the UHV ehamber, the turbomoleeular pump and the gas handling system. The differentially pumped sliding seal is loeated between the high-pressure eell and the UHV ehamber [37],...

Figure B2.3.3. Crossed-moleciilar beam apparatus employed for die study of the F + D2 —> DF + D reaetion. Indieated in the figure are (1) the effusive F atom soiiree (2) slotted-disk veloeity seleetor (3) liquid-nitrogen-eooled trap (4) D2 beam souree (7) skimmer (8) ehopper (9) eross-eorrelation ehopper for produet veloeity analysis and (11) rotatable, ultralrigh-vaeuum, triply differentially pumped, mass speetrometer deteetor ehamber. Reprinted with pemrission from Lee [29], Copyright 1987 Ameriean Assoeiation for the Advaneement of Seienee.

Differential pumping of the two hexapole sections keeps the second at a pressure of about ICk mbar. The two consecutive hexapole sections are sometimes described as bridges between the pressure in the inlet and the following vacuum in the TOP mass analyzer. 

Moving-belt (ribbon or wire) interface. An interface that continuously applies all, or a part of, the effluent from a liquid chromatograph to a belt (ribbon or wire) that passes through two or more orifices, with differential pumping into the mass spectrometer s vacuum system. Heat is applied to remove the solvent and to evaporate the solute into the ion source. 

Lithium fluoride is transparent down to about 105 nm, below which windowless systems must be used with differential pumping. 

As time goes on, the ultimate resolution of the SEM operated in these modes will probably level out near 1 nm. The major growth of SEMs now seems to be in the development of specialized instruments. An environmental SEM has been developed that uses differential pumping to permit the observation of specimens at higher pressures. Photographs of the formation of ice crystals have been taken and the instrument has particular application to samples that are not vacuum compatible, such as biological samples. 

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

Fig. I. Experimental setup the clusters are emitted from the cluster condensation cell, passing as a particle beam through a differential pumping stage into the focus of a time-of-flight mass spectrometer, where they are ionized by a laser pulse.

After condensation, the clusters are transported by the He-flow through a nozzle and a differential pumping stage into a high vacuum chamber. For ionization of the clusters, we used excimer and dye laser pulses at various wavelengths. The ions were then mass analyzed by a time-of-flight mass spectrometer, having... 

Our SIMS data is taken using a VGQ8 quadrupole with unit mass resolution between 0-300 amu and a differentially pumped argon ion gun used in a defocused mode. A current of l-2x 10 A in a 0.S cm spot area is used. 

The chemical compositions of the isolated Au SR clusters were investigated by mass spectrometry [15,16,18, 22,32-35]. TEM was used to confirm that the species detected by the mass spectrometer represents the clusters in the sample. Figure 3a is a schematic representation of the top view of the mass spectrometer, which consists of five stages of differentially pumped vacuum chambers. The apparatus accommodates two t5 pes of ion sources, electrospray ionization (ESI) and laser-desorption ionization (EDI), and a time-of-flight (TOE) mass spectrometer with a reflectron. Details of the apparatus and the measurement protocols are described below. 

The DBMS setup and experimental procedures used in this study were the same as described in more detail elsewhere [Jusys et al., 2001]. Briefly, the DBMS setup consisted of two differentially pumped chambers, a Balzers QMS 112 quadrupole mass spectrometer (MS), a Pine Instruments potentiostat, and a computerized data acquisition system. 

The partial pressures of the stable neutral molecules in the discharge (silane, hydrogen, disilane, trisilane) can be measured by a quadrupole mass spectrometer (QMS). The QMS usually is mounted in a differentially pumped chamber, which is connected to the reactor via a small extraction port [286]. In the ASTER system a QMS is mounted on the reactor that is used for intrinsic material deposition. The QMS background pressure (after proper bake-out) is between 10 and 10 mbar. The controllable diameter in the extraction port is adjusted so that during discharge operation the background pressure never exceeds 10"" mbar. 

Which directs them toweurds the analyzer slits. Alternatively, they may be extracted by the field penetration of the high voltage on the focusing electrodes. In both instances the ion beam is usually focused, collimated and accelerated to provide a beam of narrow energy dispersion that is capable of traversing the analyzer section of the mass spectrometer. In modern mass spectrometers the ionization source and analyzer sections are usually differentially pumped, allowing the source to operate at a distinctly higher... 

Figure 7. Schematic diagram of a flowing-afterglow electron-ion experiment. The diameter of flow tubes is typically 5 to 10 cm and the length is 1 to 2 meters. The carrier gas (helium) enters through the discharge and flows with a velocity of 50 to 100 m/s towards the downstream end of the tube where it exits into a fast pump. Recombination occurs mainly in the region 10 to 20 cm downstream from the movable reagent inlet, at which the ions under study are produced by ion-molecule reactions. The Langmuir probe measures the variation of the electron density in that region. A differentially pumped mass spectrometer is used to determine which ion species are present in the plasma.

Joyner and Roberts (28) have gone further by constructing a sample cell, differentially pumped, within the spectrometer. Gas is leaked at a constant rate into the sample region. There is a small hole about 2 mm above the surface of the sample through which the electrons excited from the sample pass into the spectrometer. X-rays are directed at the surface through an aluminum window a second small opening is provided if exitation is by UV radiation. In this way the system pressure can be kept at 10-3 Pa while there is a steady pressure of 100 Pa above the sample. 

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