Ultrahigh vacuum sublimation

For the production of pressures in the ultrahigh vacuum region, sputter-ion, and sublimation pumps, as well as turbomolecular pumps and cryopumps, are used in combination with suitable forepumps. The pump best suited to a particular UHV process depends on various conditions (for further details, see Section 2.5). 

The high thermal stability of large unsubstituted PBAHs such as 8846 228 permits fractional sublimation at 550—650 °C under ultrahigh vacuum conditions. 

The preparation of well-defined thin or ultrathin crystalline films by sublimation or molecular-beam epitaxy is a technology which depends upon a large number of experimental parameters, which we cannot treat in detail here. Ultrahigh vacuum is in any case a prerequisite. More on this subject can be found for example in the review articles by Forrest [18]. 

Conventional TOF-SIMS requires measurement under an ultrahigh vacuum condition. Therefore, samples are dried before the measurement or are measured under frozen. One of the common methods for preserving cells for analysis in vacuum is freeze-drying of cells on a flat substrate such as a Si substrate. This procedure involves removing the substrate from the culture media, with no rinsing, followed by plunging the chip directly into liquid N2 coolant. Once frozen, the substrate is transferred to a vacuum chamber for several hours at a modest pressure of 10 Torr, where the water is removed by sublimation. The chamber remains at room tempera-... 

A photograph and a schematic diagram of this system are shown in Figs. I and 2, respectively. It consists of a conventional stainless-steel UHV chamber within which an isolation cell for high-pressure studies is situated. The chamber is cylindrical in shape and has a diameter of 12 in. Ultrahigh vacuum is obtained by a mechanically backed Varian I 200 1/s diffusion pump with a liquid nitrogen trap, in combination with a water-cooled titanium sublimation pump. The background pressure of the system is lower than 1 X 10" Torr. 

The student should be aware that it takes special vacuum equipment to maintain an ultrahigh vacuum. First, a clean surface must be inside a special vacuum chamber that has no leaks—which is easier said than done. In addition, special vacuum pumps must be used to get to such high vacuums and stay there. The normal oil-filled rotary vacuum pump can only maintain a vacuum of about 10 torr or so, a full five orders of magnitude higher than what is necessary for ultrahigh vacuum. Special vacuum pumps (like turbomolecular pumps, titanium sublimation pumps, or liquid-helium-based cryopumps) are needed and can be extremely expensive. 

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