Ultrahigh vacuum systems

Ultrasound frequencies can be introduced into the walls of the vacuum system. If a source of ultrasound is placed on the wall of an ultrahigh vacuum system, a large hydrogen peak is observed. Related phenomena, presumably from frictional effects, are observed if the side of a vacuum system is tapped with a hammer a desorption peak can be seen. Mechanical scraping of one part on another also produces desorption. 

The pore size of Cs2.2 and Cs2.1 cannot be determined by the N2 adsorption, so that their pore sizes were estimated from the adsorption of molecules having different molecular size. Table 3 compares the adsorption capacities of Csx for various molecules measured by a microbalance connected directly to an ultrahigh vacuum system [18]. As for the adsorption of benzene (kinetic diameter = 5.9 A [25]) and neopentane (kinetic diameter = 6.2 A [25]), the ratios of the adsorption capacity between Cs2.2 and Cs2.5 were similar to the ratio for N2 adsorption. Of interest are the results of 1,3,5-trimethylbenzene (kinetic diameter = 7.5 A [25]) and triisopropylbenzene (kinetic diameter = 8.5 A [25]). Both adsorbed significantly on Cs2.5, but httle on Cs2.2, indicating that the pore size of Cs2.2 is in the range of 6.2 -7.5 A and that of Cs2.5 is larger than 8.5 A in diameter. In the case of Cs2.1, both benzene and neopentane adsorbed only a little. Hence the pore size of Cs2.1 is less than 5.9 A. These results demonstrate that the pore structure can be controlled by the substitution for H+ by Cs+. 

In conclusion, TDS of adsorbates on single crystal surfaces measured in ultrahigh vacuum systems with sufficiently high pumping speeds provides information on adsorbate coverage, the adsorption energy, the existence of lateral interactions between the adsorbates, and the preexponential factor of desorption, which in turn depends on the desorption mechanism. Analysis of spectra should be done with care, as simplified analysis procedures may easily give erroneous results. 

Pressure regulation in high and ultrahigh vacuum systems... 

Production of strand breaks by very low energy electrons (5-25 eV) in thin solid DNA films using ultrahigh vacuum systems have been reported in a number of studies [107-109]. Such studies have demonstrated the efficiencies of low energy electrons and photons to induce DNA damage. In the vacuum ultraviolet (UV) region, examination of experimental data [86,110,111] shows that the induction of strand breaks depends on the absorption spectrum of the components in the medium and the sensitivity spectrum of DNA [112]. Introduction of a variable with the wavelength for the induction of SSB by OH radicals, in conjunction with a fixed value for the quantum efficiency for the production of OH radical (sensitivity spectrum for induction of SSB in aqueous system [112]. 

The results discussed in this article were mostly obtained with ultrahigh vacuum systems at total pressures not exceeding 10"4 Torr, whereas real catalysis is performed in the atmospheric pressure regime. This general pressure gap raises the serious question to which extent experiments of the type described using the spectroscopic techniques of "surface science are relevant at all for real-life catalysis. A general answer to this problem can certainly not yet be offered. However, a rather favorable situation is found in the present case, as long as the discussion is confined to temperatures below 7 ax at which the reaction rate reaches is maximum rmax (cf., for example, Fig. 35). This situation has been discussed in detail in Section IV for palladium and holds as well for the other platinum metals since the shape of the r(T) curve is always quite similar. It has been shown that the kinetics may then approximately be described by... 

Back migration can also be caused by improper cooling around the orifice of the pump. With this problem, pump oil that had condensed begins to re-evaporate. This problem is more significant with ultrahigh-vacuum systems. 

Ultrahigh Vacuum System, Review of Scientific Instruments, 36, 854 (1965). 

There are several reports in the literature on the performance of electrochemical studies in vacuum systems. These reports include studies by Yeager [7] and Bard et al. [8-10], Of special importance are recent reports on electrochemical measurements in ultrahigh vacuum systems by Scherson et al. [11-13],... 

Ion Pump. A pump now much used in ultrahigh vacuum systems is the ion pump, of which the Varian Vac-Ion pump is an example. In this pump a gas discharge such as is commonly observed in air and other gases at about 10 Torr (e.g., neon signs, mercury and sodium lamps) is generated by a strong electric field and maintained all the way down... 

Figure 10 Typical ultrahigh vacuum system used for surface science studies of catalytic reactions on model systems. The combined development of new preparation methods for realistic catalytic samples and in situ spectroscopies for the molecular level characterization of surface species during catalysis promises to advance the basic understanding of catalytic processes...

Wagener (31) reported that the sticking probability of CO on titanium at room temperature was very close to unity. In consequence, since CO is one of the primary residual components in most ultrahigh vacuum systems, titanium has received extensive use in recent years in ion and getter pumps for attaining this type of vacuum condition. It... 

These severe requirements have been met in this Laboratory by an ultrahigh vacuum system of the type sketched in Fig. 70. The system consists of two distinct parts the gas handling train and the microscope proper. These two can be pumped out independently with mercury diffusion pumps, and are separated by an ultrahigh vacuum valve V4, which controls the admission of gas to the sample. In the gas train, the pressure either of helium or of the gas under study, can be regulated by adjustment of the valve V, or Va leading to the spectroscopically pure gas supply (Airco) and by valve V3 to the pumps. 

The implementation of this golden rule dictates the form assumed by an ultrahigh vacuum system. To maximize pumping speed, duct work is kept at a minimum and the diameter of the connections is maximized... 

The components necessary for a laboratory sized ultrahigh vacuum system can be conveniently considered under three headings ... 

Fia. 71. Triple stage mercury diffusion pump for ultrahigh vacuum system with 42 mm diameter pumping conduits. To prevent excessive mercury losses, cooling of both high and low vacuum conduits is desirable. 

A typical ultrahigh vacuum system incorporating these features is shown in Fig. 72. The system is evacuated by a three stage mercury diffusion pump, backed up by a rotary pump. This is attached to the glass tubing through flexible stainless steel bellows. To minimize the... 

Just as in ordinary vacuum technology the primary instrument for leak detection in a glass ultrahigh vacuum system is the Tesla coil. Care is necessary in sparking graded seals. In the vicinity of field emission sources, the induction coil should not be employed since the emitter may be destroyed by arcing. 

Almost all pressure gauges customary in vacuum work can be incorporated in an ultrahigh vacuum system, provided they have been adapted to withstand the rigors of baking. Although they may lack the versatility and simplicity of the ionization gauge they are important for specialized measurements. Commercially available types are therefore briefly listed. 

Greased stopcocks have no place in an ultrahigh vacuum system. This is not too great a loss since even in ordinary vacuum practice they are far from trouble-free. There are several simple and reliable ways of introducing and controlling gases. 

The lowest pressures attainable are around 10 13 mbar. The residual gas in a clean ultrahigh vacuum system consists practically only of hydrogen. It appears that hydrogen is the most universal contaminant, permeating practically all materials. There is no way known to condense or trap it completely. 

---