Vacuum methods, ultrahigh

Tn recent years ultrahigh vacuum methods have been applied to cata-lytic studies on initially clean metal surfaces having low surface area. In several instances (the hydrogenolysis of cyclopropane over platinum (I) and the catalytic methanation reaction over rhodium (2) and nickel (3)) a link between ultrahigh vacuum methods and conventional catalytic measurements was established. That is, specific reaction rates over low area cm ) catalyst samples agreed with specific reaction... 

Special Chemical Methods are particularly important in cases in which impurities are difficult to remove using physical methods. In these cases, it is sometimes possible to resynthesise the substance to be purified into a material which is more readily purified. After the successful purification, the original substance must then be reconstituted. Chemical impurities can also be produced by oxygen, water, other solvents or by light, i.e. photochemically. Many organic compounds react sensitively to these influences. It is therefore often necessary to work under vacuum, in ultrahigh vacuum (UHV) or under inert gas, and to exclude light. 

Other Continuous Processes. Various pasteurization heat treatments ate identified by names such as quick time, vacuum treatment (vacreator), modified tubular (Roswell), small-diameter tube (MaHotizer), and steam injection. The last three methods are ultrahigh temperature (UHT) processes (see Fig. 3). Higher treatment temperatures with shorter times, approaching two seconds, are preferred because the product has to be cooled quickly to prevent deleterious heat effects. 

Commercial versions of PR are available. Other contactless methods of electro-modulation are Electron-Beam Electro-reflectance (EBER) and Contacdess Electroreflectance (CER). In EBER the pump beam of Figure 2 is replaced by a modulated low-energy electron beam (- 200 eV) chopped at about 1 kHz. However, the sample and electron gun must be placed in an ultrahigh vacuum chamber. Contactless electroreflectance uses a capacitor-like arrangement. 

A major advantage of static SIMS over many other analytical methods is that usually no sample preparation is required. A solid sample is loaded directly into the instrument with the condition that it be compatible with an ultrahigh vacuum (10" —10 torr) environment. Other than this, the only constraint is one of sample size, which naturally varies from system to system. Most SIMS instruments can handle samples up to 1-2 inches in diameter. 

Modern Methods in Surface Kinetics Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques Gert Ehrlich... 

Harrington, D. A. Ultrahigh-Vacuum Surface Analytical Methods in Electrochemical Studies of Single-Crystal Surfaces 28... 

Soriaga, M. P., D. A. Harrington, J. L. Stickney, and A. Wiekowski, Ultrahigh-vacuum surface analytical methods in electrochemical studies of single-crystal surfaces, in Modem Aspects of Electrochemistry, J. O M. Bockris et al., Eds., Vol. 28, Kluwer, New York, 1996, p. 1. 

GDS instruments are viable alternatives to the traditional arc and spark-source spectroscopies for bulk metals analysis. Advantages of GDS over surface analysis methods such as AES, XPS and SIMS are that an ultrahigh vacuum is not needed and the sputtering rate is relatively high. In surface analysis, GD-OES, AES, XPS and SIMS will remain complementary techniques. GD-OES analysis is faster than AES (typically 10 s vs. 15 min). GD-OES is also 100 times more sensitive than... 

Since the work function is very sensitive to contaminants, the most reliable measurements are done in ultrahigh vacuum conditions. From the determination of the electron work functions of Fe, Co, Ni, Cu, Au, and other metals in the presence of water adsorbed from the gas phase, it follows that water molecules are oriented with oxygen atoms toward the metal surface. The method is very sensitive to the presence of water. For example, upon adsorption of 3 x lO molecules of water per square centimeter of Co film (4% of a monolayer), the work function value is decreased by ca. 0.3 eV. However, these measurements were done at 77 K, meaning that adsorbed water was likely to be in a crystalline or amorphous ice form. Hence, the quoted results are of limited value to understanding the metal-water system in electrolyte solutions. 

The techniques used in studying interfaces can be classified in two categories in situ techniques and ex situ techniques. In situ methods are those where a surface is probed by one or several techniques while immersed in solution and under potential control. In contrast, in ex situ methods, an electrochemical experiment is first carried out. Then the electrode is removed from solution and examined by one or several spectroscopic techniques, which generally require ultrahigh vacuum (UHV) conditions. Figures 6.10 and 6.11 show some of the most common ex situ and in situ techniques applicable to the study of the metal/solution interface. 

Several factors have contributed to this goal in the recent past development of electrochemical techniques for the study of complex reactions at solid electrodes, use of physical methods such as ESCA, Auger, LEED, etc. for the study of surfaces in the ultrahigh vacuum (UHV) environment and in situ techniques under the same conditions as the electrode reaction. Ellipsometry, electroreflectance, Mossbauer, enhanced Raman, infrared, electron spin resonance (ESR) spectroscopies and measurement of surface resistance and local changes of pH at surfaces were incorporated to the study of electrode kinetics. 

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