Ultrahigh vacuum , conditions

For example, energy transfer in molecule-surface collisions is best studied in nom-eactive systems, such as the scattering and trapping of rare-gas atoms or simple molecules at metal surfaces. We follow a similar approach below, discussing the dynamics of the different elementary processes separately. The surface must also be simplified compared to technologically relevant systems. To develop a detailed understanding, we must know exactly what the surface looks like and of what it is composed. This requires the use of surface science tools (section B 1.19-26) to prepare very well-characterized, atomically clean and ordered substrates on which reactions can be studied under ultrahigh vacuum conditions. The most accurate and specific experiments also employ molecular beam teclmiques, discussed in section B2.3. 

Instmmentation for tern is somewhat similar to that for sem however, because of the need to keep the sample surface as clean as possible throughout the analysis to avoid imaging surface contamination as opposed to the sample surface itself, ultrahigh vacuum conditions (ca 10 -10 Pa) are needed in the sample area of the microscope. Electron sources in tern are similar to those used in sem, although primary electron beam energies needed for effective tern are higher, typically on the order of ca 100 keV. 

Another exception to the rule of contaminated surfaces involves very small particles, generally referred to as nanoclusters. These are generally formed and the adhesion of the.se particles to substrates studied in situ, under ultrahigh vacuum conditions. Owing to the vacuum and the short existence of these particles prior to deposition, it is possible for chemistry to occur. 

The flash desorption technique is applied usually in ultrahigh vacuum conditions. Then all the mentioned contributions to S and F should be accounted for in the evaluation of the experimental desorption curves. The effect of Sw on the results of desorption measurements is discussed in... 

Mechanistic smdies are needed on a select nnmber of electrochemical reactions, particularly those involving oxygen. These smdies are far from routine and reqnire advances in knowledge of molecular interactions at electrode surfaces in the presence of an electrolyte. Recent achievements in surface science under ultrahigh vacuum conditions snggest that a comparable effort in electrochemical systems would be equally fmitful. 

In summary, we have shown that metal carbonyls formed in situ by adsorption of CO under ultrahigh vacuum condition can serve as a very sensitive tool for monitoring the nucleation site as well as the environment of the metal atom. It was shown that low coordinated metal atoms, in particular... 

Mdssbauer sources for the 6.2 keV resonance of Ta have mostly been produced by diffusing, under ultrahigh vacuum conditions, neutron-activated into... 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

Figure 10.30 STM images (50 X 50 nm) of the same area of a Au/TiO2(110) sample acquired at 450 K (left) under ultrahigh vacuum conditions and (right) during exposure of the sample to a CO/02 mixture. (From Goodman, D.W., J. Catai, 216, 213-222, 2003. Used with permission from Elsevier Scientific Publishers.)...

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. 

One approach is to carry out reactions of interest in attached chambers under conditions approaching atmospheric and then do the actual surface analyses under ultrahigh-vacuum conditions. For example, XPS has been used to follow the formation of nitrate on the surface of NaCl exposed to HNCL (Laux et al., 1994, 1996 Vogt et al., 1996 Hemminger, 1999). Figure 5.30 shows the apparatus used to dose known quantities of HN03 onto the NaCl surface (Laux et al., 1994). After each dose, the loss of Cl and uptake of N and O... 

Acetylene forms spontaneously an ordered (2 X 2) surface structure on the Pt(l 11) surface at 300 K, at low exposure under ultrahigh vacuum conditions. The intensity profiles reveal that this structure is metastable, and upon heating to 350-400 K for one hour, it undergoes a transformation to a stable structure with the same (2 X 2) unit cell. Ethylene adsorbs on the Pt(l 11) surface and at 300 K, it forms an ordered (2 X 2) surface structure that is identical to the stable acetylene structure as shown by the intensity profiles. 

In order for the adsorbent surface to be well characterized, it must be extremely clean. Surface scientists have developed a number of techniques for producing clean surfaces. These include heating, cleaving crystals, and bombarding with high-energy ions. All of these processes must be carried out under ultrahigh-vacuum conditions, to avoid immediate contamination of the surface. 

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

Lackinger M, Mueller T, Gopakumar TG, Mueller F, Hietschold M, Flynn G W (2004) Tunneling voltage polarity-dependent submolecular contrast of naphthalocyanine on graphite. An STM study of close-packed monolayers under ultrahigh-vacuum conditions. J Phys Chem B 108(7) 2279-2284... 

In 1969 Benninghoven introduced the concept of static SIMS for the determination of the chemical composition of the uppermost monolayer of a material [33]. This technique required ultrahigh vacuum conditions and very low primary beam currents, 10-9 A/cm2, so that less than 1 % of the surface was removed dur-... 

Fig. 4.4. Layout of the integrated surface analysis and preparation system DAISY-MAT (Darmstadt integrated system for materials research). A photoelectron spectrometer is connected by a sample handling system to various deposition and surface treatment chambers. Preparation and analysis can be repeatedly performed under controlled ultrahigh vacuum conditions...

Precise data on the collisional shifts in In+ are presently not available and an estimate can only be based on data from comparable atomic systems. Assuming a shift coefficient of 10-10 cm3/s and ultrahigh vacuum conditions (5 10-8 Pa), the collisional frequency shift would be 1 10-18. 

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