UHV Systems

TPD is frequently used to detenuine (relative) surface coverages. The area below a TPD spectrum of a certain species is proportional to the total amount that desorbs. In this way one can detennine uptake curves that correlate gas exposure to surface coverage. If tire pumping rate of the UHV system is sufiBciently high, the mass spectrometer signal for a particular desorption product is linearly proportional to the desorption rate of the adsorbate [20, 21] ... 

On all the Ti02(100) crystals further Investigated, maximum C/Ti ratios between 0.2 and 0.3 were sometimes observed after standing for long periods at 300K in the UHV system and before high temperature treatments, as shown in Figure la, for example. 

Figure 1 shows two reactor configurations we have used to measure reaction rates on clean surfaces. In Figure 1(a) is shown a high pressure cell inside the UHV system ( ) with the sample mounted on a bellows so it can be moved between the reaction cell and the position used for AES analysis. In Figure 1(h) is shown a reaction cell outside the analysis system with the sample translated between heating leads in the reactor and in the UHV analysis system ( ). 

FIGURE 27.39 Schematic diagram of a surface electrochemistry apparatus, showing UHV system, transfer manipulators, and interlocks. 

The experiments were performed in a combined system for UHV and electrochemical measurements. It consists of a UHV system equipped with standard facilities for surface preparation and characterization and a pocket-size scanning tunneling microscope (STM) [Kopatzki, 1994], a pre-chamber containing a flow cell for electrochemical measurements, which was attached to the main UHV system via a gate valve, and... 

Low contamination levels are readily achieved in laboratory scale UHV systems. Very high costs inhibit the use of UHV in industrial scale systems, however, so another, local-UHV approach has been proposed, viz. the plasma box reactor [152]. The substrate is mounted in a box, which is surrounded by a shell, which is pumped to a low pressure. The process pressure in the box is maintained by a throttle valve. As the pressure in the box is larger than the pressure in the surrounding shell, contaminants diffuse outwards and the incorporation of contaminants in the deposited layer is low. 

Fig. 9. LEED Auger electrochemical UHV system with electrochemical thin layer cell. After [39].

Figure 7.1 Pictures of the high-pressure high-temperature STM reactor/UHV system.

Figure 7.2 Schematic diagram of the high-pressure high-temperature STM reactor/UHV system. (1) View window, (2) mounting framework, (3) docking scaffold, (4) docking disk, (5) high-pressure reactor (STM body...

Atomically resolved images of inert HOPG under 700 Torr N 2 and of hex- Pt(l 0 0) prepared in UFIV can be routinely obtained with this homebuilt high-pressure high-temperature STM reactor/UHV system. 

In one of the earliest reports of the use of clean evaporated alloy films in surface studies, Stephens described the preparation and characterization of Pd-Au films and presented some results for the adsorption of oxygen on them 46). Films of pure Pd and 60% Au were evaporated directly from wires, while films of 80% Au and pure Au were evaporated from a pre-outgassed tungsten support wire. The films were evaporated in a UHV system and the pressure was kept below PC8 Torr during evaporation. After evaporation, the films were stabilized by cycling between —195° and 30°C four times. They w ere characterized by X-ray diffraction and chemical analysis surface areas were measured by the BET method using krypton adsorption. 

To obtain these startling new results with Pt single crystals, Clavilier employed a novel surface preparation technique that did not require a complex and expensive UHV system, but did require considerable skill and experience. Clavilier s original papers... 

The electrochemical experiments were conducted in an apparatus consisting of an electrochemical cell attached directly to a UHV system and has been described in detail elsewhere (16). The transfer between UHV and the EC was accomplished via a stainless steel air lock vented with ultra-pure Ar. Differentially pumped sliding teflon seals provided the isolation between UHV and atmospheric pressure. The sample was mounted on a polished stainless steel rod around which the teflon seals were compressed. All valves in the air lock were stainless steel gate valves with viton seals. Details of the electrochemical cell and conditions are contained in reference 16. Electrochemical potentials are referred to a saturated calomel electrode (SCE). 

The metal surfaces are always covered with a monolayer of CO upon evacuation of the reactor and transfer to the UHV system. On both Pd and Ir the CO, which desorbs as CO2 when reacted with the oxide species, desorbs at a much higher temperature than CO from the clean surface. This result implies that the oxide species forms an inactive complex with CO upon adsorption of CO under reaction conditions. While the presence of the oxide species reduces the overall rate of reaction, the activation energy is unchanged, suggesting that oxygen serves as a simple site blocker on the surface. 

NEG pumps are mostly used in combination with other UHV pumps (turbomolecular and cryopumps). Such combinations are especially useful when wanting to further reduce the ultimate pressure of UHV systems. 

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