UHV chamber

Once a sample is properly oriented and polished, it is placed into a UHV chamber for the final preparation steps. Samples are processed in situ by a variety of methods in order to produce an atomically clean and flat surface. Ion bombardment and aimealing (IBA) is the most conunon method used. Other methods include cleaving and film growth. 

Modem UHV chambers are constmcted from stainless steel. The principal seals are metal-on-metal, thus the use of greases is avoided. A combination of pumps is nomially used, including ion pumps, turbomolecular pumps, cryopumps and mechanical (roughing) pumps. The entire system is generally heatable to 500 K. This bakeout for a period of... 

A modified immersion method has been used by Hamm et al.140 to obtain electrochemical cell by a closed-transfer system, and immersed in 0.1 M HCIO4 solution at various . was derived from the charge flowing during the contact with the electrolyte under potential control. For the reconstructed Au(l 11M22 X Vayo.l M HCIO4interface, =0.31 0.04V (SCE) (Table 9). Using the impedance method, = 0.34 V (SCE) for recon-... 

The apparatuses used for the studies of both ammonia synthesis emd hydrodesulfurization were almost identical, consisting of a UHV chamber pumped by both ion and oil diffusion pumps to base pressures of 1 x10 " Torr. Each chamber was equipped with Low Energy Electron Diffraction optics used to determine the orientation of the surfaces and to ascertain that the surfaces were indeed well-ordered. The LEED optics doubled as retarding field analyzers used for Auger Electron Spectroscopy. In addition, each chamber was equipped with a UTI 100C quadrupole mass spectrometer used for analysis of background gases and for Thermal Desorption Spectroscopy studies. 

HREELS experiments [66] were performed in a UHV chamber. The chamber was pre-evacuated by polyphenylether-oil diffusion pump the base pressure reached 2 x 10 Torr. The HREELS spectrometer consisted of a double-pass electrostatic cylindrical-deflector-type monochromator and the same type of analyzer. The energy resolution of the spectrometer is 4-6 meV (32-48 cm ). A sample was transferred from the ICP growth chamber to the HREELS chamber in the atmosphere. It was clipped by a small tantalum plate, which was suspended by tantalum wires. The sample was radia-tively heated in vacuum by a tungsten filament placed at the rear. The sample temperature was measured by an infrared (A = 2.0 yum) optical pyrometer. All HREELS measurements were taken at room temperature. The electron incident and detection angles were each 72° to the surface normal. The primary electron energy was 15 eV. 

Samples may either be those in which the surface of interest has been exposed to the environment before analysis, or the surface to be examined is created in the UHV chamber of the instrument. The latter method is generally preferable, and also argon-ion bombardment is commonly used to clean sample surfaces in situ in the spectrometer. In metallurgical studies, the fracture sample is particularly important the sample is machined to fit the sample holder, and a notch is cut at the desired point for fracture. The fracture stage is isolated from the analytical chamber and is pumped down to UHV. Liquid nitrogen cooling is often provided, as this encourages... 

After adsorption of CO and solution exchange with pure base electrolyte, the oxidation of adsorbed CO during a triangular potential scan is observed (see Fig. 1.4a). In a second run after adsorption of CO the electrode is emersed and transferred to the UHV chamber in the same way as in the normal experimental procedure. The electrode is then transferred back to the cell and re-immersed in the base electrolyte. A potential scan is applied to oxidize the adsorbate. Fig. 1,4b shows... 

In order to check the survival of methanol adsorbate to the transfer conditions, the following experiment was performed. After adsorption of methanol and solution exchange with base electrolyte, the Pt electrode was transferred to the UHV chamber over a period of ca. 10 min, then back to the cell where it was reimmersed into the pure supporting electrolyte. A voltammogram was run and compared with that of an usual flow cell experiment. The results, (see Fig. 2.5a,b), show that the transfer procedure is valid. The areas under the oxidation curve are the same. As in the case of adsorbed CO on Pt (see Fig. 1.4), the change in the double peak structure indicates that some surface re-distribution may occur. 

Fig. 2.5. Test for the survival of methanol adsorbate in the UHV the potential scan was applied a) after adsorption in 5 x 10 J M C H3OH5 x 10 2 M H2S04 followed by electrolyte exchange with base solution (b) after adsorption, transfer in the UHV chamber and reimmersion in base electrolyte dotted line base voltammogram sweep rate 62.5 mV/s.

The electrochemical cell is built into the UHV chamber, the solution is removed by suction or blowing and direct transfer is then effected. 

As was discussed above, it is essential to determine the effect, if any, that the emersion process has on the double layer. To do this, Wilhelm and colleagues have performed the definitive type of blank experiment. CO was adsorbed onto the Pt working electrode from sulphuric acid electrolyte. After adsorption, the CO-saturated solution was replaced with pure electrolyte. The potential of the electrode was then ramped in order to oxidise off the adsorbate, as C02, and the voltammogram so obtained is shown in Figure 2.118(a). The experiment was then repeated CO was adsorbed as before, but the electrode was emersed and transferred into the UHV chamber, before being re-immersed and the potential ramp applied. The voltammogram so... 

Fig. 3.8. Left schematic illustration of TRPE. The IR pump pulse (hi/1) perturbs the electronic states of the sample. The photon energy of the UV probe pulse (h.1/2) exceeds the work function and monitors changes in occupied and unoccupied states simultaneously. Right experimental setup for TRPE. Pairs of IR and UV pulses are time delayed with respect to each other and are focused onto the sample surface in the UHV chamber. The kinetic energy of photoelectrons is analyzed by an electron time-of-flight spectrometer (e-TOF). From [23]...

Two-phase particles ranging from 10 to 20 microns in size, supported on a graphite substrate, were observed in-situ in the UHV chamber of a scanning Auger microprobe. Both surface composition analysis and imaging of the particles could be undertaken. The preparation of the samples has been described in detail elsewhere. ... 

The experiments were performed in stainless steel UHV chambers which were equipped with the instrumentation necessary to perform Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), UV Photoelectron Spectroscopy (UPS), Low Energy Electron Diffraction (LEED), work function measurements (A( )), High Resolution Electron Energy Loss Spectroscopy (HREELS), and Temperature Programmed Desorption (TPD). The Au(lll) crystal was heated resist vely and cooled by direct contact of the crystal mounting block with a liquid nitrogen reservoir. The temperature of the Au(lll) crystal was monitored directly by means of a... 

The atomically clean, well-ordered Ni(lOO) surface was obtained by using a standard cleaning procedure as described in detail elsewhere [17]. The deposition of potassium was achieved by evaporation from a SAES getter source [18]. Reagent grade (99.999% purity) gases of O2, H2 and CH4 were introduced into the UHV chambers through leak-valves. 

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