Electron diffraction apparatus

The physical laboratory, complete with x-ray apparatus, electron diffraction, viscosity and osmotic measurement equipment, was under the direct guidance of Karl Wolf When Mark took over it was not the only department of the Central Research Laboratory. There was a synthetic organic laboratory under the leadership of Heinrich Hopff, the personal assistant of K.H. Meyer, and a technical station, called Technion, for the synthesis of amounts of polymer substances sufficient to permit study of their properties. The latter group was under Manfred Dunkel. The research group of Otto Schmidt studied catalytic reactions and synthetic rubber chemistry. ... 

Photo 11 Graduate student Lawrence D. Brockway (SP 59, SP 60, SP 61) operating electron-diffraction apparatus in the mid 1930 s. 

We wish to thank Dr. G. H. Cady for lending us his apparatus, Professor Don M. Yost and Mr. A. Beerbower for preparing the fluorine nitrate used in this work, Mr. K. S. Palmer for assisting in the preparation of the electron diffraction photographs, and Dr. Sidney Weinbaum and Mrs. M. Lassettre for assisting in their interpretation. 

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. 

In 1930, R. Wierl and Mark studied N. Davidson and J. Germer s experiments on electron diffraction. Employing their wide experience in instrumentation, they promptly constructed an improved electron scattering apparatus. With this instrument, they determined the interatomic distances in a number of molecules and published a series of papers on the technique and their findings (17, 18, 19). Mark s contributions to the field of crystal structure are discussed in a later chapter of this volume and will not be covered in more detail here (see Pauling, L. "Herman Mark and the Structure of Crystals", this volume.). 

All the above considerations were taken into account in the coupling of the Soviet-made type EG-IOOA electron diffraction apparatus and type NZ-850 quadm-pole mass spectrometer developed and built in the Institute of Nuclear Research (ATOMKI), Hungarian Academy of Sciences, Debrecen Fig. 9 illustrates the combined experimental setup. 

Initial attempts to prepare Cf metal using metallothermic reduction methods (Section II,A) were less than successful due to the high vapor pressure of Cf metal 28, 46). Reduction of californium oxide with La metal (Section II,B) and collection of the product Cf metal on a fused silica fiber (in the apparatus shown schematically in Fig. 15), were found to give metal with usable X-ray diffraction patterns (5). Later, the same method was used to collect Cf metal both on a fused silica fiber for X-ray diffraction analysis and on an electron microscopy grid for electron diffraction analysis 56). As more Cf became available, preparations via this method were carried out on 0.4-1.0-mg samples of californium oxide (55), using fibers of quartz. Be, or C (suitable for direct X-ray diffraction analysis) to collect the product Cf metal. 

Mass spectrometers Molecular beam apparatus Ion sources Particle accelerators Electron microscopes Electron diffraction apparatus Vacuum spectographs Low-temperature research Production of thin films Surface physics Plasma research Nuclear fusion apparatus Space simulation Material research Preparations for electron microscopy... 

FIG. 9.13 Schematic illustration of a low-energy electron diffraction (LEED) apparatus. (Redrawn with permission from Atkins 1994.)... 

By using the presented second-generation gas electron diffraction apparatus, it would also be possible to probe vibrational motion in real time. Especially when a molecule is photodissociated, a series of snapshots of a diffraction pattern would facilitate understanding the photodissociation process because it describes how a molecule vibrates in the course of the separation of two frag-... 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

Fig. 2.4 Low-energy electron diffraction (LEED). (a) Apparatus, showing how electrons reflected from a surface are detected by a fluorescent screen, (b) LEED pattern obtained from the surface of a tungsten oxide crystal. The bright spots show reflected electron beams. Measurement of their angles and Intensities gives information about the positions of atoms on the surface.

Fig. 12.13. Schematic view of electron diffraction apparatus for low-energy electron diffraction (LEED) and reflection high-energy electron diffraction...

The general features of the experimental tube (Figure 1) are similar to others used in this laboratory for low-energy electron diffraction, with added provisions for measuring the photoelectric work function, diffraction apparatus has been described (2). 

X-ray powder diffraction data of the zeolite samples were collected before and after the catalytic reactions with a Diano-XRD 8000 X-ray powder diffraction apparatus. Electron paramagnetic resonance samples were sealed off in quartz tubes on a vacuum line after various treatments and analyzed with a Varian E-3 spectrometer at room temperature. 

I am grateful to my colleagues at the Du Pont Experimental Station listed below for their help and support to apply the novel concepts described in this report to produce viable catalysts. Brian S. Malone and Rashmi M. Contractor conducted the attrition resistance and activity/selectivity measurements of the VPO-PSA catalysts in apparatuses of their own design. William J. Linn tested catalytic performance of the MCM-PSA products in the acrylonitrile process. EPMA micrographs were obtained by Joseph W. Brennan. SEM pictures were taken by Michael L. Van Kavelaar. The late Gunther Teufer conducted the X-ray diffraction and electron diffraction analysis. 

Fig. 2. Pyrex glass apparatus used (a) for the synthesis and sampling of a base-free gallane and (b) for the admission of the gallane vapor to the chamber of the electron-diffraction apparatus. In (a) A is a sample of [H2GaCl]2 Bj, B2, and B3 are greaseless valves C is freshly prepared LiGaH4, LiBH4, or [Bu"4N][B3H8] Dj, D2, and D3 are U-tube traps for fractionation of the volatile components of the reaction mixture and E is an NMR tube (reproduced with permission from Ref. 56 copyright 1991, American Chemical Society).

---