Instruments electron diffraction

No. My first project was to design and construct an electron diffraction instrument, which would be used for gas diffraction. I did that from 1947 to 1948, when the instrument was constructed by the engineers at NRL. 

FIGURE 7.1 The electron diffraction instrument consists of three main components the UHV chamber housing the diffraction beamline, the time-of-flight mass spectrometer, and the sputter-aggregation cluster source. The ion beam propagation and various components are... 

The other type of x-ray source is an electron syncluotron, which produces an extremely intense, highly polarized and, in the direction perpendicular to the plane of polarization, highly collimated beam. The energy spectrum is continuous up to a maximum that depends on the energy of the accelerated electrons, so that x-rays for diffraction experiments must either be reflected from a monochromator crystal or used in the Laue mode. Whereas diffraction instruments using vacuum tubes as the source are available in many institutions worldwide, there are syncluotron x-ray facilities only in a few major research institutions. There are syncluotron facilities in the United States, the United Kingdom, France, Genuany and Japan. 

In electron-optical instruments, e.g. the scanning electron microscope (SEM), the electron-probe microanalyzer (EPMA), and the transmission electron microscope there is always a wealth of signals, caused by the interaction between the primary electrons and the target, which can be used for materials characterization via imaging, diffraction, and chemical analysis. The different interaction processes for an electron-transparent crystalline specimen inside a TEM are sketched in Eig. 2.31. 

The diffraction pattern obtained in the detector plane when the beam scan in a STEM instrument is stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 3X in diameter. In order to form a probe of this diameter it is necessary to illuminate the specimen with a convergent beam. The pattern obtained is then a convergent beam electron diffraction (CBED) pattern in which the central spot and all diffraction spots from a thin crystal are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for particular postulated distributions of atoms. This has been attempted, as yet, for only a few cases such as on the diffraction study of the planar, nitrogen-rich defects in diamonds (21). 

Characterization methods. The 100 kV Vacuum Generator HB-5 STEM was used to mlcroanalyze samples. The HB-5 has a KEVEX SI(LI) energy dispersive X-ray spectrometer (EDS) and micro area electron diffraction (MAED) capabilities In conjunction with simultaneous bright and dark field Imaging capabilities. A more detailed explanation of the Instrumental operation can be obtained In a publication by C. Lyman(12). 

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.). 

Electron diffraction patterns are usually produced with transmission electron microscopes. These instruments are composed of several magnetic lenses. The main lens is the objective lens, which, in addition to forming the first magnified image of the specimen, also produces the first diffraction pattern. This original pattern is then magnified by the other lenses of the microscope so as to produce the final diffraction patterns on the screen or on a camera. 

New Instrumentation For Tern Electron Diffraction Structure Analysis 171... 

Electron diffractometry system with the combination of the precession technique can be very perspective experimental instrumentation for precise structural investigations. The technique can now be adapted in a commercial TEM (previously applied uniquely to electron diffraction cameras) taking advantage of the small beam size and can measure reflections in the ED pattern with same required precision for structure analysis. 

Small structural differences should, of course, always be treated cautiously in electron diffraction work. It is also to be realized, however, that the reliability of the determination of these small differences depends not only on the determination of the parameters directly involved in describing them but contributions from other, usually nonbond distances may play important role. This was clearly seen in the determination of the ring deformatin of p-xylene where the long C C distances were particularly instrumental ... 

Acknowledgments These studies were supported primarily by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number EEC-0647560. This work made use of the J.B. Cohen X-ray Diffraction facility and the Electron Probe Instrumentation Center (EPIC) and Keck Interdisciplinary Surface Science (Keck-II) facility of NUANCE center at Northwestern University. 

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 first instruments applying differentially pumped vacuum columns were mainly used for low-magnification imaging and electron diffraction investigations... 

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