Electron diffraction interpretation

In the process of MBE, the surface structure can be investigated by reflected high energy electron diffraction (RHEED). During MBE growth, one often observes an oscillation in the intensity of the specular reflected beam as a function of time. This is interpreted to be due to the layer-by-layer growth of a two-dimensional island. 

It is an intensely reactive and hygoscopic yellow-brown substance (m.p. 75-78°C) its volatility suggests a low molecular mass Mossbauer spectra indicate 6-coordinate gold while the Raman spectrum is interpreted in terms of cw-bridged octahedral units. In the gas phase at 170°C, it comprises dimers and trimers [29] (electron diffraction). 

For < z = 75-5°, < n = 44-5°. and Kn/Kl = 1-063, < is found to be 590 15, and for KjjIkj = 0-941, < is 6o° 45. It is seen that the bending of two valencies affects the disposition of the other four to only a very small extent. There are at present no published data by which this conclusion can be tested, and since the introduction of substituents in order to obtain such data from, say, measurements of electric dipole moment or electron-diffraction photographs would usually cause complications, as most substituents set up resonance of types other than that considered above, the interpretation of the results might be by no means straightforward. ... 

SP 58 The Radial Distribution Method of Interpretation of Electron Diffraction Photographs of Gas Molecules... 

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 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). 

Both trimethylsilylamides M[N(SiMe3)2]2 (M = Cd and Hg) have been studied in the gas phase by electron diffraction. The diffraction data are best interpreted assuming monomers with linear N—M—N cores and two planar M—NSi2 groups in staggered conformation.147,148... 

During the last decades, a large body of structural information has been derived from gas-electron diffraction studies. The corresponding results are nearly exclusively reported in the literature in terms of r distances, or the equivalent thermal average intemuclear distances, which are denoted r. The r distances are defined by the relation, r = r — If. Alternative methods for interpreting gas-electron diffraction data are possible, for example, in terms of -geometries5, but they are currently too complex to apply in routine stmctural analyses, because they require detailed information on the molecular potential energy surface which is not usually available. 

K. Kuchitsu and L. S. Bartell, Effects of Anharmonicity of Molecular Vibrations on The Diffraction of Electrons. II. Interpretation of Experimental Structural Parameters, J. Chem. Phys., 35 (1961) 1945-1949. 

Thus, although the initial X-ray study of Cotton, Dollase and Wood (21) was interpreted (22) in terms of a uniform C—C distance and an overall D6h symmetry, Jellinek (23, 24) concluded from his crystallographic data that the benzenoid rings showed alternating C—C bond lengths, with an effective symmetry no higher thanZ)3d. Finally however more accurate low temperature crystal data due to Keulen and Jellinek (25), and electron diffraction measurements by Haoland (26) showed conclusively that bis-benzenechromium has in fact sixfold, D6h, symmetry in both crystal and the gas phase. 

Bartell and coworkers investigated the structures of a series of noncyclic alkanes by means of gas electron diffraction (14, 44, 45) and invoked for the interpretation of their results a simple force field which contained to a high extent vibrational spectroscopic constants of Snyder and Schachtschneider. This force field reproduces bond lengths and bond angles of acyclic hydrocarbons well, energies of isomerisation satisfactorily. As an example, Fig. 8 shows geometry parameters of tri-t-butylmethane as observed by electron diffraction and calculated with this force field (14). 

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