Diffraction atoms, molecules

We performed a series of theoretical studies on pump-probe diffraction patterns with a twofold objective the first aim is to evaluate the effect of electronic and vibrational excitation on electron diffraction patterns, compared to that of structural rearrangements that are the primary goal for observation in structural dynamics measurements. Secondly, we wish to explore to what extent electronic and vibrational probability density distributions are observable using the pump-probe electron diffraction methodology. Previously we have discussed the effect of electronic excitation in atomic systems,[3] and the observability of vibrational excitation in diatomic and triatomic systems.[4,5] We have now extended this work to the 8-atomic molecule s-tetrazine (C2H2N4). 

Electron diffraction Diffraction (atom or molecule) mainly by nuclei, but also by electrons (A = 0.01-0.1 nm) Scdar distances due to random orientation 10- s but averaged over vibrational motion 100 Pa (1 ton) Thermal motions cause blurring of distances. Preferably only one (small) species present. Heavy atoms easy to detect... 

The same holds for the distances found by electron diffraction for molecules in the vapour state. While the normal distance between two oxygen atoms, which do not belong to one group, amounts to at least 2.8 to 2.9 A, a shorter distance points to the formation of a hydrogen bond. 

The structures of gaseous tetrafluorohydridophosphane, HPF4, and trifluorodihydridophosphorane H2PF3, have been determined by electron diffraction. Both molecules adopt thp structures with the hydrogen atoms occupying equatorial sites in conformity with C2y symmetry. Theoretical... 

Glusker JP, Traeblood KN (1985) Crystal Stmctnre Analysis. 2nd edition. Oxford Univ Press, Oxford, UK Gnutzmaim V, Vogel W (1990) Surface oxidation and reduction of small platinum particles observed by in situ X-ray diffraction. Z PhysikD (Atoms, Molecules, Clusters) 12 597-600 Greenwood NN (1970) Ionic Crystals Lattice Defects and Norrstoichiometry. Butterworths, London Guinier A (1963) X-ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies. W H Freeman, San Francisco... 

Diffraction (atom or molecule) mainly by nuclei, but also by electrons (A = 0.01-0.1 nm)... 

Until recently, analytical investigations of surfaces were handicapped by the lack of suitable methods and instrumentation capable of supplying reliable and relevant information. Electron diffraction is an excellent way to determine the geometric arrangement of the atoms on a surface, but it does not answer the question as to the chemical composition of the upper atomic layer. The use of the electron microprobe (EMP), a powerful instrument for chemical analyses, is unfortunately limited because of its extended information depth. The first real success in the analysis of a surface layer was achieved by Auger electron spectroscopy (AES) [16,17], followed a little later by other techniques such as electron spectroscopy for chemical analysis (ESCA) and secondary-ion mass spectrometry (SIMS), etc. [18-23]. All these techniques use some type of emission (photons, electrons, atoms, molecules, ions) caused by excitation of the surface state. Each of these techniques provides a substantial amount of information. To obtain the optimum Information it is, however, often beneficial to combine several techniques. 

Here total means that g(r) pertains to all the diffracting atoms. Application of x-ray diffraction results in the structure of liquid water, in terms of g(Ow-Ow, r), since only the oxygen atoms, but not the hydrogen atoms, diffract x-rays. This function resembles to some extent that of liquid argon, a non-structured liquid by aU accounts (Fisenko et al. 2008), as demonstrated by Marcus (1996). There is, thus, more in the notion of the structure of water than what is measurable by g(r), which is dominated by the strong repulsion of molecules that are too closely packed together. 

Fourier transformation. The electron density map shows the location of atoms. A 2D electron density map is produced for each angle. The computer program uses the 2D maps plus the rotation angle data to generate the 3D coordinates for atoms (molecules, ions) in the crystal. The mathematical treatment of the experimental data to produce a crystal structure from an unknown single-crystal diffraction pattern is complicated and beyond the scope of this text, but an example of the results will be shown in the following. 

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