Determination of interatomic distances

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

This step is important in that it prevents the large-amplitude oscillations (typically present at low k) from dominating over the smaller ones (typically at high k). This is critical since the determination of interatomic distances depends on the frequency and not the amplitude of the oscillations. Other approaches having the same effect have also been employed.37,47... 

The use of electron beams for diffraction studies on gases or vapours is one of the best known methods for the accurate determination of interatomic distances in simple molecules. There is an extensive literature on this subject, and no attempt will be made here to describe the method. A recent review article by Bastiansen and Skancke (1961) describes the method fully and illustrates the precision that can now be attained in the determination of intemuclear distances. 

The Determination of Interatomic Distances from the Radial Distribution Curves of Scattered X-Rays. Soviet Phys. — Tech. Phys. (English Transl.) 1, 409 (1956). 

The multiplicity of the modes of vibration and rotation considerably complicates the interpretation of experimental data, and little progress has so far been made with the analysis of the electronic spectra of polyatomic molecules. The study of vibrational-rotational (infra-red) spectra is less difficult and has led to the determination of interatomic distances, valency angles and vibrational frequencies of some of the simpler polyatomic molecules. 

To obtain structural information on platinum-iridium clusters from EXAFS data, we concentrate primarily on the determination of interatomic distances. To obtain accurate values of interatomic distances, we need to have precise information on phase shifts. In this regard, we are fortunate that the phase shift functions of platinum and iridium are not very different. 

The determination of interatomic distances and other parameters involves fitting of the expression for n Eq. 4.13, utilizing Eqs. 4.16 and 4.17... 

Figure 4.30 Determination of interatomic distances in platinum-iridium catalysts by evaluation of the quality of fit of the expression for the sum of the platinum L EXAFS function and the function I,(K) arising from the iridium L m EXAFS to the corresponding sum derived from experimental data (48). (Reprinted with permission from the American Institute of Physics.)...

Pt(2—amino—6—mercaptopurine riboside)2 [Pt(2—A—6—MPR)2l, using Extended X-ray Aborption Fine Structure (EXAFS) spectroscopy. The EXAFS technique (24-M) allows an accurate determination of interatomic distances between an X-ray absorbing atom and its near-neighbors as well as the coordination numbers. 

Lytle FW (1966) Determination of interatomic distances from X-ray absorption fine structure. Advances in X-ray Analysis 9 398-409... 

Vapor phase measurements of relatively simple molecules can lead to determination of interatomic distances from rotational structures of bands. 

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