Evaluation of Structural Information from Gaseous Electron Diffraction

Critical Evaluation of Structural Information from Gaseous Electron Diffraction 

For the nonexpert to learn how to evaluate structural information from reports of gas electron-diffraction experiments is not an easy task, nor is it always easy for the expert either (as those who are called upon to referee manuscripts will know). In any structural analysis there are a large number of factors-some experimental, some related to the procedures used-that can affect the results. The reader must determine whether the author has recognized all these factors, many of which are different for different molecules. He must then determine whether the author has investigated or estimated their effects and appropriately taken them into account. In the event that the reader is unsatisfied with the treatment, he must attempt to determine the extent to which the structural claims are to be relied upon. 

Because the factors mentioned above are closely connected to the nature of both the electron-diffraction experiment and the analysis of the data, the first three sections to follow are meant to provide a necessary background of knowledge about the method.1-5 Nonessential details have been ignored in these sections, and to minimize confusion I have tried to avoid commonly used symbols for intensity functions unless these functions are indeed those usually identified with the symbol. Various aspects of the method and the limitations they impose on the structural results are next discussed.6-8 Finally, I note some points that, 

FIGURE 2 A typical electron-diffraction photograph. The substance was SiF4. 

In the background subtraction method one calculates a function slm (s) given by 

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