Automatic background subtraction in gas electron diffraction: The covariance matrix in least-squares structure-parameter analysis

Conclusions

The present study is part of a large program being conducted at the All-Union Scientific-Research Center for Properties of Surfaces and Vacuum involving electron-diffraction studies on polyatomic inorganic molecules. The unique data on the structures of oxygen-acid salts and complex halides have been included in reference tables [9] and used in calculating thermodynamic functions [10]. The difficulties in obtaining electron-diffraction patterns for these compounds do not allow one to perform a complete statistical analysis in each case, so the results here are of interest not only in themselves but also as regards interpreting molecular constants previously measured.

It was possible to perform a complete statistical analysis for the molecular constants of HgBr₂ because of the software for continuous automatic structure-parameter definition. It would be completely unrealistic to process separately the data from the 40 experiments without it.

The main results are as follows.

• 1. An effective method has been tested for distinguishing the molecular component of the scattering intensity.

• 2. A unified software suite has been written for interpreting electron-diffraction patterns based on a 3CS microdensitometer and an HP 21MX minicomputer (Hewlett Packard).

• 3. Complete statistical analysis for the HgBr₂ molecule has shown that the methods used at the center for processing electron-diffraction patterns lead to undistorted values for the internuclear distances and mean-square vibration amplitudes, together with somewhat underestimates of the errors.