Low pressure gas electron diffraction: An experimental setup and case studies.

Principles of low pressure gas electron diffraction are introduced. An experimental setup has been constructed for measuring the electron diffraction patterns of gaseous samples at pressures below 10 ^-3 mbar. Test measurements have been performed for benzoic acid at T = 287 K corresponding to a vapor pressure of the substance P = 2 × 10 ^-4 mbar, for iodoform CHI ₃ at T = 288 K (P = 4 × 10 ^-4 mbar), and for carbon tetraiodide CI ₄ at T = 290 K (P = 1 × 10 ^-4 mbar). Due to the low experimental temperature, thermal decomposition of CI ₄ has been prevented, which was unavoidable in previous classical measurements at higher temperatures. From the obtained data, the molecular structures have been successfully refined. The most important semi-empirical equilibrium molecular parameters are r _e (C _ar -C _ar ) _av = 1.387(5) Å in benzoic acid, r _e (C-I) = 2.123(3) Å in iodoform, and r _e (C-I) = 2.133(7) Å in carbon tetraiodide. The determined parameters showed consistency with the theoretically predicted values. A critical comparison with the results of the earlier investigations has also been done.