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 CHI₃ at T = 288 K (P = 4 × 10^-4 mbar), and for carbon tetraiodide CI₄ at T = 290 K (P = 1 × 10^-4 mbar). Due to the low experimental temperature, thermal decomposition of CI₄ 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.