Low Pressure Gas Electron Diffraction: an Experimental Setup and Case Studies

Principles of low pressure gas electron diffraction(LPGED) are introduced. An experimental setup has
been constructed for measuring 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 = 290K (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 re(Car–Car)av = 1.387(5) Å in benzoic acid, re(C–I)
= 2.123(3) Å in iodoform and re(C–I) = 2.133(7) Å in carbon tetraiodide. The determined parameters
showed consistency with theoretically predicted values. A critical comparison with results of the earlier
investigations has also been done.