Electron diffraction studies of hot molecules. I. Observed and calculated thermal expansions of SF6, CF4, and SiF4

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Abstract
A new method is described for the rapid heating (10−5–10−6 s) of gas molecules for study by electron diffraction. Laser irradiation of the tip of a micronozzle makes it possible to determine structures, amplitudes of vibrations, and aspects of anharmonicity of molecules at temperatures much higher than those at which decomposition occurs in conventional oven nozzles. The vibrations and thermal expansions of SF6, CF4, and SiF4 have been investigated up to 1700, 1600, and 1200 K, respectively. Clear evidence for effects of anharmonicity was observed in amplitudes of vibration as well as mean bond lengths. Various models proposed for the treatment of increases in bond length have been assessed, among which an anharmonic Urey–Bradley field accounted well for results. Comparisons are made with the predictions of Heenan and Robiette based on spectroscopic analyses. The diffraction approach offers a promising method for augmenting spectroscopy in the investigation of intramolecular forces