Combined Gas Electron Diffraction and Mass Spectrometric Experimental Setup at Bielefeld University

We have designed and constructed a combined experimental setup for synchronous measurements of electron diffraction patterns and mass-spectra of gas samples. Test measurements have been performed for acetic acid at two temperatures, 296 and 457 K, respectively. Electron diffraction data have been analysed taking into account mass spectra measured in the same experiments. From the diffraction intensities molecular structures and mole fractions of the acetic acid monomer and dimer have been refined. The obtained results demonstrate the importance of measuring mass spectra in gas electron diffraction experiments. In particular, it is possible to detect the sample decomposition, which can be used for the optimization of experimental conditions and for the data interpretation. The determined in this work length of the hydrogen bond in the acetic acid dimer, re(O^...H) = 1.657(9) Å, is in good agreement with modern theoretical predictions. We recommend to measure diffraction patterns of acetic acid for the calibration of the sample pressure in the diffraction point.