10−7 mbar. The electron beam hits a perpendicular stream of a gaseous sample effusing from a nozzle of a small diameter (typically 0.2 mm). At this point, the electrons are scattered. Most of the sample is immediately condensed and frozen onto the surface of a cold trap held at -196 °C (liquid nitrogen). The scattered electrons are detected on the surface of a suitable detector in a well-defined distance to the point of scattering. The scattering pattern consists of diffuse concentric rings (see Figure 2). The steep decent of intensity can be compensated for by passing the electrons through a fast rotation sector (Figure 3). This is cut in a way, that electrons with small scattering angles are more shadowed than those at wider scattering angles. The detector can be a photographic plate, an electron imaging plate (usual technique today) or other position sensitive devices such as hybrid pixel detectors (future technique). The intensities generated from reading out the plates or processing intensity data from other detectors are then corrected for the sector effect. They are initially a function of distance between primary beam position and intensity, and then converted into a function of scattering angle. The so-called atomic intensity and the experimental background are subtracted to give the final experimental molecular scattering intensities as a function of s (the change of momentum). These data are then processed by suitable fitting software like UNEX for refining a suitable model for the compound and to yield precise structural information in terms of bond lengths, angles and torsional angles. Theory GED can be described by scattering theory. The outcome if applied to gases with randomly oriented molecules is provided here in short: Scattering occurs at each individual atom (), but also at pairs (also called molecular scattering) (), or triples (), of atoms. is the scattering variable or change of electron momentum, and its absolute value is defined as with being the electron wavelength defined above, and being the scattering angle. The above-mentioned contributions of scattering add up to the total scattering where is the experimental background intensity, which is needed to describe the experiment completely. The contribution of individual atom scattering is called atomic scattering and easy to calculate: with , being the distance between the point of scattering and the detector, being the intensity of the primary electron beam, and being the scattering amplitude of the i-th atom. In essence,