Gas electron diffraction

Gas electron diffraction (GED) is one of the applications of electron diffraction techniques. The target of this method is the determination of the structure of gaseous molecules i.e. the geometrical arrangement of the atoms from which a molecule is built up. GED is one of two experimental methods (besides microwave spectroscopy) to determine the structure of free molecules, undistorted by intermolecular forces, which are omnipresent in the solid and liquid state. The determination of accurate molecular structures by GED studies is fundamental for an understanding of structural chemistry.
Introduction.
Diffraction occurs because the wavelength of electrons accelerated by a potential of a few thousand volts is of the same order of magnitude as internuclear distances in molecules. The principle is the same as that of other electron diffraction methods such as LEED and RHEED, but the obtainable diffraction pattern is considerably weaker than those of LEED and RHEED because the density of the target is about one thousand times smaller. Since the orientation of the target molecules relative to the electron beams is random, the internuclear distance information obtained is one-dimensional. Thus only relatively simple molecules can be completely structurally characterized by electron diffraction in the gas phase. It is possible to combine information obtained from other sources, such as rotational spectra, NMR spectroscopy or high-quality quantum-mechanical calculations with electron diffraction data, if the latter are not sufficient to determine the molecule's structure completely.
The total scattering intensity in GED is given as a function of the momentum transfer, which is defined as the difference between the wave vector of the incident electron beam and that of the scattered electron beam and has the reciprocal dimension of length. The total scattering intensity is composed of two parts: the atomic scattering intensity and the molecular scattering intensity. The former decreases monotonically and contains no information about the molecular structure. The latter has sinusoidal modulations as a result of the interference of the scattering spherical waves generated by the scattering from the atoms included in the target molecule. The interferences reflect the distributions of the atoms composing the molecules, so the molecular structure is determined from this part.
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 (formula_1), but also at pairs (also called molecular scattering) (formula_2), or triples (formula_3), of atoms.
formula_4 is the scattering variable or change of electron momentum, and its absolute value is defined as
with formula_6 being the electron wavelength defined above, and formula_7 being the scattering angle.
The above mentioned contributions of scattering add up to the total scattering
where formula_9 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 formula_11, formula_12 being the distance between the point of scattering and the detector, formula_13 being the intensity of the primary electron beam, and formula_14 being the scattering amplitude of the "i"-th atom. In essence, this is a summation over the scattering contributions of all atoms independent of the molecular structure. formula_1 is the main contribution and easily obtained if the atomic composition of the gas (sum formula) is known.
The most interesting contribution is the molecular scattering, because it contains information about the distance between all pairs of atoms in a molecule (bonded or non-bonded):
with formula_17 being the parameter of main interest: the atomic distance between two atoms, formula_18 being the mean square amplitude of vibration between the two atoms, formula_19 the anharmonicity constant (correcting the vibration description for deviations from a purely harmonic model), and formula_20 is a phase factor, which becomes important if a pair of atoms with very different nuclear charge is involved.
The first part is similar to the atomic scattering, but contains two scattering factors of the involved atoms. Summation is performed over all atom pairs.
formula_3 is negligible in most cases and not described here in more detail. formula_9 is mostly determined by fitting and subtracting smooth functions to account for the background contribution.
So it is the molecular scattering intensity that is of interest, and this is obtained by calculation all other contributions and subtracting them from the experimentally measured total scattering function.
Results.
Some selected examples of important contributions to the structural chemistry of molecules are provided here: