ELECTRON DIFFRACTION

Just as the crystals can act as diffraction grating for X-rays and produce diffraction effects, the diffraction of a beam of electrons can also be used to study the internal structure of a crystal might be obtained in a manner analogous to X-rays diffraction by crystals. It was indeed In 1925. W. Elasser first suggested that the evidence of the wave nature of electrons Davisson and L. H. Germer, and by the diffraction of electrons by a thin film, about 10-6 demonstrated by the diffraction of beams of electrons by a crystal of nickel in 1927 by CJ cm thickness of metal by G. P. Thomson (1927).

When a high energy electron strikes the material, the scattering is caused primarily by interaction with atomic nuclei, ut the seattering of low energy electrons, such as electron beam with a wavelength of 1 A is caused by the interaction with outer electrons of the atoms as well as with the nuclei The electron beam with a wavelength of less than 1A 40,000 V) is used in electron diffraction studies. Since the low energy electrons can be easily absorbed by matter, samples used are usually thin films er gases and vapours.

When electron beam is passed through a gas or vapour it produces a series of concentric rings on a photographic plate due to the diffraction by the atoms within the molecule, i.e., the diffraction depends on the distances between atoms in the molecule. The dark rings on the photographic plate represent the position of maximum scattering and the lighter portions in between correspond to angles for which the electron scattering is minimum. Because of the appreciable amount of the background scattering of the electron beam, the diffraction of the techniques in electron diffraction measurements, the resolution of the bonds has been considerably improved. The scattering of electrons of two types incoherent or inelastic, due to change in equivalent wavelength of the electrons, and the coherent or elastic scattering.

Whereas the inelasting scattering is not important for the present purpose, the elastic scattering is responsible for the appearance of concentric rings on a photographic plate.

The diffraction of electrons, as in X-rays depends on spacings between targets.

EXPERIMENTAL METHOD

The instrument used for electron diffraction studies consists of

(1) an electron gun (a hot anode or cathode),

(ii) lenses to focus the electron beam on to the desired area of volume of the specimen and

photographic screen Photographie means of viewing the pattern, usually recording of the image is usually carried out by direct impart of the electron bean upon the photographic film.

In addition, the instrument is also provided with related electric power supply, vacuum pumps and controls. Most commercial electron microscopes have been used far electron diffraction studies with slight modifications. However, commercial electron microscopes are available which are exclusively used to obtain an electron diffraction pattern without moving the specimen from its normal position. The complete instrumem is large, complex and expensive.

A fine stream of accelerated electrons using a potential of about 50,000 volts is allowed to fall on a very thin film about 10 cm thickness of metal or to meet at right angles a stream of the gas or vapour at a low pressure, about 10-3 mm. Since the electrons interact very strongly with the molecules of the sample and greatly affect the photographic pate, the emerging electron beam is allowed to fall on a photographic plate for a short time, e.g. 0.5 to 5 seconds. On development, series of concentric rings is observed on the photographic plate. The plate is then held to a strong light and position of the apparent maxima and minima of scattering intensity are marked. Photometric methods have also been used.

The accuracy in estimated bond lengths and the bond angles obtained from diffraction studies for simple molecules is comparable to that obtained by X-ray diffraction studies. The accuracy is usually not better than 0.01A in cases of estimated bond lengtha for O-H. NH bonds but under favourable conditions these can be used as good as ± 0.024.

Electron diffraction studies have a number of important applications. The most important application involves the study of the diffraction of elements by substances as vapours at low pressures with a view to evaluate the bond lengths and bond angles in relatively simple molecules and to determine the molecular configurations. Electron diffraction studies on a large number of compounds, simple and complex, have been carried out which have proved to be very important to understand the structure and geometry of these compounds.