No abstract

A cylindrical specimen chamber and camera have been used to study the high-angle Kikuchi patterns obtained by reflexion of electrons, of energy 6 to 50 keV, from the cleavage surfaces of crystals with the sodium chloride structure. Angles of scattering ranging from 0 to 164° were covered. The relative intensity of the pattern at different scattering angles was measured using a photographic technique. The intensity distribution was found to become less steep as the energy of the incident electrons decreased. In photographs taken with a large value of the glancing angle of incidence, defect bands were found, starting near the shadow edge of the pattern; these changed to excess bands at higher angles of scattering. The most striking feature of the results is the remarkable intensity and clarity at the highest scattering angles of the pattern produced by crystals such as lead sulphide and potassium iodide, the constituents of which have a relatively high elastic scattering cross-section. In marked contrast, a relatively low intensity and low clarity was found at these angles for lithium fluoride under the same experimental conditions. An investigation of the width of Kikuchi bands, visible over the whole available angular range, showed that the electrons forming these bands had the same energy as that of the incident electrons within the experimental error of 10%. A possible mechanism is discussed by means of which electrons can be diffused through large angles with high efficiency, relative to small angles, and with relatively little loss of energy.

¦ theoretical investigation has been made of obliquely propagating electrostatic solitary structures in a hot magnetized three-component dusty plasma, which consists of a negatively charged hot dust fluid, Blotzmann distributed electrons, and free and trapped ions. It has been found that owing to the departure § from the Boltzmann ion distribution to a trapped ion one, the dynamics of small but finite-amplitudë electrostatic waves in a magnetized hot dusty plasma is governed by the modified Korteweg-de Vries © K dV equation. The latter admits a stationary solitary wave solution that has a larger amplitude, a smaller width, and higher propagation velocity, than that involving Boltzmann distributed § ions. It has been shown that the effects of obliqueness, external magnetic field, trapped ions,

A rigorous theoretical investigation has been made of obliquely propagating electrostatic solitary structures in a hot magnetized dusty plasma which consists of a negatively charged, extremely massive, hot, dust Ñuid and Maxwellian distributed ions and electrons. The reductive perturbation method has been employed to derive the KortewegÈde Vries (KÈdV) equation which admits a solitary wave solution for small but Ðnite amplitude limit. It has been shown that the e †ects of dust-temperature, obliqueness, magnetic Ðeld and free electrons have changed the nature of the electrostatic solitary structures. The result of the present investigation for unmagnetized case, parallel propagation, no free electron and cold dust Ñuid completely agrees with the existing published results (Mamun et al. [4]).

A theoretical investigation has been made of propagation of low frequency electrostatic modes in a cold magnetized gravitating dusty plasma, which consists of extremely massive, negatively charged, cold dust grains, nonthermally distributed ions, and Boltzmann distributed electrons, where the gravitational force is comparable to or greater than the electrostatic or electromagnetic force. It is found that the e †ects of gravitational force and free electrons make these electrostatic modes unstable, whereas the e †ects of magnetic Ðeld and nonthermal ions play a stabilizing role and counter the gravitational condensation of the dust grains. The relevancy of this result to some astrophysical situations is brieÑy discussed.