RETARDING FIELD ELECTRON DIFFRACTION SPECTROMETER HAVING IMPROVED RESOLUTION

摘要

1277466 Electron microscopes VARIAN ASSOCIATES 17 Oct 1969 [21 Oct 1968] 51146/69 Heading HID In an electron diffraction apparatus comprising means such as the guns 2 and 2SP1/SP for directing a stream of mono-energetic electrons on to a target 1, a spherical fluorescent screen 3 for displaying a diffraction pattern and concentric spherical grids 5SP1/SP and 9, the resolution is improved by preventing field penetration of the retarding grid 5SP1/SP and by making the radius of the earthed grid 9 between 30% and 80% of that of grid 51 instead of at least 95% as hitherto. The grid 5SP1/SP may comprise two concentric grids, as shown in Fig. 3, separated radially by at least one third the distance between grid wires; alternatively it may comprise a metallic cellular structure having passageways of a diameter less than three times their length. An earthed grid 11 may be provided adjacent the screen 3 to reflect secondary electrons emitted by the screen, and spherical electrodes 26 and 27 are placed at the edges of grid 9 and provided with intermediate potentials to preserve the shape of the equipotentials in the marginal areas. When observing inelastically scattered electrons produced by the impact of electrons from the oblique gun 2 operated at - 100 volts, the voltage of the retarding electrode 5 may be swept synchronously with a recorder and modulated by 1 volt at 100 Hz., the current collected from the electrode being fed through a phasesensitive detector to the recorder (Fig. 1, not shown). The second input of the phase-sensitive detector may be the modulation signal, or the second harmonic of the modulation signal derived by feeding the signal through a diode multiplier; in the latter case the output of the detector corresponds to the first derivative of the energy distribution. When observing elastically scattered electrons the gun 2SP1/SP is used; since its outer tube 28SP1/SP is connected to the innermost grid 5SP1/SP the latter is then unable to operate as a retarding grid, but the grid 11 in conjunction with the outermost grid 5SP1/SP may be used for the retarding system. The fluorescent screen 3 is observed through a window (not shown) disposed to the left of the target 1.