This paper describes theoretical modelling of electrostatic lenses based on 3, 4 and 5 closely spaced cylindrical electrodes, respectively. In each case, modelling is carried out numerically using commercial packages SIMION and LENSYS, and a variety of performance parameters are obtained. These include the magnification, the 3 rd order spherical and chromatic aberration coefficients. Special cases such as zoom lens (i.e., lenses whose magnification may be changed without losing focus) are considered. Results are obtained as a function of the ratios of the electrode lengths and gaps, and as a function of ratios of the controlling voltages.As a result, it is shown that how a multi-element lens system can be operated with the whole focal properties in a useful mode for using in experimental studies.
A specially designed hemispherical deflector analyzer (HDA) with 5-element input lens having a movable entry position R0 suitable for electron energy analysis in atomic collisions was constructed and tested. The energy resolution of the HDA was experimentally determined for three different entry positions R0 = 84, 100, 112 mm as a function of the nominal entry potential V(R0) under pre-retardation conditions. The resolution for the (conventional) entry at the mean radius R0 = 100 mm was found to be a factor of 1.6-2 times worse than the resolution for the two (paracentric) positions R0 = 84 and 112 mm at particular values of V(R0). These results provide the first experimental verification and a proof of principle of the utility of such a paracentric HDA, while demonstrating its advantages over the conventional HDA: greater dispersion with reduced angular aberrations resulting in better energy resolution without the use of any additional fringing field correction electrodes. Supporting simulations of the entire lens plus HDA spectrometer are also provided and mostly found to be within 20%-30% of experimental values. The paracentric HDA is expected to provide a lower cost and∕or more compact alternative to the conventional HDA particularly useful in modern applications utilizing a position sensitive detector.
For the last 50 years, there has been considerable interest in the possibility of observing the equivalence of a Young's double slit wave interference at the quantum level for diatomic molecules. For electron-impact ionization of diatomic molecules, indirect evidence for this type of interference has been found by changing the energy (wavelength) of the ejected electron while keeping the incident projectile scattering angle fixed. The present work represents an experimental and theoretical collaboration to better understand the physics of this type of interference. In addition to examining the effect of changing the ejected electron energy for fixed scattered projectile angle, we have also examined the effect of keeping the ejected-electron energy fixed while varying the projectile scattering angle. Model calculations are performed for three different types of possible two center interference effects and it is found that the most important one is diffraction of the projectile off two scattering centers.
The energy resolution of a hemispherical deflector analyser (HDA) can be substantially improved by using its entry fringing fields advantageously, rather than trying to eliminate them—the traditional approach. The intrinsic lensing properties of these fringing fields, as shown in simulations, are able to not only restore, but even improve first-order focusing at the 180° deflection plane in a controlled way, without the use of any additional field correction electrodes. This is accomplished by changing the entry radius R0 and bias from their conventional values of , the mean radius and to new values with or with . An HDA with , ΔR = R2 − R1 = 58.4 mm and maximum entry angle αmax = 2° demonstrates the impressive resolution gains that can be attained, 34 for a point entry (Δr0 = 0) and 4.2 for an aperture diameter of Δr0 = 1 mm, over corresponding conventional entry conditions.
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