Melike Ulu

Multi-element cylindrical electrostatic lens systems for focusing and controlling charged particles

Abstract This paper describes theoretical modeling of electrostatic lenses based on three, four and five closely spaced cylindrical electrodes. In each case, modeling is carried out numerically using commercial packages SIMION and LENSYS, and a variety of performance parameters are obtained. Special cases such as a 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, how a multi-element lens system can be operated with the whole focal properties in a useful mode for experimental studies is shown.

Novel and traditional fringing field correction schemes for the hemispherical analyser : comparison of first-order focusing and energy resolution

Strong fringing fields at the entry and exit of a real hemispherical deflector analyser (HDA) significantly degrade the 180° first-order focusing conditions, one of the central advantages of the ideal-field HDA. Over the past 50 years, traditional approaches to cure this problem have primarily sought to suppress these fields by improving field termination conditions typically requiring the unwieldy use of additional electrodes. Recently, Zouros et al (2006 Meas. Sci. Technol. 17 N81–N86) have shown in simulation that a simple repositioning of the HDA entry when appropriately biased results in the effective utilization of the intrinsic lensing properties of these fields to restore and even improve first-order focusing. Here, we investigate in simulation the efficacy of the new controlled lensing approach and compare it to the traditional Herzog and Jost field corrector approach. HDA focusing properties and energy resolution are reported as a function of entry angle, source extent and hemispherical interelectrode separation. For all cases considered, HDAs using controlled lensing always came out ahead demonstrating superior focusing along the 180° deflection plane and improved energy resolution.

Using the fringing fields of a hemispherical spectrograph to improve its energy resolution

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.

Double Differential Cross-Sections for Electron Impact Ionization of Atoms and Molecules

The single ionizing collision between an incident electron and an atom/molecule ends up two kinds of outgoing electrons called scattered and ejected electrons. As features of electron impact ionization, these two types of electrons are indistinguishable. Double differential cross-sections (DDCS) can be obtained by measuring the energy and angular distributions of one of the two outgoing electrons with an electron analyzer. We used He, Ar, H2, and CH4 targets in order to understand the ionization mechanisms of atomic and molecular systems. We measured differential cross-sections (DCS) and double differential cross-sections at 250 eV electron impact energy. The elastic DCSs were measured for He, Ar, H2, and CH4, whereas the inelastic DCSs of He were obtained for 21P excitation level for 200 eV impact electron energy.

Experimental and theoretical investigation of (e, 2e) ionization of Ar(3p) in asymmetric kinematics at 200?eV

Experimental and theoretical studies of electron impact ionization of the 3p orbital of argon are reported. The relative triple-differential cross sections were measured in the coplanar asymmetric geometry. The experimental data at an incident electron energy of 200 eV, ejected electron energies of 15 and 20 eV, and scattering angles of −10°, −15°, and −20° were obtained using a conventional crossed-beam (e, 2e) spectrometer. The experimental data are compared with predictions from first- and second-order hybrid distorted-wave plus R-matrix models, as well as a fully nonperturbative B-spline R-matrix approach.

Young Double Slit Interference Effects at Quantum Level

The currently accepted model for quantum interference resulting from the emission of electron waves from two scattering centers induced by either light or charged particle impact is analogous to Youngs emission of two light waves from two slits. In this work we show that this simple classical wave model is incomplete and that there is a more complicated quantum interference pattern for low energy ionization caused by electron impact.

Experimental and theoretical double differential cross sections for electron impact ionization of methane

Experimental and theoretical double differential cross sections (DDCSs) for electron-induced ionization of methane (CH4) are here reported for primary energies ranging from 50 eV to 350 eV and ejection angles between 25° and 130°. Experimental DDCSs are compared with theoretical predictions performed within the first Born approximation Coulomb wave. In this model, the initial molecular state is described by using single center wave functions, the incident (scattered) electron being described by a plane wave, while a Coulomb wave function is used for modeling the secondary ejected electron. A fairly good agreement may be observed between theory and experiment with nevertheless an expected systematic overestimation of the theory at low-ejection energies (<50 eV).

Double Differential Cross Section Measurements for Electron Impact Ionization of Atmospheric Gases

We present comprehensive experimental measurements of double differential cross sections (DDCS) for atmospheric gases, Ar, N2, CO2 and CH4. DDCSs were obtained by measuring the energy and angular distributions of one of the two outgoing electrons which are indistinguishable by use traditional electron spectrometer with an electron analyzer.

Experimental and theoretical investigation of (e,2e) ionization of Ar(3p) in asymmetric kinematics at intermediate energies

Experimental and theoretical studies of electron-impact ionization of the 3p orbital of argon are reported. Good agreement is obtained with other benchmark experiments and state-of-the-art calculations.

Double Differential Cross Sections for Methane Molecules at Intermediate Energies

Double differential cross sections (DDCS) can be obtained by the measurements of energy and angular distributions of one of the two outgoing electrons by a detector. In this pespective, we used methane molecule as a target that is reasonable to expect to understand ionization mechanisms of polyatomic molecular systems.