Fringing, lens

In order to observe fringes, the screen should be placed in the regime of Fraunhofer diffraction where F/B B/X. In practice, such an interferometer can be realized by placing the stop immediately in front of a collecting optics, e. g., a lens or a telescope, and by observing the fringes in its focal plane (F = fes). 

Figure 4, Area of benzene covered gold (111). surface, for two different objective len.s defooi a.s required for unique image interpretation (see 2 ). Tri a) the gold atomic columns are black, in b) white. Moire fringes, rather than any true structural image, result from the benzene monolayer. Simulations (right) have benzene overlay on top surface only.

Figure 5. Schematic arrangement for hologram formation with an electron biprism. A plane wave illuminates the specimen placed off-axis. After the object lens a wire is placed between two earthed plates. The wire is the electron optical analog of a Fresnel biprism and causes the unperturbed and perturbed waves forming the electron hologram to interfere. The object phase-shift causes a displacement in the hologram fringes, and is thus observable.

Coma is due to the electron beam being tilted away from the optical axis of the objective lens, the coma-free axis. The coma results in lattice fringes related to +g and -g being shifted by the objective lens. Lattice fringes belonging to different beam pairs, +g and -g, are shifted differently resulting in an asymmetry of the HRTEM image. 

LITs two modes for the mass selective ejection of ions are used either the ions are expelled axially using fringe field effects by applying AC voltages between the rods of the linear trap and the exit lens, or slots are hollowed out in two opposite rods and mass selective radial expulsion of ions is obtained by applying an appropriate AC voltage on these two rods. 

As shown in Figure 2.32, the ions are expelled axially using fringe field effects by applying AC voltages between the rods of the linear trap and the exit lens. 

This instrument can be operated as a normal triple quadrupole with all its scan modes or as a trap in various combinations with the use of the other quadrupoles. If a slow scan rate is used to expel the ions a resolution up to 6000 FWHM can be reached by scanning at 5 Th s 1 using q2 and at 100 Th s 1 using Q3, which is at a lower pressure. As fringe field effects are used, only ions close to the exit lens are expelled. In consequence, mass selective ejection in the axial direction based on this technique is characterized by low ejection efficiency. For instance, an ejection efficiency of less than 20 % is achieved at 1 Th ms-1 scan rate. Different techniques have been proposed to improve the axial ejection efficiency [21], but the most promising technique for mass selective axial ejection is the technique named axial resonant excitation (AREX) [22]. Lenses are introduced between each rod of the quadrupole... 

Instruments using as analyzer a B,E arrangement are consequently called double-focusing mass spectrometers (see Fig. 2.5). Under these conditions, what are called first-order aberrations are overcome however, further aberrations of the ion optics are still present, due to the fringing fields present either at the magnetic or electrostatic sector levels. To reduce these undesired effects, usually electrostatic lens are placed in the field-free region of the instrument. 

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