Defocus positive

High-resolution electron microscopic studies employed a modified JEOL-JEM200CX (8) operated at 200 kV with objective lens characteristics Cs = 0.52 mm, Cc = 1.05 mm leading to a theoretical point resolution as defined by the first zero in the phase contrast transfer function of 1.95 A at the optimum or Scherzer (9) defocus position (400 A underfocus). 

FIGURE 1.10 Transfer function calculated for optimum contrast (a) Reproduction of the phase shift sin % from Scherzer (1949) in case of optimum contrast, usually improperly called the extended Scherzer plateau (at 870 A defocus position for the high-resolution transmission electron microscopy used), (b) Effect of defocus on cos %. (Adapted from N. Uyeda et al. Molecular image resolution in electron microscopy. J. Appl. Phys. 43, 5181-5189 (1972). With permission.) (c) Scherzer plateau calculated for Philips CM 20 (200 kV, Cj = 1.2 mm). (From O. Scherzer. The theoretical resolution limit of the electron microscope. J. Appl. Phys. 20, 20-29 (1949). With permission.)... 

Figure 7 Contrast transfer functions T(u) at defocus values e = -850 A, -165A and -525A. The optical parameters are from a Philips CM30/ST microscope U = 300 kV, Cs=1.15 mm, A = 70 A and a = 1.2 mrad. All the three contrast functions have a common first crossover position at u = 0.272 A. The defocus value -850 A was determined to be the correct defocus...

The amplitudes of the Fourier transform u) are zero where fmxiu) = 0, i.e. no information at these positions in reciprocal space are transferred to the image. These positions are zero-crossover(s) of the CTF. The defocus value(s) can be estimated from the position(s) u of the CTF crossover, and C and X (Zou etal, 1996). 

The fact that all deconvoluted images given in the middle row of Fig. 3 are very similar to one another in contrast and all metallic atoms are resolved as black dots with correct positions denotes that the image deconvolution technique is powerful to transform the image taken at an arbitrary defocus into the structure image. It is for the first time to clarify that the deconvoluted images still reveal the projected structure even if some reflections fall in the vicinity of zero cross of CTF. 

The most recent metastable defocusing technique, which is referred to as mass-analyzed ion kinetic spectroscopy (MIKES) by Beynon (77, 270, 277) and as direct analysis of daughter ions (DADI) by Maurer (278) requires the interchange of the source and collector positions in a double-focusing mass spectrometer of Nier-Johnson geometry. With... 

Figure 6.3. High Tq cuprate superconductors (HTSC) as catalysts (a) structural schematic diagram of YBa2Cu307 ( 123 ) HTSC with Cu02 sheets (b) HRTEM atomic image of the 123 phase in [010] projection with ED pattern. The image is recorded near the Scherzer defocus. The positions of the Y, Ba and Cu atom columns are indicated. The layer separation is "- 1.18 nm and the unit cell is outlined. (After Gai and Thomas 1991.)...

If the specimen is moved away from the focal position, then this will cause a phase shift that depends on 6. If the wavenumber in the coupling fluid is k = 2n/Xo, then the z component of the wavevector is kz = k cos 6. Defocusing the specimen by an amount z causes a phase delay of 2zkz, or 2zk cos 0 (the factor of two arises because both the incident wave and the reflected wave suffer a change in path length). Expressing this phase delay as the complex exponential of a phase angle, the response of the microscope with a defocus z is... 

The limits zmjn and zmax are the values of defocus at which the reflected rays just fill the area of the transducer thus for zmin < z < zmax all the reflected rays that enter the lens fall on the transducer, while for values of defocus outside that range some of the rays miss it altogether. The value z0 is the defocus at which the geometrically reflected rays are focused on the transducer at this point, as indeed at z = 0, ray optics breaks down because it does not allow for diffraction, although it does correctly predict the position of a minimum in V(z) at z0. The approximate expressions are valid when D/n > q, as is usually the case in a high-resolution acoustic microscope. In the approximation for z0, the quantity Ft is the ratio of the separation between the transducer and the back focal plane of the lens D — q/n) to the Fresnel distance for the transducer (na /Ao),... 

Fig. 8.6. Time-resolved measurements S(t, z) separating specular (geometrical) from Rayleigh reflections horizontal axis is time t vertical axis is defocus z the value of S(t, z) is indicated by the intensity, with mid-grey as zero and dark and light as negative and positive values of S (Weaver et al. 1989).

It is possible to use a hybrid of the plane wave and focused methods to study cells, by operating at modest values of positive defocus (Litniewski and Bereiter-Hahn 1990). Provided the reflecting surfaces, starting with the top of the cell, are a few wavelengths or more away from the focus of the lens, the effect of defocus is primarily to reduce the effective aperture of the lens. 

In order to illustrate this, Fig. 9.12 shows three pictures of a specimen of enamel that was specially prepared so that the left half was slightly demineralized and the right half was sound. V(z) curves were measured over each half of the specimen, and these are shown as a continuous curve for the sound enamel and a broken curve for the demineralized enamel. At focus (and indeed for a fair range of z either side of focus, especially positive defocus) the two V z) curves lie almost on top of each other. This implies that at focus the two halves of the specimen will give almost identical contrast, and this is what is seen in the top picture. By a defocus of z = -10 pm, the two curves have separated, with the sound enamel curve lying above the demineralized curve. This corresponds to the demineralized enamel appearing darker than... 

This gives the contrast in an imaging acoustic microscope due to a crack in a specimen, for any defocus and lens position. Once again a great advantage of... 

Fig. 12.8. Short incipient fatigue cracks in stainless steel (a) negative defocus, z = —20 (tm (b) positive defocus, z = +32 m 0.37 GHz (Rowe et al. 1986).

When a positive ion enters the filter, maintained under vacuum, at the origin O, its velocity vector in xyz space will determine its trajectory. The central portion of the quadrupole behaves like a tunnel in the O-z axis. The walls of the tunnel can either attract or repel an ion depending on the ion s position. The two positively charged electrodes will focus the ion in the O-z axis, corresponding to a potential valley (stability zone) while the two negatively charged electrodes will have a defocusing effect (potential maximum, unstable y-O-z plane). 

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