Probe Formation

The basic purpose of the lenses in a scanning electron microscope (SEM) is to form a focused beam spot at the specimen. By simple geometry. 

The STEM is in principle optically the same as the SEM, but the TEM/STEM is a little different. Here a special objective lens can be highly excited, so that it acts as two lenses in STEM mode [10]. The part before the specimen acts as a final condenser and the remainder acts 

It is important to remember that image resolution depends on the interaction volume, which may be much greater than the probe diameter (see Section 2.3 and Section 3.2.2). Because brightness B is fixed by the electron source, the geometric (or Gaussian) focused spot size can be given in terms of the probe current i using Eq. (3.14). 

Here AE is the energy spread in the beam in e V, Cs the spherical aberration coefficient of the final condenser lens, and Ccits chromatic aberration coefficient. 

The first, geometric, term is related to noise in the image through the probe current, / the next two are the diffraction and spherical aberration limits to resolution that were discussed in Section 3.1.4.1 for the TEM. The last term is the probe size due to chromatic aberration. The theoretical minimum probe diameter can be calculated by finding the value of a that minimizes this expression [10, 59]. This is a lower limit to the size of the resolved detail. Alternatively, the equation can be used to calculate the current obtainable in a probe of a given size. 

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Aside from the obvious improvement in the s/n ratio, which arises in part from concentrating the sample from 145 to 40 pL as well as from having all of the sample in the rf coil rather than 70%, the ketone carbonyl resonance at 219.5 ppm was observed with reasonable signal intensity whereas its presence was questionable at best in the 3 mm micro-dual probe 13C spectrum shown in the top panel. The s/n ratios of the overnight acquisitions were 5.5 1 for the 3 mm micro-dual probe vs. 11 1 for the Nano-probe . There is no obvious reason why the carbonyl resonance was not observed in the 3 mm micro-dual 13C spectrum, as the relaxation delay between acquisitions was set identically in the two experiments. These data also suggest that on a per-transient basis, that the performance of the Nanoprobe is perhaps somewhat lower than that of the 3 mm micro-dual probe. This inference is based on the nearly 4-fold concentration difference between the two probe formats. Assuming probe efficiencies were identical, an s/n ratio approaching 22 1 should have been observed for the Nano-probe based on the concentration increase. 

One other application where Nano-probe technology excels relative to conventional tube or flow NMR probe formats is in the area of heterogeneous samples. One of the early applications demonstrated for the Nano-probe was the acquisition of NMR spectral data for chemically modified polymer beads used in solid-phase-assisted peptide synthesis and related chemical transformations.21 23 When chemically modified beads are interrogated in a conventional NMR sample tube, the resin bead behaves as an insoluble material and at best very broad and poorly resolved spectra may be recorded. In contrast, when the same beads are placed in a Nano-probe and spun at several kHz at the magic angle, there is sufficient solvation of the pendant chemical moiety and the linker to resin bead nucleus to allow the modified portion to behave as if it is in pseudo solution, which allows reasonable NMR spectra to be recorded. Various factors affect the quality of the NMR data that can be obtained for the pendant molecule, which include the tether length and the solvent used for the measurement.23 There have been a diverse assortment of applications of Nano-probe applications reported in the literature that are discussed in further detail in Section 6.3. 

Reactant gases can be introduced into the microreactor as steady flow s or transient inputs with pulse widths (FWHH) of 250 fdsec. Reaction products are analyzed in realtime using a quadrupole mass spectrometer. Pulses from separate valves can be introduced as sets of pulses of predetermined length or in a pump-probe format alternating between two valves. 

Not shown in Fig. 3.1 are other analytical detectors normally found in STEM, because of lack of space. The principal omission is an X-ray detector that analyses X-rays emitted by the specimen, others include detectors designed to collect cathodoluminescence, specimen current. Auger electrons, etc. The lack of physical space around the specimen is often the deciding factor on which detectors are employed on a particular machine. In general, dedicated STEM has room to employ more detectors. In these respects, STEM is very much about detection systems, and not so much about the probe formation. 

Having said that, the electron-optics of STEM is dedicated to the production of a small probe. Before we review the probe formation, it is profitable to ask what determines the useful probe size for a realistic sample with finite thickness Part of this question can be answered through an understanding of the physics of electron scattering in solids, and its effect on the beam broadening. 

Proett, M.A., and Chin, W.C., Advanced Permeability and Anisotropy Measurements While Testing and Sampling in Real-Time Using a Dual Probe Formation Tester, SPE Paper No. 64650, Seventh International Oil Gas Conference and Exhibition, Nov. 2000, Beijing, China. 

HCN has been detected in the interstellar medium. Since then, extensive studies have probed formation and destruction pathways of HCN in various environments and examined its use as a tracer for a variety of astronomical species and processes. HCN can be observed from ground-based telescopes through a number of atmospheric windows. The J=1 0, J=3— 2, J= 4—>3, and J=10 9 pure rotational transitions have all been observed. 

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