Atom probe configurations

FIGURE 40.22 Schematics of different atom probe configurations. (A) Original atom probe (B) 3D atom probe and (C) LEAR Adapted from Reference [344]. 

While a proper aiming of the atom-probe can be experimentally determined, information on field lines and on equipotential lines is difficult to derive with an experimental method because of the small size of the tip. Yet this information is needed for interpreting quantitatively many experiments in field emission and in field ion emission. We describe here a highly idealized tip-counter electrode configuration which may be useful for describing field lines at a short distance away from the tip surface but far enough removed from the lattice steps of the surface. The electrode is assumed to consist of a hyperboloidal tip and a planar counter-electrode.30 In the prolate spheroidal coordinates, the boundary surfaces correspond to coordinate surfaces and Laplace s equation is separable, so that the boundary conditions can be easily satisfied. 

The reactivity of a surface depends on many factors. These include the adsorption energies of chemical species and their dissociation behavior, their diffusion on the surface, the adatom-adatom interactions, the active sites where a chemical reaction can occur, and the desorption behavior of a new chemical species formed on the surface. The site specificity depends on at least three factors the atomic configuration of the surface, the electronic structures of the surface, and the localized surface field. In atom-probe experiments, the desorption sites can be revealed by a timegated image of an imaging atom-probe as well as by an aiming study with a probe-hole atom-probe, the electronic structure effect of a chemical reaction can be investigated by the emitter material specificity, and the surface field can be modified by the applied field. 

Figure 5. Set of time-resolved UV-near-IR spectroscopic data (3.44-0.99 eV) following the femtosecond UV excitation of an aqueous sodium chloride solution ([H20]/[NaCl] = 55). An instrumental response of the pump-probe configuration at 1.77 eV (n-heptane) is also shown in the middle part of the figure. The ultra-short-lived components discriminated by UV and IR spectroscopy correspond to low or high excited CTTS states (CTTS, CTTS ), electron-atom pairs (Che pairs), and excited hydrated electrons (ehyd )- The spectral signature of relaxed electronic states (ground state of a hydrated electron, (ehyd) electron-cation pairs, a e hyd) observed in the red spectral region.

In scattering Intensity Is treated here. It would be wrong to Imply that other factors, such as shadowing and the target atom s electron configuration, do not play a role. Clearly, they do, but their effects do not alter our conclusions. Thus, It appears that LEISS may also be quite valuable as a qualitative probe for changes In surface morphology.(16)... 

The Use of Diastereotopic Groups as a Probe for the Determination of the Configurational Stability at the Tin Atom... 

In a second approach of the reactivity, one fragment A is represented by its electronic density and the other, B, by some reactivity probe of A. In the usual approach, which permits to define chemical hardness, softness, Fukui functions, etc., the probe is simply a change in the total number of electrons of A. [5,6,8] More realistic probes are an electrostatic potential cf>, a pseudopotential (as in Equation 24.102), or an electric field E. For instance, let us consider a homogeneous electric field E applied to a fragment A. How does this field modify the intermolecular forces in A Again, the Hellman-Feynman theorem [22,23] tells us that for an instantaneous nuclear configuration, the force on each atom changes by... 

A metal-nucleotide complex that exhibits low rates of ligand exchange as a result of substituting higher oxidation state metal ions with ionic radii nearly equal to the naturally bound metal ion. Such compounds can be prepared with chromium(III), cobalt(III), and rhodi-um(III) in place of magnesium or calcium ion. Because these exchange-inert complexes can be resolved into their various optically active isomers, they have proven to be powerful mechanistic probes, particularly for kinases, NTPases, and nucleotidyl transferases. In the case of Cr(III) coordination complexes with the two phosphates of ATP or ADP, the second phosphate becomes chiral, and the screw sense must be specified to describe the three-dimensional configuration of atoms. 

The oxidation state of Ge atoms and linking metal atoms in the framework structure was probed by X-ray photoelectron spectroscopy (Fig. 15). The XPS spectra suggest that the Ge and linking metal atoms are in relatively low oxidation state and have electronic configuration similar to those observed in metal-alloy phases [57]. 

Studies that attempted to probe the electronic structure of the dithio complexes include the correlation of the electronic spectra in a series of R2 Dtc complexes of closed-shell configuration (584). In this study three major electronic absorptions in the spectra were found to be essentially unaffected by the central metal atom of the complexes and were considered intraligand transitions. 

The atomic form factor accounts for the internal structure of the different atoms or molecules. It will also be different for X-rays and neutrons, since the former probe the electron distribution of the target, while the latter interact with the nuclei of the atoms. Therefore, the analysis of the positions of the reflexes indicates mainly the lattice constants and angles. The intensity of the reflexes contains mainly information about the atomic configuration within an unit cell (structure factor) and the scattering behavior of the single atoms (form factor). 

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