Atom probe position sensitive detection

The Chevron channel plate ion detector assembly of an imaging atom-probe can also be replaced by a position sensitive particle detector combined with a data processor, as reported by Cerezo etal.5s (A position sensitive detector was used earlier for the purpose of field ion image recording and processing.59) With such a detector both the chemical identity and the spatial origin on the emitter surface can be found for each field evaporated ion. This position sensitive atom-probe can be used to study the spatial distribution of different ion species on the emitter surface as well as inside the bulk of the emitter with a spatial resolution nearly comparable to the FIM. For such a purpose, one carries out the field evaporation at an extremely slow rate so that no more than one ion is detected from the entire field ion emitter surface in each pulsed field evaporation. From the flight time of the ion its chemical species is identified, and from the location of the detector where the ion is detected the spatial origin of the ion is located. With a fast data processor, a two-dimensional distribution of chemical species on the tip surface can be... 

Figure 23.14 Schematic of double-molecular beam apparatus. Counter-propagating laser beams intersect pulsed molecular beams the pump (photolysis) beam produces atomic Cl that expands outwards and crosses the R H molecular beam. The HCl product is state-selectively photoionized by the probe laser using (2 + 1) REMPI. The resulting ions are detected with a 2D position-sensitive imaging detector after passage through a linear TOF mass spectrometer. Reproduced from Toomes and Kitsopoulos, Phys. Chem. Chem. Phys., 2003, 5 2481, with permission of the PCCP Owner Societies...

The major role of TOF-SARS and SARIS is as surface structure analysis teclmiques which are capable of probing the positions of all elements with an accuracy of <0.1 A. They are sensitive to short-range order, i.e. individual interatomic spacings that are <10 A. They provide a direct measure of the interatomic distances in the first and subsurface layers and a measure of surface periodicity in real space. One of its most important applications is the direct determination of hydrogen adsorption sites by recoiling spectrometry [12, 4T ]. Most other surface structure teclmiques do not detect hydrogen, with the possible exception of He atom scattering and vibrational spectroscopy. 

In this wet calcite system, it was also possible to obtain atomic resolution using the AFM. This observation was followed up in detail by Ohnesorge and Binnig in 1993. They showed that the force on the probe tip was important if such atomic resolution was to be achieved. If the force exceeded O.I nN, then the tip would wipe away the steps and leave a perfectly flat and ordered crystal. To show the effect of the force on the atomic imaging, they moved the tip very sensitively towards the surface and produced scans showing the surface atoms as the tip position was changed above the surface. TTie first force could be detected at a separation of 1.2 nm between tip and crystal surface, when the cantilever force constant was 0.2Nm. A sharp tip of diameter 20 nm was used, giving a typical force curve as shown in Fig. 6.20. 

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