Position sensitive atom probe

The atom probe field-ion microscope (APFIM) and its subsequent developments, the position-sensitive atom probe (POSAP) and the pulsed laser atom probe (PLAP), have the ultimate sensitivity in compositional analysis (i.e. single atoms). FIM is purely an imaging technique in which the specimen in the form of a needle with a very fine point (radius 10-100 nm) is at low temperature (liquid nitrogen or helium) and surrounded by a noble gas (He, Ne, or Ar) at 10 -10 Pa. A fluorescent screen or a... 

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. Position-sensitive atom probe images across a grain boundary in the nickel-base alloy Astroloy, for Al+Ti, Cr, Mo and B+C.

Various ion-optical tricks have to be used to compensate for the spread of energies of the extracted ions, which limit mass resolution unless corrected for. In the latest version of the atom probe (Cerezo et at. 1988), spatial as well as compositional information is gathered. The hole in the imaging screen is dispensed with and it is replaced by a position-sensitive screen that measures at each point on the screen the time of flight, and thus a compositional map with extremely high (virtually atomic) resolution is attained. Extremely sophisticated computer control is needed to obtain valid results. 

D information became possible with the development of position-sensitive detectors. The energy-compensated 3D atom-probe (or ECOPoSAP) had a -10 x 10 nm field of view and used a combination of a parallel timing system and a Si photodiode array camera allowing multihit events to be deconvoluted. It was also equipped with a refiectron ... 

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. 

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