Focusing and scanning

If you have a particle or a field of some kind whose interaction with material objects varies with their constitution, the chances are that you will be able to fashion a microscope from the phenomenon. (John Maddox 1985) 

He murmured almost to himself, sixteen surfaces, all opaque, all misaligned. (Calvin Quate 1985) 

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Figure 1. Setup of the far-field diffraction experiment. Fullerenes are sublimated, collimated by two narrow slits and diffracted at a nanofabricated SiN grating. The ionizing laser is tightly focused and scans over the molecular density distribution.

Alternatively, the electric focusing potential E can be changed, but this method needs another ion collector sited at the electric-sector focus, and it must be a collector that can be raised out of the ion beam when the mass of the ion being examined is required. This arrangement is not convenient. A better solution is obtained by linked scanning of the E/V voltages (see later discussion). 

Electron impact (El) ion sources are the simplest type. O2, Ar, or another (most often noble) gas flows through an ionization region similar to that depicted in Eig. 3.30. Electrons from an incandescent filament are accelerated to several tens of eV by means of a grid anode. A 20-100 eV electron impact on a gas atom or molecule typically effects its ionization. An extraction cathode accelerates the ions towards electrostatic focusing lenses and scanning electrodes. 

Both LIBS and LA-ICP-MS offer spatial and depth resolution. A single laser shot provides an instantaneous measurement of approximately several nanograms to micrograms of sample. For spatial analysis, the laser is focused to several tens of microns and scanned across a surface. For depth analysis (including inclusions), the laser is pulsed repetitively at a single location to drill a channel into the sample (Fig. 5). 

Focused X-ray beam (at Beam Line 15A) was used for this study. In order to improve the resolution, the focused beam (about 1 x 1.5 mm) was cut into 0.9-0.5 mm O by apertures which were set just in front of the cell. Modulation frequency was 10 Hz. Scanning X-Y stage which was originally developed for the laser microscopy was set perpendicular to the surface of the iron-base table and scanning and data acquisition were controlled by PC-9801 VM2 microcomputer (NEC Co. Ltd.) with the original program Various size and shape of metal foils were glued on the paper to have a model patterned sample. 

Spatial Variation of Organic Sulfur. The excellent spatial resolution of focused electron beams offers the possibility of examining variation of organic sulfur within macerals. The electron microprobe and scanning electron microscope allow resolution of a few microns (14-16). The transmission electron microscope allows even better resolution (less than 1 pm) because the thin foils and powders produce less electron scattering. We have used this capability to measure the distribution of S in a number of coals. 

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