Experiments with Supersonic Beams as a Source of Cold Atoms

successful. Any water that remains on the crystal can be removed by blowing it off
with argon. Storing the crystal in an argon environment until it can be inserted into
vacuum preserves the pristine state of the surface.
3.2.3 Crystal Characterization
To characterize the passivated surfaces, they are examined using an atomic
force microscope (AFM). The AFM is operated in tapping mode, and scanning 2μmby
2μm regions of the wafer was found to provide a good compromise between examining
a large area and scan speed. Figure 3.2 shows AFM images of a wafer that was wet
etched. Image (a) shows a clean surface, with large atomically flat regions. The steps
seen in the image are bilayer steps from one crystal plane to another, and are 3.8 ˚A
high. The image is of the wafer 15 minutes after removing it from the etching solution.
Image (b) shows the same wafer after waiting an additional 28 minutes. The apparent
increase in surface roughness is likely due to the oxidation of the crystal. The height
profiles shown in images (c) and (d) are taken along the white lines shown in (a) and
(b) respectively. The surface roughness shown in (c) is less than an ˚A in magnitude,
and is probably noise in the AFM rather than real artifacts on the crystal surface.
However, the increased surface roughness shown in (d) shows grains approximately 2˚A
in height, and is likely showing real contamination of the surface. This indicates the
importance of minimizing the time the crystals spend in atmosphere before inserting
them into the vacuum chamber.
3.3 The Experimental Apparatus
This section describes the apparatus constructed to create, slow, and detect
a beam of supersonic helium. The description of the apparatus follows the path of
the beam, starting with the vacuum system, followed by the slowing apparatus, and
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