Recently it has become possible to photograph the probability densities of
electrons confined in a two-dimensional region. The tip of an electron microscope
was used to place 48 individual iron atoms on a metal surface in a ring or
“corral” of radius 7.13 nm that formed the walls of a potential well, as shown in
Figure 5.18. Inside the ring, the waves of probability density for electrons trapped
in the potential well are clearly visible. The potential well is circular, rather than
square, but otherwise the analysis follows the procedures described in this section;
when the Schr¨odinger equation is solved in cylindrical polar coordinates with the
potential energy for a circular well, the calculated probability density gives a close
match with the observed one. These beautiful results are a dramatic confirmation
of the wave functions obtained for the two-dimensional potential energy well.