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The meta-stable state is
illustrated above, where an isosurface of charge density (0.25
electrons/Å3) is plotted (coloured by height)
along with a ball-and-stick model of the structure. Here is a
clearer ball-and-stick model, only showing the top two layers of
the Si substrate:
The key points about this structure are: the
hydrogen is bonded to a substrate dimer; the clean end of the
ad-dimer is bonded to a second-layer atom; and the
substrate dimer which the hydrogen is bonded to has broken its
bond to the second-layer atom.
Here is a contour map of the charge density:
This is in the plane
of the diffusing hydrogen, and clearly shows the bonding
situation (note that the bond from the substrate dimer to the
second layer atom is an artefact of the imaging software: there
is no charge, and hence no bond, here). The scale gives the
values in electrons/Å3.
Finally, here is another contour map in a different plane:
This makes clear the lack
of bond to the second-layer atom by the substrate atom: although a
bond is displayed, there is no charge (compare with the other side,
where the charge is clearly visible).
