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Both diffusion and desorption are dependent on the temperature of the
system and on an activation energy. The idea is that in order for an
atom to move from its present location it must acquire enough energy
to over come a barrier to get to the new position. The barrier is
defined as
. The concept is illustrated in the Figure 4.
The beginning and ending energies are not necessarily the same for the
atom positions (denoted by 
). This also is included in Figure 4 (note
the low and high trough regions). The other component of the
diffusion rate is the prefactor. This gives the jumping rate of the
atom. The diffusion equation is given by:
Desorption of 
from the surface of the silicon is a 1st order
kinetic reaction. Though both atoms are required to desorb, it is
believed that the position of the dimer rows allows the hydrogen atoms
to already be paired and therefore a 1st order reaction happens
instead of a 2nd order reaction. The desorption of hydrogen from a
terrace is given by
