Chemistry Reference
In-Depth Information
bottom to top. The coverages are 1.22ML in Fig.
3.7
a and 1.6ML in Fig.
3.7
b which
are lower and higher than
θ
c
, respectively. It is important to notice that the ordinate
axis scale is six times larger (0-600 s) for the (7
α(
√
3
×
√
3
×
7) than for the
)
α(
√
3
×
√
3
(0-100 s). This by itself shows that the mobility is higher on the
)
since
τ
increases as
θ
is reduced below
θ
c
; in particular,
τ
is higher on the (7
×
7) than
α(
√
3
×
√
3
on the
θ<θ
c
in the latter case. The purpose of Fig.
3.7
is
not to compare the exact values of
)
despite
on the two phases (since
this requires very fine control of the coverage to show it is the same in the two
experiments), but simply to conclude which refilling speed and mobility is larger.
The faster speed on the
τ
for the same
T
and
θ
-phase is in good agreement with the STM observations
discussed earlier (namely, the interconnected “continents” on the
α
α(
√
3
×
√
3
)
vs.
separate individual islands on the (7
×
7) for the same
θ
). In addition, Fig.
3.7
shows
the dependence of
-phase because the
data follow a more clear noise-free dependence. They can be fitted to a power law
dependence
τ
on the hole radius
r
0
especially for the
α
r
0
with the exponent
n
close to 2.8. This is another illustration
of the constant refilling speed because from the LEEM intensity calibration it was
found that increasing the hole radius (with laser power) also increases the depth of
the profile hole,
τ
∼
kr
0
, so the total amount desorbed is proportional to
r
0
and therefore the measured exponent is very close to the expected
n
θ
depth
≈
3 exponent.
For completion, the results of earlier SPA-LEED experiments that explored the
phase transformations in Pb/Si(111) after Pb deposition at low temperatures are
shown in Fig.
3.8
. In these experiments, a uniform 2-d phase exists over the whole
=
Fig. 3.8
1-d diffraction profiles along the [11 2] direction at 190 K. Initially the surface is prepared
in the
β(
√
3
×
√
3
)
with
θ
=
1
/
3ML(
bottom curve
), phase with 1
×
1 diffraction pattern after
∼
2
/
3ML (second curve),
α(
√
3
√
3
)
phase after
×
∼
1ML (third curve), well-annealed
α(
√
3
×
√
3
)
(after heating 1.7ML to 500 K)
