Chemistry Reference
In-Depth Information
hole is not flat; the coverage is lower at the center than at the edges. The structure
of the film within the hole consists of the more strongly bound Pb
γ(
√
3
×
√
3
)
β(
√
3
×
√
3
(
3ML) phases in some proportion that depends
upon the amount of Pb that was desorbed, which is controlled by the laser power. By
varying the power, the radius of the hole
r
0
can be also varied from 5 to 25
θ
=
1
/
6ML)or
)
(
θ
=
1
/
m which
at the same time increases the coverage difference between the center and the hole
edge,
μ
θ
depth
, proportionally as
θ
depth
=
kr
0
. The presented results were obtained
for 17
μ
m
≤
r
0
; for 17
μ
m, the coverage difference between hole center and hole
edge
31ML but the difference between the outside coverage and the
coverage at the hole center is more than 0.75ML. Because of this large gradient,
the finite
θ
depth
is
∼
0
.
θ
depth
and the non-flatness of the bottom of the profile should make no
difference in the evolution of
(
r
,
t
).
Although the diffusion of the
θ
-phase wetting layer was studied and reported on
more extensively [
3
], we focus here on the behavior of the amorphous wetting layer
on the (7
α
7) surface to show the similarity between diffusion on the two interfaces
and to make a closer link with the previously discussed STM experiments.
Figure
3.4
shows refilling of the hole in the amorphous layer at
×
θ
=
.
1
4MLon
×
the Si(111)-(7
7) surface at 292 K. The surface prior to the application of the laser
pulse is shown in Fig.
3.4
a, followed by 11 images separated by 2.75 s for a total
refilling time of
30 s. The moving boundary divides the shrinking low coverage
phase at the center (dark) and the surrounding brighter region. (The bright rim and
the faint ring outside the hole do not change appreciably with time which indicates
that a different phase is far away from the hole vs. at its edge.) Similar observations
∼
a
b
c
d
e
m
μ
20
f
g
h
i
j
k
l
Fig. 3.4
Refilling of the initial hole generated by the laser pulse on Si(111)-(7
7). The tempera-
ture is 292 K and coverage 1.4ML. The length scale is seen by the
white bar
in (
a
). The first image
is before the application of the laser pulse and the time between each image is
×
75 s for a
total time of 30 s. An outward almost stationary pulse is not related to the refilling of the hole
t
=
2
.
