Chemistry Reference
In-Depth Information
Fig.
3.5
. Both conclusions about non-classical diffusion described before also hold
for Fig.
3.6
which confirms the similar behavior of the wetting layer diffusion on the
two interfaces. As will be discussed below, the laser pulse heats the surface within
the hole and desorbs partially Pb to a local coverage
7ML less than the coverage
outside. This initial phase generated by the pulse is the same for both Pb-(7
∼
0
.
×
7) and
α
-phases. As Pb enters to refill the hole it generates the same sequence of phases
with
θ
:firstthe(1
×
1) at
θ
≈
1ML followed by the SIC phase at
θ
≈
4
/
3ML.
For the wetting layer on the (7
×
7), this means that although the phases outside
and inside the hole have the same
, their structure is different: outside the hole the
structure is the initial amorphous layer with 4/3ML and inside the hole is a weakly
ordered SIC phase with 4/3ML.
Another unusual observation about these experiments already discussed in [
3
]
for the
θ
α
-phase layer is the existence of a sharp transition of the refilling time
τ
vs.
θ
(of the initial wetting layer outside the hole) at fixed
T
. There is a critical coverage
θ
c
that was estimated in [
3
] to be 1.3ML (close to the coverage of the SIC phase)
below which there is dramatic increase of the refilling time. This divergence can be
described by power law reminiscent of critical phenomena 1
/τ
∼
(θ
c
−
θ)
−
κ
for
θ<
θ
c
. (More detailed recent measurements have identified from the diffraction patterns
the numerous “devil's staircase” phases that are known to form in this system in the
range 1
.
2
<θ<
4
/
3ML and have located with higher precision the exact value
of
θ
c
[
47
].) Although the nature of this dependence is not fully understood, such
a critical sensitivity of
τ
or
D
c
(θ)
with small changes of
θ
as low as 0.01ML is
unique to the Pb wetting layer.
Figure
3.7
shows the results of several refilling experiments on the two interfaces.
The refilling time is plotted as a function of hole radius at the same four tempera-
tures. (As described before the hole radius is changed by adjusting the laser power.)
Figure
3.7
a corresponds to the refilling on the Pb-
α(
√
3
×
√
3
)
layer and Fig.
3.7
bon
the Pb-(7
×
7) layer. The four temperatures shown are 286, 276, 264, and 259 K from
30
600
100
500
400
300
50
200
100
0
0
0
5
10
15
20
25
0
5
10
15
20
25
Radius (
μ
m)
Radius (
μ
m)
α(
√
3
×
√
3
Fig. 3.7
(
a
) Refilling on the
c
) for 286 K (
), 279 K (
),
269 K (
•
), 262 K (
)and(
b
)onthe(7
×
7) for
θ
=
1
.
6 ML (above
θ
c
) for 286 K (
), 276 K (
),
264 K (
•
), 259 K (
). Note: the ordinate axis scales for the (7
×
7) and the
α
(
√
3
×
√
3) differ by a
factor of 6
)
for
θ
=
1
.
22ML (below
θ
