558
Langmuir 1989,5, 558-562
A Molecular Dynamics Study of Small Rafts of Xenon Physisorbed on Platinum( 11 l ) + J. E. Black* and A. Janzen Department of Physics, Brock University, St. Catharines, Ontario L2S 3A1, Canada Received October 27, 1988. I n Final Form: January 9, 1989 We have examined the behavior of small rafts of xenon atoms physisorbed on Pt(ll1) with the technique of molecular dynamics. We do not observe the v'3Xv'3R30° structure observed experimentally. We do observe uniaxial strain events, which reduce the raft size below the 4.8 A needed for a commensurate structure. The events are seen at higher xenon platinum potential corrugations. They may distort the raft temporarily and withdraw or may lead to a translation of the raft to a new position.
Introduction In a recent experiment using high-resolution He scattering, Kern et al.' found that rafts of xenon atoms adsorbed on a Pt(ll1) surface exhibited a nearest-neighbor spacing of 4.8 8, and were rotated with respect to the substrate by 30'. Linear dimensions from 50 to 500 8,were estimated for the rafts. In technical terms, the xenon atoms were commensurate with the platinum surface and exhibited a d 3 X d 3 R30' structure. The commensurate phase was observed for temperatures in the range 60-99 K. An incommensurate phase was observed at lower temperatures. This incommensurate phase was also rotated by 30' with respect to the substrate. Black and Bopp2 examined the behavior of small rafts of xenon atoms (linear dimensions of the order of 35 8,) using the technique of molecular dynamics. They found that these small rafts were rotated by 30' with respect to the substrate. They did not find the rafts to be commensurate. Specifically, the atom spacing remained less than the 4.8 8, needed for the rafts to be commensurate with the substrate. Their results suggested the xenonxenon spacing was controlled by the temperature rather than the corrugation of the xenon platinum potential. Comsa and Kern3 suggested that the corrugation of the xenon platinum potential used by Black and Bopp might be too small. Their experimental data suggested that the energy difference between the hollow and bridge sites of the platinum surface could be as much as 350 K, whereas a maximum corrugation of 28 K was used by Black and BOPP. The present study was undertaken with the idea of seeing what the effects of increasing the xenon platinum potential corrugation would be on small xenon rafts. The plan of this paper is as follows. In the first section, the potentials studied in the present work are presented. We then describe the effects of varying the corrugation on a raft of 49 atoms under a variety of temperatures in the Results section. Finally, some concluding remarks are given. Potentials Black and Bopp2 attempted to simulate the observed v'3Xv'3R30° structure of Xe on Pt. They used a Lennard-Jones atom-atom potential between the Xe atoms and the first Pt layer to produce the corrugation. The interaction between the Xe and the remaining Pt atoms was represented by a z-dependent (hence uncorrugated) 'Presented at the symposium on "Adsorption on Solid Surfaces", 62nd Colloid and Surface Science Symposium, Pennsylvania State University, State College, PA, June 19-22, 1988 W. A. Steele, Chairman.
Morse potential. The potential parameters were chosen so that (i) the potential minimum of a Xe atom in a hollow site was at a selected height zmin (either 3.55 or 3.10 8,) above the Pt surface, (ii) the hollow site energy u h (zmin) was the negative of the observed desorption energy ud, (U,,, = 295 meV), and (iii) the perpendicular frequency wI E [az~h/(Maz2)]'/21z& was the measured value (nu, = 3.25 meV).3 The larger value of zmh is the measured Xe-Ag(ll1) height, while the smaller value is an estimated Xe-Pt(ll1) height. The Pt-Pt interaction used by Black and Bopp was between nearest neighbors only and was quadratic in R R,, where R and R, are respectively the instantaneous and equilibrium Pt-Pt separations. There were four layers of Pt atoms, the bottom layer being coupled by springs to an infinitely massive substrate. The Xe-Xe interaction consisted of the pair potential of Barker et al.4 in conjunction with a substrate-mediated potential of the Maitland-Smith form, as described by Brucha6 The simulations reported by Black and Bopp show that a weak corrugation is sufficient to rotate small Xe rafts to the observed 30' orientation. However, even the strongest corrugation they used (28 K between bridge and hollow sites) was not sufficient to lock the Xe into the observed 4.8-A spacing of the d3Xd3R3O0 phase. We have studied four forms of X e P t potentials with the aim of finding one with sufficient corrugation to lock the Xe into the 4.8-8, spacing while maintaining Ud-, wI, and zmin. These were the Morse, generalized Morse, Lennard-Jones, and exponential potentials. One general finding was that when ud, and zminwere fixed, wI and the corrugation could not both be chosen arbitrarily. (The corrugation C is defined to be min( Ub) - min(Uh),where Ub and Uh are the total potentials above bridge and hollow sites, respectively, and the minimization is with respect to 2.) Specifically,when C was significantly larger than the Black and Bopp value of 28 K, hwI was found to be larger than the measured value3of 3.25 meV. This is partly explained by the fact that C and wL2 both scale with the energy prefactor of the pair potential. In the four forms of pair potential studied herein, it proved impossible to find potential parameters which simultaneously gave large C and correct wI. We found a numerical routine6 for solving simultaneous nonlinear equa(1) Kern, K.; David, R.; Palmer, R. L.; Comsa. G. Phys. Reu. Lett. 1986, 56, 620.
(2) Black, J. E.; Bopp, P. Surf. Sci. 1987, 182, 98. (3) Comsa, G.;Kern, K. Structure of Solid Surfaces II; SpringerVerlag: Heidelberg, 1988. (4) Barker. J. A,: Watts. R. 0.: Lee. S. K.: Schafer. T. P.: and Lee. Y. T.J. Chem. Phys. 1974, 61, 3081. ( 5 ) Bruch, L., private communication.
n742.74~2im m n ~ i - n ~ i ~ i ~.1mzinni 0 1989 Amprirnn Chpmiral Snriptv
Langmuir, Vol. 5, No. 3, 1989 559
Molecular Dynamics of Xenon on P t ( l l 1 ) 0
0
oab 0 0
0 0
o 0
0
o 0 0
o 0
0
0
0 0
0 0
0
0
0
5 >
o 0
0 0
0
0
0
0
0
0
e
0
0
0 0
0
0
0 0
0
0
0
0
0
0
0 0
0
0
0 0
0 0
e
0
0
0
0 0
0
0
0 0
0 0 0
0 0
0-0
0
0'0
0
0
e 0
0
e
0 0
0.0
0
0
0.0 0
0
0 0'0
e
0
0.0
0 0'0
0
0 0 0
0
e
0.0'0
0 0
0 ' 0
0
0'0
o,o*o e
0
0.0
0
0
0,o.o
0 ' 0
om0
0.0
0
0
0
0
0
0
0
0
010 0
on0
0
0
0
0
0.0
0'0
0
0
0 0
0
0'0
0 0
0
0 0
0 0
0
0
0
0 0
0 0
0 0
0
0
0
0
0
0 0
0
0
0
0
0
0 0
0
0 0
0 0
0 0
0
0 ' 0
0
0 0
0 0
0.0
0.0
e
0
e 0
0
0 0
0 ~ 0 , o 0'0
0
0.0
0
0 0
0 0
0
P o . 0
0
30
0
0.0
0
0-0
0'0,.
0'0
0.0
0
0.0
0.0
0
0
0 0
Table 1. Average Spacing (A) of Atoms in the 49 Atom Rafts at Various TemDeratures and Corrugations corrugation, K T.K 0.00 11.14 34.70 66.30 109.30 92.5-95.0 90.0-92.4 87.5-89.9 85.0-87.4 82.5-84.9 80.0-82.4 79.5-79.9 75.0-77.4 72.5-74.9 70.0-72.4 67.5-69.9 65.0-67.4 62.5-64.9 60.0-62.4 57.5-59.9 55.0-57.4 52.5-54.9 50.0-52.4 47.5-49.9 0-10
24.78
4.62
4.68 4.67 4.62
4.68 4.62 4.62
4.55 4.67 4.45
4.39
4.51
4.58
4.605
4.43
4.49
4.53
4.64 4.60
0 0
0
0
0
0.0
0
0
0.0
0 0
0
0 0
0
0
0
0 0
0
0
0 0
0
0
0 0
0
0
0
0
0
0
0
0
0
0
P o
0
0 0
0
3
o 0
0
0
0
0:o
010
0.0
0
0
0:o
0
0'0
0
0 ' 0
0
0
0
0
010 0
0 0
o 0 0
e
o 0
0
0
0
0
0
o
o
0
0
0
0:o
o:e 0
0 0
0 : o :
0:o
0
0 0
0
0
0
0
0
0 0
0.0
0 8 0
0
0
0 0
0
o 0
0
0
0
0
0
0.0
0
o l o
0
0
0.0
0.0
0 0
0
0.0
o*o
0
0
o 0
0
0
o
0
,a '.,
0
0
0.0
0
0 ' 0
0 0
oto
O D 0
0 8 0
0
0
0
0
0
0
0
0
0
o o
060
o*o
oao
0 0
0
0
o o
0 0
0
0
0
0
0
0 0
!bo
0
o
o
0.'.
0:o
0.0
o:o
0
0
0
0
0
0
0
0
0
o
0
0
0:o
0
0
OCO
e
0.0 0
0
a
0
o
0
0
0
0
0
0
o
0:o
0.0
0
0
0'0
o
0.0'0
0
0
0
0
ob0
0
0 ' 0
0
0
0 0
0
0
0
0
0 0
0
0
o ,
ob0
0.0'0
0
0
0
0.0
0
o,o*', 0 0'0 o,o*'o 0'0
0
0
0
os0
o,o*:
0
0
0 ~ 0 , o ob0 0
0
0
0
0
e*'o
0.0'0
' o
o
D
0
om.*'.
0 0
0
0
0':
0
0
o 0
0
0
e
0
0
0':
0
0
o o o
0
o 8'0 P o 0
Q
0
* o b 0
0
0 ' 0
0
0'0
0-0
0.0
0
a o ' o
0
0 0
*
0.0
0
6
0
0
p
0
0
0 0
0
0.0
060
0
0 ' 0
0 0
0
o d o o o o o o t o 0 L'O 0 0 0 0
o
0'0
0.0
o
o
0
0
*bo
0
0'0
0
0
o o o
0:o
0'0
0
0.0
0
O.O
o 0
0 0
00:
0
os0
b
0-0
0
o
o
0
0
0
0 ' 0
0
0
0
0
010
0 .go o*@o 0 0 o,o*'o
0
0:o
0
o
o
0 O:O
o h
o*a
o
0
o
0
o
0
0
o
o'm
0's
0 0
o
obb 0
0
0
o 0
0
0
o 0
0
0
o 0
o
0.:
0
0
D
0
0
o
0.0
0
0
0
o
0
0.0
0
0 0
0
o
.a'*
0
0
0
o
0
o
0.8
o
0
o
0 0
x (A,
Figure 1. Final positions of the xenon atoms after quenching for corrugations of (a) 0, (b) 34.7, and (c) 109.3 K are indicated by solid squares. The solid circles indicate the commensurate raft of Figure 4. Open circles indicate the equilibrium position of the platinum atoms in the first layer.
tions very useful in obtaining the parameters. The potentials chosen for the MD simulations reported here were four Morse potentials. Each has a zmh of 3.1 A and Ud, of 295 meV, with h a , ranging from 3.25 to 6.5 meV as the corrugation ranged from 11 to 109 K. Results The molecular dynamics simulations were carried out with a platinum substrate 18 by 18 atoms across and 4 layers deep. The top layer is shown in Figure 1. Periodic boundary conditions were used, and nearest-neighbor forces linked the platinum atoms. An Adam-BashforthMoulton type of predictor corrector algorithm was used to integrate the equations of motion. This is described by Hamming.' Time steps of 4 X ps were used. The reader is referred to Black and Bopp for further details. The starting configuration of the rafts is also shown in Figure 1. The atoms are separated by 4.8 A, and therefore
we are starting with commensurate rafts which are rotated 30' with respect to the substrate. Behavior of Rafts at Very Low Temperature. In this study the initial temperature of the raft was 0 K, and the substrate was frozen at 0 K and not moving. The raft was allowed to move for a number of time steps and then the kinetic energy was removed. This process was repeated until the raft reached what appeared to be an equilibrium configuration. Several different approaches to equilibrium were tried, and they led to almost identical results. For corrugations of 0,11.14,34.70,66.30,and 109.30 K the final average atomic distances were 4.39, 4.425,4.49,4.53, and 4.60 A, respectively. Note that even with the strongest corrugation the spacing is clearly less than the 4.8 A required for the raft to be commensurate. Note also that the 0 K raft was started at the Barker-Bobetic spacing of 4.36 A and actually expanded (one would expect a slight contraction if only the Barker-Bobetic potential was used) by 0.03 A. The expansion is caused by the substratemediated potential. Finally, note that in the 0 K case a z-dependent holding potential was introduced as described in the Black-Bopp paper. In Figure 1 the final position of the raft of xenon atoms is shown for corrugations of 0, 34.7, and 109.3 K. Since there is no reference point in the 0 K corrugation, we have placed the central atom of the raft at the initial position of the central atom. At 0 K one sees that the relaxation takes place more-or-less inward toward the center of the raft. Note that the raft has only one mirror plane. Now consider the case of the large corrugation. Here we see that the raft relaxes inward along the channels formed along hollow-bridge-hollow directions of the substrate. The atoms at the top and bottom of the left side of the raft (and the atom at the center of the right side of the raft) are then "pinned" due to the fact that they would have to move toward the high potential of substrate top sites if they moved toward the center of the raft. It is in part this pinning which stops the raft from shrinking to the value of 4.49 8, observed by Kerna at 20 K. In the case of the intermediate corrugation, the results are intermediate between the two extreme cases described above. Here we see evidence of the motion both toward the center and also along hollow-bridge-hollow directions. Somehow the atoms at the center of the top and bottom
(6)IMSL routines ZSPOW and ZXLSF.
(7)Hamming, R. W. Numerical Methods for Scientists and Engineers; McGraw-Hill: New York, 1973.
(8) Kern, K., private communication.
Black and Janzen
560 Langmuir, Vol. 5,No. 3, 1989
2
*
..
*
,
I
,
I
*
I
I
D
*
*
I
*
.'L *
I
,
*
I
*
*
I
I
,&,
*
.
n
I
.
I
I
*
I
*
I
I
I
s
0.0
0
0
0
0.0
0
0
0
0
0
0
0.0
0
0
0
0
0
0
0
0
0
0
0
0
0
s
I
S
10
15
20
25
N
35
40
0
0
45
$0
sequent discussion.
I
*
I
I
0
0
x ti) Figure 3. Atoms which leave their initial upward pointing triangle have 0 replaced by the number of the triangle which they enter, as shown in the lower right. h can be seen, the atoms in the lower right of the raft have moved up to the left for the example shown. Atoms are numbered along the central row for use in the sub-
I
a
I
I
I
0
0
I
I
*
8
s
I
I
I
*
8
I
I
I
, $ I
*Ho
.
*
I
I
*
I
-
*
I
.PI
I
.
0
I
I
*
I
I
" '
I
0
I
8
I
*
0
*
g , l
>
*
*
I
8
.F?
.#,
*@. 7
. & ? e
6
2'
-
.4f?
- 2 ,p. .t. , . .e .e? .e .p. - 0 . . . . .r! . . 4 . ,*. .e ,& . b. . ,k **. ?P,+ . . . - 2 k -+*.k4 ,e, ..*,9 ,3. . . .a.a B c . + . f. . .*.< . . p , . # , . , . il
$5
.
*
0
I
spacing used in Table I was that obtained only for bonds linking pairs of atoms where each atom itself was sur8 rounded by six neighbors with bonds less than the specified 8 1 . . . . . . . . 1 - ,,.. * * * * distance. The specified distance used in this study was * * * 8 .C)# the average of the nearest-neighbor (4.36 A) and second2 8 . . ~ . ' ? ' , j . ' ~ ' , ~ , ' ~ ' . ~ .nearest-neighbor distance (7.55 A) and is 5.96 A. Note: we have also examined the length of the bonds linking the ., -1. - 8 , * +*!',;.'!* central atom of the raft to its nearest neighbors. We find .*. .a E' * that these lengths are also reduced below 4.8 A. Thus, it .e. & * .# - 9 . .+. q..;4 * is not simply that the outer bonds of the raft are being : . Y o ' + *.1. .'I. y. , .-A . m a R . 3 . reduced in length. , , , .*. * :. From Table I we see that even with the largest corru.*. , gation at 60 K the average spacing is 4.67 A, 0.13 A less .e, ,-. * . * !. * * than the 4.80 A required for the raft to be commensurate :.'.-.','.'.&*. , with the substrate. We also can see that the temperature controls the spacing for all but the highest of the corru. . . . . . ma gations. There is no indication that above a particular temperature the spacing locks into a value of 4.8 A and stays there over a range of temperatures. This is what one would expect on the basis of the Kern et al. experiment. While such behavior may well occur if we increase the corrugation still further, we are reluctant to do so due to the rather large discrepancy between the measured and calculated perpendicular vibration frequency, which our larger corrugations will give. Behavior of Rafts for T = 50 K. Dynamics of Atoms. In spite of the fact that our rafts are not commensurate with the substrate with the largest corrugation, in apparent disagreement with the experimental results, they do exhibit some rather interesting properties. In Figure 2 we show the trajectories of atoms for 20 ps with corrugations of 11.14 and 66.3 K. Note that with the low corrugation almost all directions of motion of the atoms are seen, while with the higher corrugation the atoms tend to exhibit motion along the hollow-bridge-hollow directions of the substrate. The initial conditions of the two 66.3 K corrugation cases were identical except in one respect. Different seeds were used in the random number generators used to determine the initial positions and velocities. This preference for hollow-bridge-hollow motion is evident in all cases of the trajectory plots studied thus far for corrugations greater or equal to 34.7 K. In an effort to explore the dependence of particle motion .
. -~. .r. '4' '4. , . . . '8'
I
.
.
I
.
>
>
.
, . , . , . . * , . . . , . * . , . ,? *. . , , . , .
.. I
9
.
.
.
a
9
0 ,
I
.. .... . . I' . . . i.'b*.*.. . . . . . . . .. . . .. .. . .. . . . . s. .. I
t
.
.
.
6
.
8
I
.
,
I
I
...
I
I
I
I
.
a
,
I
.
,
I
0 0
.
,
I O
.
.
-
.
-
,DO
.
-
I,O
,
-
,
-
,OD
.
.
.
,so
,
-
I
I '
( >
a
I
.
I
~
I
* #
s
I
*
I
I
I
I
I
.
I
I
"
I
.
,
,
.
I
.
,
I
I
.
I
.
.
-
IS0
.
I
(00
.
I
I
.
.
.,I
.
8 . )
Molecular Dynamics of Xenon on Pt(ll1)
a)
0
0
0
o
e
0
d
Langmuir, Vol. 5, No. 3, 1989 561
0
%
0
0
0
o
0
e
e
m
0 0
0
d
-
0 .
"4
. 0
0
0
0 .
'0
0
oo
1
d 00
e
0 0
a
o
c
0
d
. 0
4
0
e
0
00
. 0
>
a
d
. 0 . 0
0
O
b
0
0
~
x (A) Figure 4. Atom positions (solid circles) are compared with their initial positions (open circles) at (a) 27.2, (b) 31.2, and (c) 92 ps. The corrugation is 110 K, and the average adsorbate temperature parallel to the surface is 59 K.
on corrugation, we prepared diagrams such as that shown in Figure 3. The diagram indicates into which triangle the atom has moved from its starting site. (The positions were averaged over the 20 points we had in the history for each 1000 time steps, i.e., a point every 50 time steps.) In the case of the higher corrugations, these diagrams are particularly useful. From these diagrams it was immediately evident that the rafts undergo motion along preferred directions. What is more striking with the high corrugations, however, is the highly corrrelated nature of the motion. As an example, we consider the case illustrated with actual snapshots in Figures 4-6. Here the corrugation is 110 K, and the average adsorbate temperature parallel to the surface is 59 K. For the first 28 ps the raft atoms did very little. In Figure 4 the raft is shown at 27.2 ps. The raft at 31.2 ps has developed a strain in the 3-direction at the lower right side. (This is the time illustrated in Figure 3, but that figure was averaged over the preceding 4 ps.) This strain penetrates deeply into the raft, waxing and waning somewhat, and reaches a maximum penetration at 92 ps. The strain then slowly leaves the raft, and by 122 ps it has vanished (no 3-type triangles
.
1
,
1
,
o
,
,
~
,
s
,
w
r
n
m
8
a
x (A) Figure 5. As in Figure 8: (a) 127, (b) 139, and (c) 160 ps.
occupied). While this strain was present, there were never more than one or two triangles of the 1or 2 type occupied. The entire event took place over 96 ps, and at its peak 60% of the atoms had shifted into the 3 triangles. We call such an event a uniaxial strain event. Following the event described above, the raft did nothing for about 5 ps, and then in 12 ps, on the left side of the raft, a 2-type strain developed very quickly, peaked later, and had almost vanished at 228 ps. The development is shown in Figure 5. Finally, a 1-type strain began in the upper right, and it developed along with a 2-type to the end of the run at 252 ps, as shown in Figure 6. We have examined in detail the nine runs of Table I (all were of about 240 ps) with corrugations of 34.7-110 K. Uniaxial strain events were observed in the four temperatures at the highest Corrugation. In the case of the corrugation of 66 K, the two lower temperature cases exhibited events. These were somewhat smaller than those of the highest corrugation. In fact, one run exhibited only very small events. For the highest temperature at a corrugation of 66 K and for all temperatures at the 34.7 K corrugation, diagrams of the type of Figure 3 were no longer helpful. The problem was that the center of mass of the raft moved substantially during the run. Thus at
~
Black and Janzen
562 Langmuir, Vol. 5, No. 3, 1989
"1
50
0
0,
n.
0
0
Q 0
(D
e
Q
0
@ o
IO
8
Q
0
0
0
8
e
0
e
0
0
0
20.
0
0
0
O@
0
0
"4
a,
5
o,
> 0
b)
40
0
a
0
o@
om
6
d
0 .
5
1
0
1
6
8
e
0 @ 0
0
$
Q
8
8
om
0
0
d
0.
O@
0
0 0.
O@ xi
0
0
0,
0,
.
5
M
2
x
5
3
0
3
5
Q
4
5
5
0
4,
Figure 6. As in Figure 8: (a) 228 and (b) 252 ps.
present it is not clear to us if events are occurring in these cases. While our results are somewhat preliminary, it is possible to put forward a summary of what we have seen thus far, which we can attempt to explore further with our subsequent studies. What is happening here is that a strain develops in a small region on the raft and in one of the three preferred directions. Through some sort of cooperative effort this strain then propagates into the raft. Once started, it seems to inhibit other strains (perhaps due to the corrugation). In the cases of high corrugation, the strain does not propagate through the raft. Eventually it relaxes out of the raft, and we have had a uniaxial strain event. In the case of the lower corrugations, the strain propagates entirely through the raft, and we have a displacement of the center of mass.
Discussion The unexpected results of this study were the uniaxial strain events. These events can be characterized as occurring in one direction (of three possible symmetry directions) at a time. They are clearly seen at the higher corrugations of 66 and 110 K. They can involve a part of the raft of xenon atoms and finish in about 100 ps, or they can be instrumental in shifting the entire raft to a new location on the platinum substrate. The strain in the events leads to the same xenon structure as that seen experimentally in the striped phase of xenon observed by Kerng and Kern et al.'O They observe the phase when the temperature is reduced below that needed to hold the commensurate phase in place. Possibly uniaxial strain events are instrumental in developing such a phase. We are in the process of examining the dependence of the events on cluster size, cluster shape, temperature, and corrugation. We find that the 30° orientation is stable in the cases we have studied. However, we find that the rafts are not commensurate with the substrate. There are two components in this discrepancy between experiment and calculation. On the one hand, there is a small strain such as seen in the raft in Figure 1. On the other hand, there are the strain events which can lead to a large reduction in interatomic spacing. It is possible that with these events eliminated the raft spacing could be quite close to its commensurate value at corrugations of 66 and 110 K. We are in the process of seeing if pinning the raft can lead to a suppression of the events. We are also examining the effects of increasing the value of the Barker-Bobetic Xe-Xe potential minimum position to see if this will be effective in cutting out the strain events in the temperature regime where the raft should be commensurate. Our preliminary data suggest that shifting the minimum from 4.36 to 4.385 A gives a transition from commensurate to incommensurate xenon at 60 K for the corrugation of 110 K. The role of the uniaxial strain events in the transition will be discussed in a future paper." One final problem remains. We find that the large corrugations cannot be obtained by using pair potentials unless we fail to fit the perpendicular vibration frequency of the xenon atoms. We are searching for a pair potential that will fit both corrugation and perpendicular vibration frequency.
Acknowledgment. We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada and the Ontario Centre for Large Scale Computing. Registry No. Xe,7440-63-3; Pt,7440-06-4. (9)Kern. K. Phvs. Reu. B 1988.35.8265.
(10) Kern, K.; D k d , R.; Zeppenfeld, P.; Palmer, R.; Comsa, G. Solid State Commun. 1987,62,391. (11) Black, J. E.;Janzen, A., to be published in Surf. Sci.
