J. Phys. Chem. 1993, 97, 3829-3837
3829
Probing Three Distinct Iodine Monolayer Structures at Pt( 111) by Means of Angular Distribution Auger Microscopy: Results Agree with Scanning Tunneling Microscopy Douglas G. Frank, Oliver M. R. Chyan, Teresa Golden, and Arthur T. Hubbard’ Surface Center and Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72 Received: March 4, 1992; In Final Form: October 26, 1992
W e report angular distributions of Auger electrons (507 and 5 18 eV) emitted from three distinct monolayer structuresof iodineatomsonsingle-crystal plathum: Pt(l1 1)(d3Xd3)R30°-I, P t ( l l l ) ( d 7 X d7)R19.1°-I, and Pt( 111)(3X3)-1. Each of these monolayers has been characterized by means of LEED, STM, AES, XPS, TDMS, and electrochemistry. Accordingly, these monolayers provide an opportunity to explore the nature of Auger electron angular distributions for surface atomic layers of known structure and to correlate the results with what is already known regarding these structures. Evident near grazing angles of emission in the measured angular distributions are intensity minima along trajectories corresponding to the 1-1 internuclear directions and intensity maxima along trajectories corresponding to the gaps between neighboring iodine atoms. Visible near the surface normal are features due to electrons emanating from the P t ( l l 1 ) substrate. Simulations of the measured angular distributions in which iodine atoms are treated as point emitters and spherical scatterers of Auger electrons are in qualitative agreement with experiment. These results suggest that iodine atoms act predominantly to block Auger electrons in this energy range, producing minima in the angular distributions. Accordingly, the locations of the minima are a direct consequence of the relative positions of atoms, making measurements of Auger electron angular distributions useful for real-space probing of surface atomic and monolayer structure.
Introduction In recent years, there has been increased interest in measurements capable of revealing the arrangement of atoms at or near the surface of a solid. The more productive approaches have been those based upon the use of multiple techniques, yielding complementary information useful for resolving ambiguities that might otherwise have remained. The present study begins with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and angular distribution Auger measurements (ADAM) for a series of well-known monola er structures: Pt(111)(d3Xd3)R30°-I, P t ( l l l ) ( d 7 X ~ ? ) R 1 9 . 1 ~ - 1 and , Pt(111)(3X3)-I.1-9 The LEED and Auger results are then compared to recent STM results obtained for the same structure^.^-^ An important outcome of this study is that the ADAM results confirm the findings obtained by the other methods. In addition, they also provide quantitative support for the corrugated structures recently revealed by STM.6-9 Auger electron a’ngular distributions are obtainedlk12 by excitation of the sample with an electron beam at fixed direction of incidence while a moving analyzer measures the angular distribution for a specific Auger electron kinetic energy. “Complete” distributions are measured and displayed over the full range of angles of emission so that the angular distribution and surface structure can be readily visualized. Recent measurements of complete distributions have led to the o b ~ e r v a t i o n lthat ~’~ intensity minima are present along interatomic directions, except when obscured by electron diffraction and/or channeling effects in single crystals.18-26Accordingly, the locations of the minima directly reveal (in real space) the relative positions of atoms near a solid surface, an hypothesis fully supported by STM result^.'^ In addition, the observation of such intensity minima in the distributions has been corroborated in at least three other laboratories.27-z9 Work is currently underway to understand the quantum-mechanical basis for these observation^.^^-^^ An exact description must include the contributions from elastic and inelastic scattering, although one or the other may predominate under a given set of conditions. Some workers initially questioned these experimental findsuggesting that our samples were misoriented. The fact
that intensity minima are observed along interatomic directions is in conflict with predictions put forward by those workers.33-35 According to their predictions, the atoms neighboring an Auger emitter should “focus” Auger electrons or photoelectrons along the interatomic directions, giving rise to intensity maxima. Recently, Terminello and Barton2’ confirmed the presence of intensity minima along internuclear directions for Auger electron emission angular distributions and reported substantialdifferences between measured Auger electron and photoelectron angular distributions, even when obtained from the same sample at the same kinetic energy. These differences were also not predicted by the earlier theoretical models, which may help to account for some of the initial confusion concerning Auger distributions, as most of the previous experimental and theoretical development was directed toward photoelectrons. The relative scarcity of Auger distribution data may also have contributed to the confusion, as previously noted.3ls32 In view of the above questions regarding the data and their interpretation, this work emphasizes measurements of complete Auger electron angular distributions for samples whose surface structures have been independently determined by means of STM and LEED. The well-defined iodine monolayers discussed in this work were chosen for several reasons: (i) The STM images of the adlayers are remarkably ~ l e a r ,confirming ~-~ the structures previously determined by LEEDi4 and leaving little doubt as to their basic structure. High-resolution XPS data are also a ~ a i l a b l e (ii) . ~ Monolayers yield relatively simple Auger electron angular distributions (compared to single crystals), simplifying the interpretation. And, (iii) the iodine Auger electron kinetic energy (507-5 18 eV) is higher than the energies employed in our previous experiments and is well within the energy range in which Auger electron “forward focusing”33-35 is predicted to occur. The results presented here reveal that the measured angular distributions of Auger electrons emitted from each of the three distinct iodine monolayer structures contain intensity minima along the iodine-iodine internuclear directions, establishing that iodine atoms act primarily to block Auger electrons rather that to focus them. Furthermore, simulations of the angular distributions (based upon the known structures) in which each iodine
0022-3654/93/2091-3829%04.00~00 1993 American Chemical Society
Frank et al.
3830 The Journal of Physical Chemistry, Vol. 97, No, 15, 1993
I
t
D
-,t
Figure 1. Iodine Auger electron angular distributions for Pt( 1 11)(d3Xd3)R30°-I. Brighter colors represent stronger Auger signals as indicated by the color bar (lower left). The center of the image corresponds to emission normal to the surface (4 = O O ) , and the edge of the image corresponds to grazing emission (4 = 90’). (A) Measured angular distribution of iodine Auger electrons (507-518 eV) emitted from the 4 3 - 1 adlattice. The edge of the distribution contains intensity minima due to scattering by neighboring atoms within the monolayer, and the center of the distribution exhibits features attributable to the Pt(ll1) substrate. (B) Digital simulation of the angular distribution for iodine Auger emission from the d 3 - I adlattice, assuming that iodine atoms behave as point emitters and spherical scatterers of Auger electrons. (C) Features of the measured 4 3 - 1 distribution can be seen more clearly following subtraction of the smoothly varying “background”. (D) Simulation of the 4 3 - 1 distribution which includes the contribution due to the Pt( 111) substrate.
atom is assumed to be an isotropic point emitter and spherical scatterer of Auger electrons closely predict the observed angular distributions. By “spherical scatterer” we mean that the atoms behave as spheres which scatter electrons both elastically and inelastically, resulting in some probability of inelastic scattering and some other probability of elastic scattering. Evidently, inelastic scattering predominates, as will be demonstrated in a future article.37 These findings point to the potential usefulness of Auger electron angular distributions for determination of the relative positions of atoms within a monolayer. Initial results of this type for the Pt( 111)(d7Xd7)R19.1°-I adlattice were reported recently,” while in the present article we include results obtained for two additional I/Pt adlattice structures. Although the individual adsorption sites of iodine atoms on the Pt( 111) surface were not determined in this study, they have been previously inferred from LEED, XPS, and STM.1-7 Direct determination of adsorption site registry from Auger electron angular distributions is the focus of an ongoing in~estigation.3~ Angular distributions from iodine monolayers are less complicated than those from multilayer system^,^^.^ 3-15 since no scattering atoms are located between the emitting atoms and the
detector over most of the forward hemisphere above the sample. Intensity minima due to scattering by neighboring atoms in the monolayer begin to appear at angles of emission within about 25’ of grazing. An easily recognized substrate pattern is also observed in the monolayer angular distributions due to the presence of weak emission and scattering of electrons by the Pt lattice. In ref 10 we presented angular distribution data for Auger electrons emitted from an iodine monolayer situated on top of a Ag monolayer. The angular distribution from the iodine monolayer was nearly featureless and was included in the paper to illustrate that emission from individualiodine atoms is essentially isotropic. Intensity minima in the distribution due to neighboring iodine atoms were not observed, since only the center portion of the angular distribution (within 70’ of the surface normal) was available at that time. Also, the data were normalized to remove the analyzer sensitivity function (a gradual, smooth variation, as described in Table I). Experimental Section The experimental apparatus employed for ADAM has been d e ~ c r i b e d . ~ ~To - ~ *stimulate + ~ ~ Auger emission, the sample was
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3031 irradiated with a glancing incidence electron beam (2 keV, 1 1 O from grazing). The incident beam and sample remained stationary while the moving analyzer scanned the angles composing the complete forward hemisphere above the solid surface. Thus, the direction of the incident electron beam remained constant throughout the measurements. It is important for this type of measurement that the sampleand incident beam remain stationary while the moving analyzer scans the angular distribution, since even minor variations in the angle of incidence can lead to substantial changes in Auger intensity.20.2133945 A small portion of the angular distribution in the vicinity of the incident beam was not accessible. The final angular distribution contained 27 331 data points ( l o resolution) and required about 3 h of measurement time. Distributionsare displayed in spherical coordinates (6, $), with the intensity along the surface normal shown at the center of the display (+ = Oo) and emission parallel to the surface (+ = 90°, grazing emission) shown at the edge. As indicated by the scales included with each distribution, +varies linearly from the center of the image to the edge. Lighter colors (white, yellow) represent larger Auger intensities, while darker colors (blue, black)
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Figure 3. Polar scans extracted from the 507-518-eV angular distributions measured for clean Pt(ll1) (lower curves) and Pt(l1 1)(d3Xd3)R3O0-1 (upper curves): (A) 8 = 30°, (B) 8 = 90°.
Frank et al.
3832 The Journal of Physical Chemistry, Vol. 97, No. 15, 1993
TABLE I: Structural Parameters of Iodine Monolavers8 relative coordinates (A) monolayer structure
I atoms/Pt atom
1-1 distance (A)
Pt( 11 1)(d3Xd3)R3O0-I Pt(l1 1)(d7Xd7)R19.l0-1
0.33 0.43
4.808 4.247
Pt( 11 1)(3X3)-I-hex
0.44
4.164
Pt( 11 1)(3X3)-I-asym
0.44
4.164
a
adsorption geometry 3-f0ld, fCC 1-fold, “A”, atop 3-f0ld, “B”, hcp 3-f0ld, “C”, fCC 1-fold, “A”, atop 2-fold, “B”, bridge 2-fold, “B”, bridge 2-fold, “B”, bridge 3-f0ld, “A”, fCc 1-fold, “B”, asym 1-fold, “B”, asym 1-fold, “B”, asym
X
0 0 2.776 5.552 0 4.164 2.082 6.246 0 4.164 2.082 6.246
Y 0 0 3.205 6.41 1 0 0 3.606 3.606 0 0 3.606 3.606
z (fO.1)
0 0.8 0.6 0 0.8 0 0 0 0 1.o 1.o 1.o
STM z value
(A)
0 0.95 f 0.20 0.71 f 0.23 0 0.83 f 0.08 0 0 0 0 1.02 f 0.12 1.02 f 0.12 1.02 f 0.12
The simulated Auger intensity for each trajectory I(&@) was calculated as follows:
where N is the number of emitting atoms in the model, T i s the transparency of the atom, mi(&@) is the number of atoms encountered by the Auger electron along the path (6,@), and I, is a normalization function which produces a smooth and gradual increase of signal with increasing polar angle of detection:
I,(@) = 1 +-
k,
+
[exp[A sin(24 270°)] - exp(-A)] 2 sinh A The functional form of I,(@) was empirically determined to yield best fit of a smooth curve to the center region (4 < 50’) of the iodine monolayer data, which is relatively featureless. The parameter A determines the shape of the analyzer function, and kl is an arbitrary scale factor. Best agreement with experiment was found for T equal to 6096, A equal to 0.4, and a scattering radius for iodine (used in evaluating mi)equal to 1.2 A. The function I,(@) closely resembles the usual sec @ dependence of the field of view of a mechanical collimator upon the takeoff angle but is more convenient computationally.
A P t ( l l l I ( d 3 x d3>R30°-I
Lli.. .
.
. . . . .
Y
M
B
equivalent to the (1 11) plane. This permits the crystal to be used as an immersed electrode46 and also makes its orientation ~ n m i s t a k a b l e . ~Prior ~ - ~to ~ preparation of the iodine monolayer, all six faces of the crystal were simultaneously cleaned by bombardment with Ar+ ions, followed by annealing in UHV. Surface cleanlinessand structural perfection of the clean Pt(II1) surface were verified by Auger spectroscopy and LEED, respectively. The iodine monolayer was prepared by exposing the clean Pt( 111) surface to iodine vapor, producing a Pt( 111)(3X3)-1 adlattice. ThePt(l1 1)(d7Xd7)R19.l0-Iadlatticewasobtained by gentle warming of the (3X3)-I in vacuum while observing the LEED pattern. The Pt(l1 1)(d3Xd3)R30°-I adlattice was obtained from the 4 7 - 1 surface by further heating. Each iodine adlattice was characterized by LEED and Auger spectroscopy before and after the ADAM measurements to verify surface structure, cleanliness, and stability. Results and Discussion
Pt (111)
Figure 4. (A) The Pt( 11 1)(d3Xd3)R30°-I adlattice consists of a flat, hexagonal layer of iodine atoms (shaded circles) rotated 30’ with respect to the Pt( 1 1 1) substrate (grid). The iodine atoms are shown in 3-fold fcc sites (triangles pointing down) on the Pt(ll1). (B) Auger electrons emitted from t h e 4 3 adlatticeof iodineatoms arescattered by neighboring iodine atoms, producing intensity minima near the horizon of the angular distribution.
correspond to lower Auger signals as shown in the scale and color bar located at the lower left of the displays. The data are normalized so that the left edge of the scale correspondsto zero signal and the right edge to the maximum observed signal. The Pt( 111) single crystal employed in this work was oriented by means of X-ray reflection photography. All six faces of the crystal were oriented and polished and were crystallographically
1. Pt( 111)(d3xd3)R30°-I. The measured angular distribution of iodine Auger electronsemitted from thed3-I monolayer (predominantly 507- and 518-eV electrons) is shown in Figure 1A. Brighter colors represent stronger iodine Auger signals, as shown in the scaleat the lower left. The data have been normalized so that the left edge of the scale correspondsto zero Auger signal and the right edge to the maximum observed signal. The data have been averaged employing the obvious 3-fold symmetry of the data. Visible in the measured angular distribution (Figure 1A) at polar angles near grazing emission ( 6 5 O < 4 < 75’) is a bright, corrugated ring of intensity. Seen closer to the center of the distribution is an hexagonal pattern of less intense maxima. The features of the distribution can be seen more clearly after subtraction of a smoothly varying “backgroundnfunction (Figure 1C). The background for each direction of emission (&4) was obtained by digitally averaging a group of adjacent data points (0 f 20°, 4 f 20°) large in comparison with the typical angular range of the structural features, producing an angular distribution which is featureless except for gradual variations. Subtraction of this background function increases the contrast of the
Iodine Monolayer Structures at Pt( 111)
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3833
-
1
Y
I
Figure 5. Iodine Auger electron angular distributions for Pt( 111)(I/7XI/7)R19.1°-I. (A) Measured angular distribution of iodine Auger electrons (507-518 eV). The distribution displays intensity minima near the horizon (4 > 65') due to scattering by neighboring atoms within the monolayer. (B) Digital simulation of the ADAM image for iodine emission from both domains of the 1/74 adlattice (compare with Figure 7B). Since the monolayer is corrugated, blocking by neighboring iodine atoms protrudes into the distribution to differing extents, producing multiple intensity minima along the nearest-neighbor directions. (C) Features of the measured 4 7 - 1 distribution are seen more clearly following "background" subtraction. (D) A simulation which includes the contribution due to the Pt( 111) substrate closely predicts the observed distribution.
distributions, reduces contributions such as those due to the angular variations of the analyzerfield of view (analyzerfunction), and does not introduce new features into the distributions. Features seen near the center of the distribution (4 < 65') are also present in the distribution measured for Pt( 111) in the absence of the iodine monolayer (Figure 2). Of course, these features are not as distinct as the iodine features because the signal from Pt is relatively weak at the KE of the iodine Auger transition (507518 eV). For this reason, the data shown in Figure 2 have been background-subtractedas described above. These features are observed regardless of which iodine structure is present, as well as in the absence of iodine. Thus, the 3-fold pattern of maxima observed in the center portion (4 < 65') of Figure 1A is attributable to emission and scattering by the Pt crystal and is not due to scattering by the iodine monolayer. Indeed, the locations of these intensity maxima correspond to the Pt-Pt internucleardirections, which are alsothe directionsof the clearest channels through the Pt single crystal. Extraction of polar scans from the complete distributions facilitates quantitative comparison of data. For example, Figure 4 shows polar scans for the d 3 - I distribution compared with scans for the clean Pt. Two azimuthal directions are shown: B
= 30' in Figure 3A and 8 = 90' in Figure 3B, as illustrated to the right of each plot. As can be seen from the data, the variations in the Pt intensity are small relative to thevariations in the iodine signal. The locations of features in the Pt polar scans for bare Pt correlate well with features in the central portion (4 < 65') of the polar scans obtained in the presence of an iodine layer. Figure 4A shows a model of the Pt(lll)(d3Xd3)R30°-I adlattice structurebased upon the findingsof LEED14 and atomic resolution STM.7 Structural parameters are collected in Table I. The structure of the sample was verified by LEED in the present work both before and after measurement of the angular distribution. A sharp (d3Xd3)R3Oo LEED pattern was obtained each time. It is generally thought that the adsorbed iodine atoms occupy 3-fold sites on the Pt( 111): but whether there is a preference for fcc or hcp sites has not yet been resolved. (The hcp and fcc 3-fold sites differ by the presence of a secondlayer Pt atom or an octahedral hole, respectively, directly beneath the site.) For illustration purposes, we assume that the iodine atoms occupy fcc hollow sites; the present work probes only the relative structure within the monolayer. Figure 4B illustrates the expected locations of intensity minima in the angular distribution based upon blocking by neighboring iodine atoms.
3834 The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 A
P t ( l l l l ( J 7 x J7)R19.lo-I
Frank et al. A .
. .
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PtClll)
Figure 6. (A) The Pt( 11 1)(d7Xd7)R19.lo-I adlattice consists of two mirror-image domains of a corrugated, hexagonal layer of iodine atoms rotated 19.1O with respect to the substrate. (B) Top view of a single unit cell. The unit cell contains three atoms, one in an atop site (“A”), one in an hcp hollow site (“B”), and one in an fcc hollow site (“(2”). (C) Side view of a single unit cell.
As shown, minima are expected near the horizon (4 > 65’) along the 8 = 30°, 90°, ..., 330’ directions. A simulation of the distribution based upon point emitters and spherical scatterers of Auger electrons is shown in Figure 1B. Parameters and equations used in the simulation are listed in Table I. As expected, the center region of the simulated distribution (4 < 65’) is essentially featureless since no atoms are situated between the monolayer and the detector along these trajectories. Features begin to appear near 4 = 75’ due to scattering by neighboring iodine atoms. Other minima in the distribution located closer to grazing emission (4 = 90’) correspond to scattering by iodine atoms other than the nearest neighbors in the monolayer. The gradual increase of intensity from the center (4 = 0’) to the edge (4 = 90’) of the simulateddistribution is based upon the empirical formulasgiven in Table I and is descriptiveof variations in signal related to the geometry of the experiment. Likewise, intensity maxima are observed in directions corresponding to the gaps between neighboring iodine atoms in the monolayer (4 = 75’, 8 = O’, 60°, ..., 300’). A more complete simulationwould includethe contribution due to the Pt substrate as attenuated by the iodine overlayer. This can be approximated by attenuating the intensities measured for the bare Pt substrate (Figure 2A) based upon path length through an iodinemonolayer [exp(0.2 sec 4)] and adding the resulting distribution to the background-subtracted monolayer simulation (Figure 1B). A simulationobtained in this fashion (Figure 1D) shows qualitative agreement with the background-subtracteddata shown in Figure 1C, although minor differencesare apparent. Differencesare to be expected, however, since the above simulation neglects backscatteringof iodine Auger electrons by the Pt substrate and assumes that Auger electrons emitted by the Pt are scattered homogeneouslyby the iodine overlayer. Investigationsemploying an atomic bilayer of Ag and iodine have sh0wn3~that Ag Auger
. . . Pt (111) Figure 7. (A) Auger electrons emitted from one domain of the Pt( 1 1 1)(d7Xd7)R19.1 O - I adlatticeof iodineatomsarescattered by neighboring iodine atoms, producing intensity minima near the horizon of the angular distribution. (B) When the intensity minima from both mirror-image domains are superimposed,a distinct pattern of “gaps” and “double hits” is produced. ,
electrons are inhomogeneously scattered by the iodine overlayer. However, in the present case, the conditions approximate homogeneity due to the myriad geometric relationships which exist between the monolayer and substrateatoms. Determination of site registries based upon angular distribution data is beyond the scope of this article and is the subject of an ongoing investigation.36 2. Pt(111)(d7Xd7)R19.1°-I. As observed for the 4 3 - 1 adlattice, the measured angular distribution of iodine Auger electrons emitted from a Pt( 111)(d7Xd7)R19.1°-I monolayer (Figure 5A) exhibits a bright, corrugated ring of intensity near grazing emission (4 = 70’) due to iodineand an hexagonal pattern of intensity maxima nearer the center of the distribution (4 < 60’) due to the Pt substrate. The background-subtracted distribution is shown in Figure 5C. Figure 6 shows models of the d 7 - I adlattice structure based upon findings from LEEDI4and STM.‘~Y*,~ The structural parameters are given in Table I. As in the case of the d 3 - I monolayer, the d 7 - I structure was investigated by LEED before and after ADAM measurements to verify the surface structure and stability. The data have been averaged employing the obvious 3-fold symmetry of the data and have been normalized so that the left edge of the scalecorresponds to zero Auger signal and the right edge to the maximum observed signal. Figure 7 aids in the visualization and comparison of the simulated (Figure 5B) and experimental (Figure 5A) angular distributions. Figure 7A illustrates the directions in which intensity minima are predicted due to scattering by neighboring iodine atoms in one domain of the 4 7 - 1 adlattice. Figure 7B superimposes both mirror-image domains of the adlayer, producing a more convoluted pattern of maxima and minima analogous to that observed experimentally. The featuresof Figure 5B accurately predict those of the experimental image (Figure 5A): bright channels along 8 = 30°, 90°, ..., 330’ directionsand “double-hit” minima along the 8 = Oo, 60°, ..., 300’ directions. STM images of the 4 7 - 1 monolayer confirm that the unit cell
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3835
Iodine Monolayer Structures at Pt( 111)
E80
D
I
0 1
-
90
- -I
J
Figure 8. (A) Measured angular distribution of 501-518-eV Auger electrons emitted from the (3x3) mixture of adlattices which forms on Pt(ll1). The distribution contains intensity minima where expected based upon the known structure of the adlattice and blocking by neighboring iodine atoms. (B) Digital simulation of the ADAM image for iodine emission from an equal mixture of the Pt( 1 1 1)(3X3)-I-hex and Pt( 1 1 1)(3X3)-I-asym adlattices (Figures 9 and 10). (C) Measured distribution following background subtraction. (D) Simulation including the contribution due to the Pt( 1 1 1) substrate.
contains three nonequivalent iodine atoms, each appearing to have a different height.6.819 The presence of three nonequivalent iodine atoms in the STM image could be due to "true topographic features of the structure", to "differences in tunneling probability resulting from iodine-substrate bonding effects",6 or to some combination of effects. However, the ADAM image (Figure 5A) exhibits features suggestive of noncoplanar iodine atoms. In particular, the observation of multiple minima along the 4 direction (4 = 66O, 5 8 O ; compare to Figure 5B) is as expected for emitters and scatterers located at different heights. Accordingly, the simulated ADAM image was computed for a structure in which the two iodine atoms situated in 3-fold hollows were lower than iodine atoms in atop sites (Figure 6 C ) . Best qualitative agreement was obtained between the features of the theoretical distribution and those present in the experimental data when iodine atoms in hcp and fcc 3-fold hollows were taken to be closer to the Pt surface by 0.6 and 0.8 A, respectively, compared to iodine atoms in 1-fold sites, reflecting the influence of second layer Pt atoms. These height differencesare reasonablefor 1-fold and 3-fold sites and are in good agreement with recent STM resultsgwhich placed the two iodine atoms in hcp and fcc 3-fold hollow sites closer to the surface by 0.7 1 f 0.23 and 0.95 f 0.20 A, respectively.
A simulation which includes the contribution due to emission from the Pt substrate as attenuated by an homogeneous iodine overlayeris shown in Figure 5D. Qualitative agreement is found between the experimental (Figure 5C) and calculated (Figure 5D)angular distributions. 3. Pt(111)(3X3)-1. The measured distribution of Auger electrons emitted from the Pt( 111)(3X3)-1 monolayer is shown in Figure 8A. The background-subtracted distribution is shown in Figure 8C. Seen at angles near grazing emission (4 > 70') are intensity minima due to scattering by neighboring iodine atoms in the monolayer. The center portion of the distribution (4 < 70') contains the hexagonal pattern of maxima due to the Pt substrate, as observed for the d 3 and d7 iodine adlattices. The data have been averaged employing the obvious 3-fold symmetry of the data and have been normalized so that the left edge of the scale corresponds to zero Auger signal and the right edge to the maximum observed signal. The (3X3)-I adlattice which forms on Pt( 111) consists of an essentially equal mixture of two distinct adlattices:8 Pt( 111)(3X3)-I-hex and Pt(ll1)(3X3)-I-asym. The adlattices are illustrated in Figures 9 and 10, and structural parameters are listed in Table I. Each lattice has the same number of iodine atoms per unit cell (four) and the same number of iodine atoms
Frank et al.
3836 The Journal of Physical Chemistry, Vol. 97. No. 15, 1993 A
Pt(111)(3 x 3)-I-hex
B
B I Pt
C
Pt
F1 / \
B
C
A / I
I o d 1 ne
B I o d i ne
Pt( I 1 I I P t ( 11 1I
Figure9. (A) ThePt( 1 1 1)(3X3)-I-hexadlatticeconsistsofacorrugated, hexagonal layer of iodine atoms. (B) Top view of a single unit cell. The hex unit cell contains four atoms, one in a top site ("A") and three in 2-fold bridging sites ("B"). (C) Side view of a single unit cell.
per Pt atom (0.44), but the site registries of the iodine atoms within the unit cells are different. The hexagonal (hex, Figure 9) lattice contains three iodine atoms in 2-fold bridging sites and one iodine atom in an 1-fold site. The asymmetric (asym, Figure 10) lattice contains one iodine in an fcc 3-fold hollow site and three atoms in asymmetric atop sites (intermediate between an atop site and a 3-fold hollow site). The existence of this asymmetricstructure was directly demonstratedrecently by STM data8+9 and was previously postulated based upon LEED results. LEED patterns for the monolayer-coated Pt( 111) surface, although clearly (3X3), contained significantly more diffuse intensity than that observed for the 4 3 and 4 7 iodine adlattices, suggesting the presence of a mixed ~tructure.~?~8 Although there are two distinct (3x3) adlattice structures present at the surface simultaneously,the rotations of the lattices and the nearest-neighbor directions for each structure are the same. Figure 11 illustrates where one expects intensity minima to occur in the angular distributions due to scattering by neighboring iodine atoms. The example shown is for the (3x3)I-hex structure, but it works equally well for the asymmetric structure by shifting (not rotating) the iodine adlattice by d 3 / 6 Pt-Pt bond lengthsalong the 8 = 90' direction (down). As shown, minima are expected near grazing emission (8 > 70') along the 8 =,'O 60°, ..., 300' directions. A simulation assuming an equal mixture of both structures is shown in Figure 8B. A simulation which includes the contribution from the Pt substrate is shown in Figure 8D, which agrees qualitatively with the backgroundsubtracted data shown in Figure 8C. Although the image for the (3x3) layer is not as sharp as that observed for the 4 3 and 4 7 iodine adlattices (presumably due to the presence of a mixture of two structures), the image clearly contains intensity minima along the nearest-neighbor directions. Accordingly, it is important to emphasize that the relative positions of atoms are revealed primarily by the locations of features, rather than by their absolute intensities.
Figure 10. (A) The Pt( 11 1)(3X3)-I-asym adlattice consists of a corrugated hexagonal layer of iodine atoms in the same rotational alignment as the hex structure (Figure 7) but is displaced vs the Pt( 1 1 1) substrate (B) Top view of a single unit cell. The unit cell contains four atoms, one in an fcc hollow site ("A") and three in asymmetric atop sites ("B"). (C) Side view of a single unit cell. : _;. . . ... :.,
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Figure 11. Augerelectronsemittedfrom both the(3X3)-Lhexand (3x3)I-asym monolayer structures are scattered by neighboring iodine atoms, producing intensity minima near the horizon of the angular distribution. Minima for the (3X3)-I-hex are illustrated; minima for the (3X3)-Iasym structure occur along the same directions, since the structures are rotationally equivalent (compare Figures 9A and 1OA).
Summary
ADAM data for three distinct iodine adlattice structures on Pt( 111) indicate that iodine atoms act primarily to block Auger electrons emitted from neighboring iodine atoms, producing intensity minima in the 507-5 18-eV iodine Auger electron emission angular distributions. The structures revealed by ADAM are in agreement with those determined by STM and contribute additional quantitative information. Therefore, ADAM is a potentially useful technique for probing monolayer atomic structure. Acknowledgment. This work was supported by the Air Force Office of Scientific Research. The instrumentation was funded by the National Science Foundation. The technical assistance of Arthur Case, Frank Douglas, Richard Shaw, and Vickie
Iodine Monolayer Structures at Pt( 111) Townsend and the generous support of Joan and George Rieveschl, Jr., are gratefully acknowledged.
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