Measurements of the spatial distribution of H2 desorbed from Ni surfaces: Effects of surface composition and crystal orientation

SURFACE

SCIENCE 29 (1972) 590-602 0 North-Holland

MEASUREMENTS OF H, DESORBED OF SURFACE

OF THE SPATIAL FROM

COMPOSITION

T. L. BRADLEY*,

Publishing Co.

DISTRIBUTION

Ni SURFACES: AND

CRYSTAL

A. E. DABIRI**

EFFECTS ORIENTATION+

and R. E. STICKNEY

Department of Mechanical Engineering, Massachusetts Institute Technology, Cambridge, Massachusetts 02139, U.S.A.

Received 21 September 1971; revised manuscript

of

received 16 November

1971

The effects of surface composition and crystal orientation on the spatial distribution of He molecules desorbed from Ni surfaces have been examined. In this continuation of the work of Dabiri et al., we have obtained data for both polycrystalline and singlecrystal [(111) and (1 lo)] Ni surfaces that are contaminated to varying degrees with impurities. The surface composition was detected by Auger electron spectroscopy and varied by ion bombardment and by controlled deposition of S, C, Si and Au. The spatial distribution of the desorbed molecules was measured with a rotatable ionization gauge. The measured spatial distributions may be described approximately by the form CO+ 0, where 0 is the angle of inspection measured from the surface normal. Prior to cleaning the Ni surfaces by ion bombardment, the distributions are essentially the same (i.e., d = 4) for the polycrystalline and single-crystal samples and for sample temperatures ranging from 925 to 1225°K. In this case, however, the Auger spectra indicate that the surfaces are unintentionally contaminated with S and, to a lesser degree, with C. After the surfaces are cleaned by ion bombardment, the distributions are essentially diffuse (i.e., d= 1). Diffuse distributions are also observed when pure layers of S, C, or Si are deposited on the Ni surfaces. These results indicate that the spatial distribution of HZ desorbed from Ni is independent of crystal orientation and temperature, but strongly dependent upon surface composition. Non-diffuse distributions (i.e., d# 1) were observed only for impure surfaces (i.e., surfaces composed of more than one element). Based on these results, we suggest that the non-diffuse distributions reported by Van Willigen, Palmer et al., and Dabiri et al. correspond to impure surfaces.

1. Introduction In a previous experimental investigation of the spatial distribution and D, molecules desorbed from a polycrystalline nickel membrane,

of H, Dabiri

t Research supported in part by the Joint Services Electronics Program [Contract DA28043-AMC-02536(E)], Advanced Research Projects Agency [Contract DAHCl5-67-C02221, and the Petroleum Research Fund. * Bell Telephone Laboratories Fellow, Present address: Bell Telephone Laboratories, Naperville, Illinois 60540, U.S.A. ** Present address: Arya-Mehr University of Technology, Tehran, Iran. 590

H2

SPATIALDISTRIBUTIONOF

DESORBED

FROM

Ni

SURFACES

591

et al. 1) observed that the distributions deviated markedly from the expected form of cos 13,where 8 is the angle of inspection measured from the surface normal. The spatial distributions were described approximately by the form cosd 0, where d was found to be 4.5kO.5 over the available temperature range of the measurements, 800 to 1300°K. Two of the possible explanations suggested by Dabiri et al. to account for the non-diffuse (i.e., d# 1) spatial distributions are: (1) the breadth of the spatial distribution may depend upon the crystallographic orientation of the surface; and (2) surface impurities may serve either as “promoters” or “poisons” of the desorption step that influences the spatial distribution. The objective of the present study is to examine these possible explanations in detail. To determine the influence of crystal orientation, we have obtained comparable data for three Ni samples having different crystallographic structures: polycrystalline, and single crystals with (111) and (110) orientations. Since the (111) orientation is the most densely packed plane for Ni, it provides the “smoothest” surface from the viewpoint of the ideal atomic structure. On the other hand, the ideal atomic structure of the (110) surface is composed of “hills” and “valleys”, and we expected that the spatial distribution may depend on the azimuthal orientation of the surface lattice relative to the plane of the detector. To examine the influence of surface impurities on the spatial distribution, we have employed Auger electron spectroscopy2-7) to detect surface impurities, while ion bombardment was used to remove impurities and vapor deposition techniques to introduce impurities. Since sulfur and carbon were the impurities that appeared naturally (unintentionally) on all of our Ni samples, we varied their concentrations by bombardment and deposition to determine the dependence of the H, spatial distributions on the concentrations of S and C. Silicon and gold were deposited on a Ni surface in an experimental test of a possible model described in section 4. The experimental data that we have obtained for H, desorbed from Ni samples having different crystal orientations and surface compositions are compared with the data reported by van Willigens), Palmer et al.s), and Dabiri et a1.l). The strong dependence of the spatial distribution on surface composition has been considered from several viewpoints, but we conclude that our present understanding of the desorption process is insufficient to enable us to establish a reliable model at this time. 2. Experimental

apparatus and procedure

2.1. GENERAL SYSTEM DESCRIPTION The principal

components

of the apparatus

are shown in fig. 1. One side

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T. L.BRADLEY,

A. E. DABIRI

AND R. E. STICKNEY

of the Ni membrane is exposed to H, (or DJ at approximately atmospheric pressure, while the other side is exposed to vacuum. Hydrogen atoms diffuse through the membrane and recombine on the surface to form molecules which desorb into the evacuated chamber. The spatial distribution of the

Fig. 1.

Experimental

apparatus for measuring the spatial distribution desorbed from a solid surface.

of molecules

desorbed molecules is measured by a conventional ionization gauge mounted on a rotatable shaft whose center line is tangential to the membrane surface at its center. (See ref. 1 for further details.) The polycrystalline and single-crystal Ni membranes are 1.27 cm diameter discs, -0.75 mm thick. Each disc is welded at the end of a polycrystalline Ni cylinder (1.27 cm O.D. and 0.317 cm I.D.) which is welded to a stainless steel tube brazed to a copper tube. As indicated in fig. I, the membrane assembly is mounted on a rotary feedthrough, and it is connected by a bellows to a tube that passes through the chamber wall to a valve and on to a flask of research grade H,. The interior volume of the assembly is evacuated before filling it with H, to a pressure of - 1 atm. The membrane is heated by radiation from a hot tungsten filament that is positioned around the Ni cylinder. The temperature is measured with a chromel-alumel thermocouple (0.254 mm diameter wire) spot welded to the Ni cylinder. As indicated in fig. I, the membrane is mounted in a manner that enables

SPATIALDISTRIBUTIONOF

HZ DESORBED

FROM

Ni

SURFACES

593

us to rotate it to various working stations, including an Auger electron spectrometer, a rotatable ionization gauge for measuring the spatial distribution, an ion bombardment gun, and a station for deposition of either carbon, sulfur, silicon, or gold. The background gas in the chamber was analyzed by a partial pressure analyser. The vacuum chamber is a stainless steel cylinder, 46 cm in diameter, rough pumped by sorption pumps and evacuated to ultra-high vacuum by a 500 liter/set ion pump. The entire system is bakable and pressures of the order of 5 x lo-” Torr could be attained; however, we generally started an experimental run when the pressure reached - lo-’ Torr. 2.2. MEMBRANEPREPARATION The polycrystalline membrane, - 0.75 mm thick, was cut from a 1.27 cm diameter Ni rod (Grade A, International Nickel Co.). The Ni(ll0) and Ni(ll1) single crystal discs, - 0.75 mm thick, were spark cut from a 1.27 cm diameter crystal. The surfaces of the discs were polished mechanically, finishing with 0.05 urn alumina. The worked surface layer of the discs was removed by chemically etching for - 1 min in a solution of 100 ml nitric acid (70x), 33 ml sulfuric acid (97x), 33 ml orthophosphoric acid (88x), and 100 ml glacial acetic acid (99.7%). The single crystal surfaces were then examined under an optical microscope to ascertain that the surfaces were free of pits and imperfections. At the conclusion of the surface preparation the single crystal surfaces were examined by Laue X-ray diffraction to verify their crystallographic orientation. We found their orientations were off by no more than - 1 degree. 2.3. DETECTION OF SURFACECOMPOSITION Auger electron spectroscopy a-7) was employed to determine, qualitatively, the chemical composition of the membrane surfaces. The apparatus is a standard LEED-Auger unit described by various investigatorss-6). The principle components are a four-grid LEED optics (Varian), an electron gun (Superior Electronics SEjSU) positioned for grazing incidence (fig. l), and the necessary auxiliary electronics to generate a second derivative plot of the collector current as described in the literature3-‘). The electron gun was typically operated to provide a beam current of - 150 uA at a beam energy of 2.7 keV. The Auger electrons were sampled in the energy range 0 to 1000 eV. The Auger spectrum generally was recorded immediately after completing a measurement of the spatial distribution. This could be done quickly because it was a simple matter to rotate the membrane from one station to another. We found that it was essential to perform the Auger measurements at the same temperature at which the spatial distribution was measured

594

T. L. BRADLEY,

A. E. DABlRl

AND

R. E.STlCKNEY

because, in a few cases, the surface composition was found to be a strong function of temperature (e.g., see refs. 10, 11, and 14). The constancy of temperature also insured that hydrogen was permeating through the membrane at the same rate during the Auger analysis as during the distribution measurements. The surface composition was found to be essentially constant during a given measurement, as demonstrated in several instances by recording the Auger spectrum both before and after the spatial distribution measurement. 2.4. SURFACECLEANING

TECHNIQUES

The problem of obtaining clean Ni surfaces has been considered by many investigators l”p13), and a variety of techniques have been examined. Various combinations of these techniques were tried, and we found that the most successful procedure was argon ion bombardment of the membrane heated to the temperature required for measurement of a spatial distribution (T>900”K). The argon pressure was -5 x 10m5 Torr and the ion bombardment gun was adjusted to provide - 15 PA/cm2 of 250 eV argon ions to the membrane surface located -8 cm from the gun. 2.5. DEPOSITION TECHNIQUES We initially tried to deposit carbon and sulfur by heating the nickel membrane to - 1100°K in gaseous acetylene and hydrogen sulfide at 5 x lo-5 Torr pressure. Since this technique did not lead to a substantial increase in the surface concentrations of these impurities, we placed a hot tungsten ribbon (-2400’K) directly in front of the membrane (fig. 1) to serve as a catalyst for decomposing acetylene and hydrogen sulfide. Deposition was performed with the membrane at temperatures greater than 800°K to insure uniform coverage. Silicon was deposited upon the membrane (Tk 800°K) by the sublimation technique described by Changl5). Since Si and Ni readily form nickel silicidesls) at the temperatures required for distribution measurements, we found that it was impossible to obtain a pure silicon surface layer unless a layer of carbon was deposited on the nickel membrane before deposition of Si. The Auger spectra for surfaces formed by this procedure exhibited no peaks other than those corresponding to Si. Gold was deposited upon the membrane (Tz 800°K) by evaporation from a tungsten boat. Since a Au-Ni alloy”) forms at the temperatures of the spatial distribution measurements, we were unable to obtain a pure Au surface even when a thick layer was deposited. Typically, we obtained a surface composition for which the Auger peak of Ni at 860 V was roughly 3 the amplitude of the Au peak at 255 V.

Ha

SPATIALDISTRIBUTIONOF

DESORBEDFROM

Mi

595

SURFACES

3. Experimental results 3.1. SURFACE COMPOSITION After outgassing a nickel membrane at llOO”K, sulfur was the only impurity observed by Auger spectroscopy when the membrane was at temperatures greater than 900°K. (See fig. 2.) The sulfur Auger peak remained essentially constant over the temperature range of our experiments (925-1225°K) until argon ion bombardment was utilized. After ion bom-

NICKEL Ill01 T=950°K

ix 0.21

S

I

ii>

IO0

! I50

I I(

100

I zoo

300

I 400

I 500 600 ENERGY kV1

I 700

I

I

8co

900

I

1000

Fig. 2. Auger spectrum for a Ni(ll0) surface at 950°K after degassing at 1100°K but before cleaning by ion bombardment. Nearly identical spectra were obtained for Ni(l10) over the temperature range 925-1225 “K and for Ni(l11) and Ni(poly) samples. (Energy and current of the primary electron beam: 2700 eV and 150 IA.)

bardment the sulfur peak was reduced, but a carbon peak appeared. Extended ion bombardment at high temperature (T> 900°K) eventually reduced the sulfur peak to - 5% of its initial value, and carbon decreased apparently by diffusion into the bulk r1~14). When the temperature was lowered after ion bombardment, the carbon peak reappeared while the sulfur peak remained relatively constant. To estimate the concentration of sulfur on the nickel surface when the carbon concentration was negligible, we adopted the calibration relation

T. L. BRADLEY,

596

A. E. DABlRI

AND R. E. STICKNEY

established by Perdereau17). Specifically, Perdereau has shown that there is a linear relationship between the concentration of sulfur on a nickel surface and Is/INi, the ratio of the Auger peak for S at 150 V to the Auger peak for Ni at 62 V. Perdereau found that this linear relationship was valid up to the “saturation limit” obtained by heating the Ni crystal in hydrogen sulfide. According to Perdereau and Oudarls), the composition corresponding to the “saturation limit” consists of equal amounts of sulfur and nickel existing as a two-dimensional nickel sulfide (NiS) surface layer. This saturation condition corresponds to the surface composition that we observe (fig. 2) after degassing the membrane in vacuum at N 1100°K. Since our Auger data for this surface composition indicates that Is/INi ~0.28, we shall assume on the basis of Perdereau’s results that OS, the fractional coverage of S on Ni, may be described approximately by the relation 0, =

0.5 0.28

IJINi = 1.791,/lNi

(1)

for the range 0<0,<0.5. Since our deposition technique enables us to attain S concentrations that are above the “saturation limit” reported by Perdereau (i.e., 8,>.5), we initially assumed that eq. (1) may be extended to higher coverages. However, this introduces the problem that 0, becomes infinite when we deposit sufficient S to form a pure sulfur surface where Is/lNi+co. To constrain Bs to finite values for convenience of plotting the data (e.g., fig. 5), we shall assume instead that eq. (1) applies only for Gs~0.5, whereas the following relation is adopted as the definition of 0s in the range 0,>0.5: us = 1 - (7.151,/1,,)-‘. The motivation at Zs/lNi=0.28

behind this choice is that it yields the same value as eq. (1) (i.e., at fI,=O.5), and it approaches unity in the limit of a

pure sulfur surface, Is/lNi-+co. We should emphasize, however, the choice of eq. (2) was based on convenience rather than on rigorous logic. Since it is known that species existing within the first few atomic layers below the surface contribute to the Auger spectrad), we suspect that eq. (2) underestimates the concentration of S on the surface because of the contribution of sub-surface Ni atoms to the magnitude of INi. 3.2. RESULTSOBTAINED

PRIOR TO CLEANING

BY ION BOMBARDMENT

Shown in fig. 3 is the spatial distribution of H, desorbed from the sulfurcontaminated Ni surface (B,=O.5) corresponding to the Auger spectrum shown in fig. 2. This surface composition was observed on every nickel membrane (polycrystalline and single crystal) after initial degassing at _ 1 lOO”K, and we believe that it occurs as a result of surface segregation

SPATIAL

DISTRIBUTION

0

0

”

OF

20

Ha

”

DESORBED

40 e (DEG)

’

FROM

‘\\

SURFACES

597

\

0

"P-Q

60

Ni

80

Fig. 3. Spatial distribution of Hz desorbed from a sulfur-contaminated Ni(ll0) surface at 950°K. The surface composition corresponds to the Auger spectrum presented in fig. 2. Essentially identical distributions were obtained throughout the temperature range 9251225°K for Ni(llO), Ni(lll), and Ni(poly) samples contaminated to the same degree (i.e., N + monolayer of S).

of trace amounts of sulfur impurities present initially in the bulk14). Notice that the experimental data for the spatial distribution deviates substantially from the cos fI relation, falling instead near the curve corresponding to cos4 8. (Since the distributions were almost perfectly symmetric in all cases, we have omitted the data for negative values of f3). The spatial distribution for this surface was independent of temperature. Essentially identical results were obtained with the Ni (111) and Ni (poly) membranes. In the case of the Ni (1 IO) crystal, spatial distributions were measured for three azimuthal orientations (see section l), and the results were identical within the present limits of detection. 3.3. RESULTS OBTAINED FOR VARYING SURFACE COMPOSITION Shown in fig. 4 is the spatial distribution for a Ni(ll0) surface after an extended period of ion bombardment at high temperature. The sulfur peak in the Auger spectrum for this surface was - 5% of the peak observed prior to ion bombardment. Notice that the spatial distribution has broadened to the extent that it is nearly diffuse (i.e., nearly equal to cos 0). The same trend was observed for the Ni(poly) and Ni(ll1) samples. Also included in fig. 4 is the spatial distribution measured after depositing a layer of carbon on the surface. The Auger analysis of this surface showed that enough carbon had been deposited to eliminate detection of the nickel substrate. Notice that the data fall almost directly on the curve cos 0. This

598

T. L. BRADLEY,

A. E. DABIRI

AND R. E. STICKNEY

experiment was also performed substituting sulfur for carbon. Although the surface was primarily sulfur, a small Ni peak was always observed regardless of the duration of the deposition process. Again, the spatial distribution approached cos 0. Fig. 5 graphically illustrates the dependence of the spatial distribution on the coverage (concentration) of sulfur on the Ni surface. (The procedure for determining the fractional coverage, 8,, from the Auger spectra was described in section 3.1.) Notice that the spatial distribution, described by cosd 0, varies from dz I in the limits of pure nickel or pure sulfur, to dz4 for a surface corresponding to -f monolayer of S. The carbon concentra-

0

20

40

60

80

BKIEG)

Fig. 4. Spatial 1100°K) and

distributions from a pure

d

‘0 B,,

of Hz desorbed from a nearly carbon surface layer deposited

0.2 FRACTIONAL

0.4

0.6

COVERAGE

clean Ni(ll0) surface (T= on Ni(ll0) (T= 975’K).

0.8 OF

S

1.0 ON

NI

Fig. 5. Illustration of the effect of sulfur concentration (coverage) on the spatial distribution of Ha desorbed from Ni. Note: d is the characteristic parameter of the spatial distribution defined by approximating the measured distributions by the form CO@ 8.

SPATIALDISTRIBUTIONOF

HZ DESORBEDFROM

Ni

SURFACES

599

tion was sufficiently low in these cases that the Auger peak corresponding to C was below the limit of detection. Although the spatial distribution was also a function of carbon concentration, a plot similar to fig. 5 could not be constructed for carbon because sulfur was generally present. However, in one instance a nickel surface characterized by an Auger spectrum exhibiting carbon and nickel peaks but no sulfur peak produced a spatial distribution of the form cos3 8. The spatial distribution measured after depositing a layer of Si on a carbon-covered Ni surface (see section 2.5) was approximately diffuse (i.e., d% 1). In this case, only silicon was observed in the Auger spectrum of the surface. In those cases where the Auger spectrum showed that nickel had diffused through the silicon to form a surface composed of both Si and Ni, the distribution corresponded to dx2. Similarly, the spatial distribution for a Au-Ni surface corresponded to dz.2.

4. Examination

of a possible model

Perdereau and Oudar have studied the S-Ni system in detail by employing a variety of techniques r7,1s). They report that when Ni is exposed to gaseous H,S, the surface concentration of sulfur attains a limiting value which they refer to as “saturation”. At saturation, they find that Is/&, the Auger signal ratio defined in section 3.1, has approximately the same value (Is/lNi =O.31+ 0.03) for all of the crystallographic orientations that we have studied [i.e., Ni(lll), Ni(llO), and Ni(poly)]. Based on their LEED results, they suggest that the structure of the saturated S-Ni surface corresponds to that of a two-dimensional nickel sulfide (NiS) layer. Our results show that approximately the same Auger signal ratio, Is/lNi z 0.28, is observed for the Ni( Ill), Ni( 1 lo), and Ni(poly) samples after heating them to - 1100°K but prior to cleaning by ion bombardment. Since this ratio is approximately equal to that reported by Perdereaul7) for the saturated S-Ni surface, it is reasonable to assume that our unbombarded surfaces have the NiS structure observed by Perdereau and Oudarrs). If this NiS structure is essentially the same on the three different Ni samples that we have studied, then we have a possible explanation of the fact that we observed essentially the same spatial distribution (i.e., dz4) for H, desorbed from all three samples prior to cleaning by bombardment. The next task is to explain why desorption of H, from the NiS surface is non-diffuse while desorption from pure Ni and pure S surfaces is diffuse. One possibility is that the adsorption of H atoms is more highly localized on NiS than on either pure Ni or pure S. Since these localized binding sites would be separated by a distance that is greater than the equilibrium distance between

600

T. L. BRADLEY,

A. E. DABIRI

AND R. E.STICKNEY

H atoms in a H, molecule, there is some reason to suspect that dissociation will be the limiting step in the adsorption processta). Therefore, by assuming that detailed balancing may be applied, we suggest that the recombination of adsorbed atoms will be the limiting step in the desorption process. That is, the proposed “model” suggests that the localized nature of H adsorption on NiS leads to a difficult (e.g., highly activated) recombination step that is responsible for the observed non-diffuse spatial distributions. (At present we shall not attempt to propose a mechanistic description of how the details of the recombination step lead to the observed spatial distributions.) To test the validity of this model, we attempted to measure the spatial distributions of H, desorbed from pure layers of Si and Au deposited on the Ni samples. Si was selected because it is believedso) that localized adsorption may exist for H on Si; Au was chosen because there is some evidence that the dissociative adsorption of H, on Au is a highly activated stepzl). Unfortunately, the results obtained for Au were inconclusive because, as described in section 2.5, we were unable to produce a pure surface layer. Results were obtained for a pure Si layer, but they do not support the model because the measured distribution was approximately diffuse. At this time we do not have sufficient information to determine whether the model is invalid or whether the results for Si do not constitute a rigorous test. (For example, it may be that the deposited Si surface is so rough and/or imperfect that the measured distribution is more diffuse than that corresponding to an ideal Si surface.)

5. Summary and conclusions The present results indicate that, for constant surface composition, the spatial distribution of hydrogen molecules desorbed from Ni is independent of the temperature and crystallographic orientation of the sample over the limited ranges of variations considered. (Although the distribution was observed to vary with temperature in a few cases where the surface composition depended on temperature, we believe that it is more accurate to interpret these cases as being dependent on surface composition rather than on temperature.) On the other hand, the results show that the spatial distribution depends strongly on the chemical composition of the Ni surfaces. The narrowest distribution (dz4) is observed when the surface is contaminated of sulfur, and this state is attained simply by heating by -3 monolayer becomes broader when the sulfur the sample to - 1100°K. The distribution concentration is either decreased (by ion bombardment) or increased (by vapor deposition), and it is essentially diffuse (dz I) in the limiting cases of a clean Ni surface and a pure sulfur surface layer deposited on the Ni

SPATIAL

substrate.

DISTRIBUTION

Diffuse distributions

OF

Ha

DESORBED

FROM

also are observed

Ni

SURFACES

601

when pure layers of carbon

and silicon are deposited on the Ni substrate, whereas the distributions are non-diffuse when the substrate is only partially covered by C, Si or Au. These results lead us to suggest the general conclusion that non-diffuse spatial distributions occur only for impure surfaces, i.e., surfaces composed of more than a single chemical element *. We suspect that the non-diffuse distributions arise from the existence of special adsorption sites (e.g., localized binding) for hydrogen on the impure surfaces, but our present knowledge is insufficient to enable us to propose a detailed model. Comparable data on the spatial distributions of H,, D,, and HD molecules desorbed from nickel surfaces have been reported by Van Willigens), Palmer et al.Q). and Dabiri et a1.t). Initially Van Willigen reported a spatial distribution of cos9 0 for H, desorbed from polycrystalline Ni, but recently he has repeated these experiments and observed a distribution of cos4 022). Palmer et a1.Q) measured the desorption of HD from the (111) face of nickel and observed a distribution of the form cos4 0 for their most perfect crystals; for less perfect crystals the form was cosd 0, with 2.5
et al., carbon and/or silicon impurities of the diffusion pump fluid.

may

References 1) 2) 3) 4) 5) 6) 7) 8)

A. E. Dabiri, T. J. Lee and R. E. Stickney, Surface Sci. 26 (1971) 522. L. A. Harris, J. Appl. Phys. 39 (1968) 1419. R. E. Weber and W. T. Peria, J. Appl. Phys. 38 (1967) 4355. P. W. Palmberg and T. N. Rhodin, J. Appl. Phys. 39 (1968) 2425. P. W. Palmberg, Appl. Phys. Letters 13 (1968) 183. N. J. Taylor, J. Vacuum Sci. Technol. 6 (1969) 241. C. C. Chang, Surface Sci. 25 (1971) 53. W. van Willigen, Phys. Letters 28 A (1968) 80.

* Note added in proof. Although the results that we have obtained subsequently for polycrystalline Fe, Pt, and Nb membranes appear to be consistent with this “general conclusion”, our recent results for Cu seem to represent an exception. Specifically, the spatial distribution of Hz desorbed from a polycrystalline Cu membrane was observed to be highly non-diffuse (d m 4) when the surface was clean (pure) to within the limits of detection of our Auger electron spectrometer.

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T. L. BRADLEY,

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AND

R. E.STlCKNEY

9) R. L. Palmer, J. N. Smith, Jr., H. Saltsburg and D. R. O’Keefe, J. Chem Phys. 53.

10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)

(1970) 1666. E. N. Sickafus, Surface Sci. 19 (1970) 181. J. P. Coad and J. C. Riviere, Surface Sci. 25 (1971) 609. L. H. Germer and A. U. MacRae, J. Chem. Phys. 37 (1962) 1382. H. E. Farnsworth, R. E. Schlier, T. H. George and R. M. Burger, J. Appl. 29 (1958) 1150. J. M. Blakely, J. S. Kim and H. C. Potter, J. Appl. Phys. 41 (1970) 2693. C. C. Chang, J. Vacuum Sci. Technol. 8 (1971) 500. M. Hansen, Constitution of Binary Alloys (McGraw Hill, New York, 1958) p. M. Perdereau, Surface Sci. 24 (1971) 239. M. Perdereau, and J. Oudar, Surface Sci. 20 (1970) 80. G. Ehrlich, J. Chem. Phys. 31 (1959) 1111. G. Ehrlich, in: Proc. Third Intern. Congr. on Catalysis, Amsterdam, July 20-25, (North-Holland, Amsterdam, 1965). D. 0. Hayward and B. M. W. Trapnell, Chemisorption (Butterworths, London, p. 75. W. van Willigen, private communication (1971).

Phys.

1039.

1964 1964)