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INFRARED SPECTRA OF ORDERED AND
DISORDERED OVERLAYERS ON METALS :
CARBON MONOXIDE ON A PLATINUM(111)
SINGLE CRYSTAL SURFACE
K. Horn, J. Pritchard
To cite this version:
JOURNAL DE PHYSIQUE Colloque C4, suppliment au no 10, Tome 38, octobre 1977, page C4-164
INFRARED SPECTRA OF ORDERED AND DISORDERED
OVERLAYERS ON METALS
:CARBON MONOXIDE ON A
PLATINUM(111) SINGLE CRYSTAL SURFACE
K. HORN (*) and J. PRITCHARD Chemistry Department, Queen Mary College,
Mile End Road, London E l 4NS, U.K.
R6sum6. - L'adsorption du monoxide de carbone sur la face (1 11) d'un monocristal de platine a 6t6 6tudiCe par spectroscopie infra-rouge et par mesure du potentiel de surface. Ce potentiel est positif et il augmente avec le degr6 de recouvrement jusqu'8 un maximum. Quand la surface se sature le potentiel tend vers 26x0. On a trouv6 une variation du potentiel de. surface avec la tempkrature de 0,36 V B 80 K jusqu'8 0,19 V B 500 K . Le potentiel dirninue parce que probablement le d6sordre thermique dans la couche augmente. Dans les spectres infra-rouges on trouve une seule bande 8 2 065-2 101 cm-', fonction du pourcentage de la surface occup6e. L'augmentation de cette bande avec le degrt de recouvrement est vraiment diff6rente B 80 K et 8 295 K. On pense que I'origine de ces differences se trouve dans la structure des couches des moltcules de CO.
Abstract.
-
The adsorption of CO on a Pt(ll1) single crystal surface has been studied by infrared reflection-absorption spectroscopy and surface potential measurements. The surface potential is positive, and after first increasing with coverage it passes through a maximum value and decreases to zero at saturation. The maximum value is temperature dependent, falling from 0.36 V at 80 K to 0.19 V at 500 K, an effect attributed to thermal disordering of the overlayer. Infrared spectra show a single, coverage-dependent band in the range 2 065-2 101 cm-'. The growth of the band at.80 K with increasing coverage differs from that at 295 K. These effects are discussed in relation to the overlayer structure.1. Introduction.
-
The infrared spectrum of ehemisorbed carbon monoxide has been recorded on several single crystal faces of copper [I-31. With increasing surface coverage structural changes in the adsorbed layer are observed by LEED but they are accompanied by rather small shifts of the infrared bands that occur in the CO stretching frequency range. In transmission infrared spectra of CO chemisorbed on supported metals much larger frequency shifts have been reported on metals such as platinum and palladium [4]. The extent to which such shifts arise from surface heterogeneity, interactions between adsorbed ' molecules via the metal, and dipole-dipole interactions, has not been established ; but results with '3CO-'2C0 mixtures [5] showed that dipole-dipole coupling was a major factor in the case of platinum and capable of accounting for the observed shift from 2 040 cm-' t o about 2 070 cm-'. The absence of comparable changes on copper, despite the similar infrared(*) Present address : Fritz-Haber-Institut der Max-Planck Gesellschaft, D-1 Berlin 33, Faradayweg 4-6, West Germany.
frequencies, suggested that a comparative study of CO chemisorption on a platinum single crystal surface would be valuable.
The properties of the Pt(ll1) surface in CO chemisorption have been investigated by several techniques. LEED observations of the CO overlayer structure have given contradictory results. Morgan and Somorjai [6] found a weak pattern corresponding to a C(4 x 2) structure, but this was not confirmed in a later study by Lang, Joyner and Somorjai [7]. Lambert and Comrie [8] could not detect an overlayer structure and suggested electron beam induced effects as a cause of the disagreement. All these investigations were carried out without cooling. Since the experimental work described in the present paper was completed, a new LEED study has been reported by Ertl, Neumann and Streit [9]. At reduced temperatures they observed a sequence of structures. These are in excellent agreement with those expected from the parallels between infrared spectra and surface potential results on Cu(ll1) and on Pt(ll1) that we discuss in this paper.
INFRARED SPECTRA O F ORDERED AND DISORDERED A combination of thermal desorption and
reflection infrared studies of CO on a recrystallised platinum ribbon of predominantly { 11 1 } orientation has been reported recently by Shigeishi and King [lo]. In their study a strong single infrared
band was detected in the range 1 500 to 2 100 cm-'
and at temperatures of 120, 200 and 300 K. At all
temperatures the band shifted smoothly to higher frequencies with increasing coverage, in accord with the behaviour originally found with supported platinum. The intensity of the band is entirely consistent with the inference from recent angle-resolved UPS studies [11] that the CO is
bonded via carbon in a vertical orientation. In the present paper we describe the results of reflection infrared spectroscopy and surface potential measurements on a Pt(ll1) single crystal,
the initial cleanness of which was established by Auger electron spectroscopy. Previous surface potential measurements on platinum have led to a variety of values, usually negative. Morgan and Somorjai [6] obtained a value of -0.17 V on Pt(1 1 1), while values of 0.00 V [12], - 0.24 V [13]
and - 0.88 V [14] have been reported with
evaporated platinum films. Ertl, Neumann and Streit [I)], however, obtained positive surface potentials, and our results confirm their findings. The unusual temperature dependence of surface potentials is accompanied by an important difference between the coverage dependence of the infrared band frequencies at low and at higher temperatures that we associate with ordering effects in the overlayer.
2. Experimental procedures. - In previous infrared studies, on copper, a very simple ultrahigh vacuum (UHV) system was used, and the progress
of surface cleaning was followed by measuring the xenon monolayer surf ace potential [ 151. Reliable
surface potential values for xenon on platinum are not available, and, in view of the reactivity of platinum and the difficulty often experienced in cleaning it satisfactorily (e.g. [ll]), a new system
was employed which included an Auger electron spectrometer of the cylindrical mirror type. The working chamber is shown diagrammatically in figure 1. In addition to the Auger spectrometer it
was equipped with a movable reference electrode for surface potential measurements by the vibrating capacitor method and with calcium fluoride windows for the infrared optical path. A 30 1 s-' ion
pump and a titanium sublimation pump evacuated the chamber, and in addition a trapped mercury diffusion pump was used to remove xenon after ion-bombardment. Following bake-out a base pressure of 2 x lo-' Pa was reached.
The Pt(ll1) crystal was elliptical in shape
(c. 17 x 9 mm) and oriented to within lo of (1 11). It
was mounted via two 0.5 mm platinum wires on two
to ion
and sublimation pump
I R window
FIG. 1. - T h e vacuum chamber (diagrammatic). ( a ) Horizontal section, ( b ) Vertical section.
tungsten pins fed through the lower end of a glass cold-finger. The crystal could be cooled to 80 K by
filling the cold-finger with liquid nitrogen. High temperatures were achieved either by resistive heating of the platinum support wires or by electron bombardment from a tungsten filament behind the crystal. Temperatures were measured by a chromel-alumel tliermocouple spot-welded to the side of the crystal
.
The maximum temperature attained was 1 400 K.C4- 166 K. HORN AND J. PRITCHARD sulphur and calcium. Cleaning was first attempted
by heating the crystal to 1 300 K in oxygen at about Pa. Although carbon was reduced, this treatment did not affect the amount of calcium and sulphur. These impurities could be removed by ion bombardment at 800 K, but prolonged treatment (20-30 hours) was necessary to remove calcium.
Infrared reflection-absorption spectra were measured in a single beam system based on a Grubb-Parsons M 2 monochromator. The optical path is represented diagrammatically in figure 2. A
H
detectorexit sll t
vi broting mlrror
UHV Section ~onochr&motor
FIG. 2.
-
Optical arrangement for reflection-absorption infrared spectroscopy with wavelength modulation.cooled indium antimonide detector (Mullard RPY 36) was used. This facilitates measurements of spectra in the derivative form [3, 161 by means of wavelength modulation, thereby reducing the noise level. Derivative spectra were stored and integrated in a Fabri-Tek 1062 signal-averaging computer but no averaging or smoothing was performed ; all spectra shown were recorded with one scan.
For the determination of isosteric heats of adsorption the method of Tracy and Palmberg [17] was employed. The surface potential was continuously recorded while, the crystal temperature was raised and lowered with a constant pressure of CO in the chamber. Adsorption isosteres were constructed from the resulting isobars.
3. Results.
-
3.1 SURFACE POTENTIALS. - The surface potential of CO adsorbed on the clean Pt(ll1) surface is shown in figure 3 as a function of exposure at several crystal temperatures. At each temperature the surface potential at first rises (work function decreases), passes through a maximum value and then decreases t o zero. On increasing the pressure at low temperature it rises again but only by about 0.02 V. Owing to the position of the ion gauge, exposures were not accurately determined, and the apparent values of-
0.35 L and 1.0 L to reach the maximum and minimum respectively are considerably too small [9]. Nevertheless, the form of the exposure scale should be the same throughout these experiments and it is therefore significant that the same exposures- are required at all four temperatures t o attain the maximum and the minimum. Variations in the maximum values at each temperature encountered in the course of manyExposure
FIG. 3.
-
Surface potential variation during exposure to CO(Pco-
3 x Pa) a t temperatures of 80 K (top), 130 K, 195 K and 295 K. A maximum value recorded under equilibrium conditions a t-
500 K is included for comparison. The exposure scale is not accurate because of the position of the ion gauge, butit is the same for the four curves.
adsorption experiments are indicated by the error limit bars.
The maximum surface potential depends strongly on the crystal temperature during adsorption. At room temperature it is 0.25V. At lower temperatures it increases and reaches about 0.36 V at 80 K. Above room temperature the variation is less, and at 500 K the maximum surface potential found in equilibrium adsorption experiments was 0.19 V. This strong dependence of surface potential on temperature is very unusual, and when first observed it was thought to be an artefact caused by adsorption on the reference electrode or contamination of the platinum surface. Neither a change in reference electrode material from stainless steel gauze to tungsten gauze, nor prolonged cleaning of the platinum, caused any change in these results, however. Before every experiment the surface cleanness was checked by Auger spectroscopy. Although Auger spectroscopy is important to determine the nature of contaminants, the surface potential was found, as in other studies [18, 191 to be a more sensitive indicator of the progress of the final stages of cleaning.
INFRARED SPECTRA O F ORDERED AND DISORDERED C4- 167 potential much easier to detect than on metals where
only a monotonic increase or decrease is found.
3 . 2 HEAT OF ADSORPTION. - Isosteric heats of adsorption have been determined in many single crystal adsorption studies by application of the Clausius-Clapeyron equation
and using constant surface potential as a criterion of constant coverage. Although the surface potential may not be a linear function of coverage, the determination of A H is not affected by this. The underlying assumption is that the surface potential at a given coverage is independent of the substrate temperature. It is evident that this is not valid for CO on Pt(l1 I ) , but over limited ranges of temperature,
particularly above room temperature, the effect should not be large. Isobars of surface potential
versus temperature were recorded as in figure 4 and then from isosten'c plots of lnp versus 1 / T at constant surface potential were obtained apparent isosteric heats as shown in figure 5 . The rather large
Temperature / K
FIG. 4. - Adsorption isobars for CO on Pt(ll1) at pressures (from left to right) of 2.7 x 8 x lo-', 2.7 x 8 x and
2.7 x 10-6Pa.
I I
50 100 150 Max 150 100 Surface potential I mV
FIG. 5. - Apparent isosteric heats of adsorption as function of surface potential.
error bars, typically * 12 kJ mol-', in this diagram allow for the range of surface potentials measured in many runs. The mean value is initially 163 kJ mol-' ;
it drops to 150 kJ before the surface potential
maximum and to 135 kJ mol-' thereafter. These apparent heats are in reasonable accord with those found in thermal desorption, e.g. initially
-
154falling to 135 k3 mol-' [ l o ] and 138 falling to 113 kJ mol-' [9]. The small change which appears
near the S.P. maximum is probably an artefact, however, and is not reflected in the thermal desorption experiments. It is clear that the temperature dependence of the surface potential would cause the true isosteric heat to be less than the apparent one at coverages before the surface potential maximum and greater than the apparent one after the maximum : The form of the S.P. curves in figure 3 with the S.P. returning to zero after the
same exposure at different temperatures, suggests that the coverage is a function of exposure only and may be used to identify the variation of S.P. with temperature at given coverages. It is then possible to make better estimates of the true isosteric heats, and apart from the initial fall the mean value is found to be the same (
-
147 kJ mol-') on each side of thesurface potential maximum in agreement with the conclusion from thermal desorption measurements.
3 . 3 INFRARED REFLECTION-ABSORPTION SPEC- TRA.
-
The infrared spectrum of CO adsorb- ed at 295 K is shown in figure 6 ( c ) . Initially a singleband appeared at 2 065 cm-' ; with increasing coverage a second band appeared at 2 082 cm-', and
this band increased while the first band decreased. As the 2082 cm-' band gained intensity it continuously shifted to higher frequencies, finally reaching 2 089 cm-'. The absorption was then 2.5 %. The crystal was then cooled to 80 K and figure 6 (b )
shows the further development of the band with additional adsorption to give a saturation value of
2 100 cm-' and an absorption of 3 %. The half-width of the bands is about 1 1 cm-'.
The development of the spectrum was significantly different when the whole adsorption was carried out at 80 K. In figure 6 ( a ) the main band is seen to appear at 2 090 cm-' and it grows to
an absorption of 2.5 % near the S.P. maximum with only a very small frequency shift. The major shift to
2 101 cm-' then takes place, and on saturation the
intensity falls slightly.
Spectra were also recorded with the crystal at
373 K. They were similar to those at 295 K but the
bands were slightly broadened. No other bands were detected in the region 1 980-2 060 cm-'. A search
below 1 980 cm-' was restricted by the strong
interference of the bands due to atmospheric water vapour in the single-beam spectrometer. No bands in the range 1 500-2 060 cm-' were found by Shigeishi and King [ l o ] , however, in their study of CO
adsorption on a { 11 1 ) oriented platinum ribbon.
4. Spectra and structure of the overlayer.
-
4 . 1 STRUCTURE.
-
The spectra of CO on P t ( l l 1 )K. HORN AND J. PRITCHARD
FIG. 6. - Infrared reflection-absorption spectra of CO on Pt(ll1). (c) CO adsorbed on clean crystal at 295 K. S.P. values (fromleft) of 0.06, 0.11, 0.16, 0.18, 0.20, 0.22 (max)andO.11 V. ( b ) additional adsorption when crystal cooled t o 80 K after adsorption a t 295 K. S.P. values (from left) 0.10,0.08,0.01 V. ( a ) C O adsorbed on clean crystal a t 80 K. S.P. values (from left)
0.10,0.15,0.20,0.24,0.32,0.05,0.0 V.
faces of copper [l,3], but a continuous shift in frequency with growth of intensity, as shown here for room temperature adsorption on platinum, has not been observed on a low index face of copper. The frequencies of the infrared bands on platinum are similar to those observed in previous reflection spectra. Low and McManus [20], using a platinum foil under ordinary vacuum conditions, detected a single band at about 2 090 cm-'. In a detailed study of adsorption on a platinum ribbon Shigeishi and King [lo] observed a band at 2 065 cm-' at low coverages that shifted continuously to 2 101 cm-' at room temperature and to 2 108 cm-' at 120 K. These large shifts are comparable to those found .with supported platinum, and, following very recent measurements with '3CO-'2C0 mixtures [21] they are convincingly explained by dipole-dipole coupling. The present results differ from them in that the initial band at 2 065 cm-' decreases in intensity at higher coverages and a new band at 2 082 cm-' emerges which shifts to 2 089 cm-' -the highest value at room temperature. Furthermore, cooling to 120 K caused little change in the result of
Shigeishi and King. The differences are quite significant and suggest that the nature of the surface of the recrystallised ribbon used by Shigeishi and King is, appreciably different from that of a (1 11) single crystal.
Following previous studies of CO adsorption on copper surfaces we were led [2] to associate infrared bands with two-dimensional surface structures rather than with individual CO groups. The present results are believed to support this view and we emphasize the parallels between CO chemisorption on copper and platinum. On Cu(ll1) at 77 K the surface potential passes through a maximum value of 0.45 V before falling steeply to about 0.09 V on saturation. The infrared bands [3] are similar in shape and frequency, except that the frequency shifts are in the opposite direction and less continuous. On Cu(ll1) the band first appears at 2 080 cm-', moves to 2 076 cm-' where it grows in intensity until the maximum surface potential is reached, and as the surface potential falls to the saturation value the 2 076 cm-' band is replaced by one at 2 070 cm-'.
The similarities led us to expect that similar LEED patterns should be observed at low temperatures on platinum although previous LEED investigations had shown no evidence for them at room temperature. The sequence of LEED patterns observed by Chesters [22,23] for CO on Cu(ll1) at low temperatures i s shown diagrammatically in figure 7. A sharp ( d 3 x
a)
R 30" net is observed at the maximum surface potential. Further adsorption gives the C(4 x 2) net and a final saturation net of hexagonal close-packing. Similar results have now been obtained by Ertl, Neumann and Streit [9] for CO on Pt(l1 I), and for the first two nets the possible structures indicated in figure7 are those proposed [9] for stages of adsorption on platinum. However, on Pt(l1l) the C(4 x 2) pattern is followed by further compression at low temperatures to aCOPPER (111) - CO
FIG. 7. - Diagrams of possible structures (top) and the actual L E E D patterns (bottom) of C O adsorbed on Cu(ll1). In the L E E D patterns heavy open circles denote substrate beams, small full circles denote first order overlayer beams and small open circles denote double diffraction beams. In the C(4 x 2) pattern
INFRARED SPECTRA OF ORDERED AND DISORDERED C4- 169 saturation coverage of E68. Another important
difference is that the ( d 3 x
a)
R 30" pattern on .platinum, even at the lowest temperature reached, 170 K, is diffuse. The diffuseness, and the temperature dependence of the surface potential maximum, are plausibly explained by thermal disordering of the overlayer. It is assumed that sites of three-fold rotational symmetry have the highest binding potential and would be exclusively occupied in the ideal(v?
x d ? ) R 30" structure at low temperature. If the binding potential of two-fold sites is only slightly less, however, some thermal distribution will exist between occupied three-fold and two-fold sites even at low coverages. The surface potential behaviour can be accounted for if the dipole of a molecule is positive when adsorbed on a three-fold site and zero on a two-fold site. As the coverage increases beyond l/3 interaction between the adsorbed molecules causes a progressive removal of molecules from three-fold sites and the exclusive occupation of two-fold sites in the C(4 x 2) structure. On the basis of the surface potential data presented here, Ertl, Neumann and Streit [9] conclude that the binding energy at a two-fold site is only 2kJ mol-' less than at a three-fold site.This model also fits well with our infrared results. The continuous shift of the band at room temperature can be associated with a continuously changing distribution between sites, as well as with increasing dipole-dipole coupling as the coverage increases. At 80 K the tendency for the 2 090 cm-' band to grow with very little frequency shift until the surface potential maximum is reached is consistent with a regular occupation of three fold sites. However, while this model can rationalise the main features of the surface potential, infrared, and thermal desorption results it does not explain the rather small frequency shifts associated with the considerable surface dipole change (from 0.2 Debye to zero 193. Furthermore there is good evidence from high resolution electron energy loss experiments [24] for two states of CO on the saturated Pt(ll1) surface, one with a vibration frequency as observed here and another lower frequency band at 1 870 cm-' (232 meV). The latter band is very broad (half-width 120 cm-' or 15 meV) and has not yet been detected in infrared reflection spectra [21] although a similarly broad band has been observed on supported platinum [4]. The two-fold sites in the C(4 x 2) structure of figure 7 are .not all crystallographically equivalent. The C(4 x 2) net requires a basis of two CO groups to give half coverage. Very recently, Crossley and King [21] have suggested that instead of both being on two-fold sites the basis molecules may be located with one on a three-fold site and one on a two-fold site, the latter accounting for the low frequency band. This modification of the model would require
a negative dipole contribution from the second molecule. Within the simple mQdel of carbon monoxide adsorption through simultaneous u-donor bond formation and dn- - p n * back donation, the frequency and negative dipole are mutually consistent with increased back donation, perhaps because of charge accumulation on surface atoms as a result of a larger a-component on three-fold sites. It is interesting to note that on silver, where the surface potential is positive and primarily weak a-bonding is expected, the infrared frequency is initially very high (2 155 cm-') and it changes continuousIy to lower values with increasing coverage 1151. In this case the heat of adsorption also falls with increasing coverage [25]. The negative dipole postulated for the low-frequency species on Pt(ll1) suggests that the sign of the nett surface potential may vary from one face to another, accounting for the negative and near zero values reported with evaporated platinum films. Again, comparison with copper suggests that the (1 11) face may be exceptional in the magnitude of the positive component. The location of the three-fold sites on the Pt(ll1) surface is far from certain. If directly above the metal atoms [21] the possible two-fold site is centrally situated between four terminal CO groups, and a symmetrical bridge-bonded species would be anticipated. Why then is the band in the energy loss spectrum so broad ? Broad bands can arise when bridge-bonding is unsymmetrical as in the compensating sets discussed by Cotton [26] which occur in polynuclear carbonyl species. Although the relative binding energy distribution over the surface unit mesh of Pt(ll1) has not been calculated accurately there is a general tendency for interstitial positions to give higher binding energies for CO than positions directly above metal atoms [27,8]. If the three-fold sites of the C(4 x 2) structure (and, of' course, the ( d 5 x ~) R 30" structure) are taken to be interstitial, the bridging sites are ineviably unsymmetrical and compensating sets of them may be formed between adjacent unit meshes
he of
the surface structure. The broad low frequency band may then be viewed as a consequence of varying bridge asymmetries dependent on co-operative interactions across the surface.C4- 170 K. HORN AND J. PRITCHARD dm -pm
*
back bonding, which strengthens themetal-C bond, weakens the CO bond and produces a frequency decrease. In addition the purely mechanical' effect of providing a force constant between the metal and the carbon atom results in an increase of the frequency of the normal vibration that is called the CO stretch. The mutual operation of such factors can conceivably give rise to frequency shifts that are positive, negative, or zero. However, a more probable reason for the growth of bands at fixed frequency, and particular where one such band is replaced by another, is a local environment that is practically independent of coverage until a coverage is reached at which a new kind of local environment is established. This implies the establishment of domains of local structure which grow in area with increasing coverage but otherwise change only gradually. On Pt(ll1) at 80 K the growth of the band at 2 090 crn-' probably reflects the nucleation and growth of domains of structure similar to that which exists in the
(4
x4)
R 30" structure which finally appears. The nucleation of such domains may be preceded by a random adsorption, thus the initial appearance of the 2 065 cm-' band probably reflects the existence of isolated CO molecules. Similarly on Cu(ll1) the initial band at 2 080 cm-' is quickly replaced by a band at 2 076 cm-' which grows without further shift until the(fi
x4)
R 30" structure is clearly established. Domains formed in this way cannot be regarded as true phases, or else the adsorption isotherm should correspond to two-dimensional condensation. I t is probable that they are defective, with short range order similar to that in the final LEED patterns but achieving full long-range order only when the surface is fully occupied. Thus on Cu(100) a band grows at 2079 cm-' until the maximum surface potential is almost reached, butwhen it is reached and the C(2 x 2) LEED pattern is clearly established, the frequency is 2 086 cm-' [I]. Such a shift is thought to accompany the establishment of long-range order. The existence of limited domains of structure is also suggested by the replacement of the band at 2 065 cm-' by that at 2 082 cm-' in the spectra of CO on Pt(ll1) at room temperature. At this temperature however; thermal disordering could well cause a continuous change in the local order as the coverage increases, resulting in the continuous shift to 2 089 cm-'. Similar effects were not observed by Shigeishi and King [lo], but
their spectra also extend to higher frequencies. It is possible that their ribbon surface is composed of stepped {I 1 1) facets. Thus with copper, the spectra of CO on the stepped (755) surface depend on coverage in a quite different way from those on Cu(ll1). As we have discussed previously [2], the steps are thought to hinder ordering in the (10
77)
direction perpendicular to the steps. Of course the step density on the (755) surface is high, but the marked effect of small departures from a low index orientation is shown in spectra of CO on Cu(100) [I].If this interpretation is correct infrared spectroscopy should prove to be a useful method of establishing the existence of short range ordering in adsorbed layers.
Acknowledgments. - We are grateful to Professor G. Ertl and Professor D. A. King for sending us preprints of their work. We are also very grateful to the British Petroleum Co Ltd for supplying the platinum crystal, to the British Gas Corporation, the Science Research Council, The Royal Society and London University Central Research .Fund for, equipment, and to the Deutscher Akademischer Austausch Dienst for a scholarship (K. H.).
References
111 HORN, K. and PRITCHARD, J., Surf. Sci. 55 (1976) 701. 1141 DROTSAERT, W. J. M., VAN REIJEN, L. L. and SACHTLER, [21 HORN, K., HUSSAIN, M. and PRITCHARD, J., Surf. Sci. to be W. H. M., J. Catal. 1 (1962) 416.
published. [I51 CHESTERS, M. A., PRITCHARD, J. and SIMS, M. L., in [31 PRITCHARD, J . , CATTERICK, T. and GUTPA, R. K., Surf. Sci. Adsorption-Desorption Phenomena (F. Ricca, ed.),
53 (1975) 1. Academic Press, London), 1972, pp. 277-290. 141 EISCHENS, R. P. and PLISKIN, W. A., Adv. Catal. 10 (1958) 1161 HORN, K. and PRITCHARD, J., Surf. Sci. 52 (1975) 437.
1. [I71 TRACY, J. C. and PALMBERG, P. W., Surf. Sci. 14 (1969) 274. [51 HAMMAKER, R. M., FRANCIS, S. A., and EISCHENS, R. P. [I81 CRISTMANN, K., ERTL, G. and PIGNET, T., Surf. Sci. 54
Spectrochim. Acta 21 (1965) 1295. (1976) 365.
[61 MORGAN, A. E. and SOMOWAI, G. A. J. Chem. Phys. 51 1191 MCELHINEY, G. and PRITCHARD, J., Surf. Sci. 60 (1976) 397. (1969) 3309. [20] Low, M. 3. D. and MCMANUS, J. C., Chem. Commun. [71 LANG, B., JOYNER, R. W. and SOMOWAI, G. A., Surf. Sci. (1967) 1166.
30 (1972) 454. I211 CROSSLEY, A. and KING, D. A., Surf. Sci., to be published. [81 LAMBERT, R. M. and COMRIE, C. M., Surf. Sci. 38 (1973) [22] CHESTERS, M. A., Thesis, University of London, 1971.
197. L231 PRITCHARD, J . , J. Vac. Sci. Technol. 9 (1972) 895.
191 ERTL, G., NEUMANN, M. and STREIT, K. M., Surf. Sci., to [24] FROITZHEIM, H., IBACH, H. and LEHWALD, S., in Photoemis-
be published. sion from Surfaces (ESTEC, Noordwijk, Holland),
1101 SHIGEISHI, R. A. and KING, D. A., Surf. Sci. 58 (1976) 379. 1976, p. 181-2. ,
[ I l l APAI, G., WEHNER, P. S., WILLIAMS, R. S., STOHR, J. and [251 MCELHINEY, G., PAPP, H. and PRITCHARD, J., Surf. Sci. 54 SHIRLEY, D. A., Phys. Rev. Lett. 37 (1976) 1497. (1976) 617.
1121 HEYNE, H. and TOMPKINS, F. C., Proc. R. Soc. London A [261 COITON, F. A., Prog. in Inorg. Chem. 21 (1976) 1. 292 (1966) 460. [271 DOYEN, G. and ERTL, G., Surf. Sci. 43 (1974) 197. [I31 DORGELO, G. H. J. and SACHTLER, M. W. H., Nature 46 1281 POLITZER, P. and KASTEN, S. D., Surf. Sci. 36 (1973) 186.
INFRARED SPECTRA OF ORDERED AND DISORDERED
DISCUSSION
J. J. FRIPIAT. - How does the absorption coefficient depend on coverage ?
J. PRITCHARD.
-
Using the surface potential as a measure of coverage during the first stage, the absorption appears to grow almost linearly with coverage, as shown for Co on Cu(100), but the precise relationship of surface potential to coverage is not established on many surfaces for which infrared spectra have been obtained.J. RUTCHARD.
-
1) Reflection infrared spectro- scopy is selectively sensitive to dipole oscillations, or components of dipole oscillations, perpendicular to the metal surface. It is not possible to observe directly the orientation of the total dipole.2) The frequencies of metal-CO vibrations are typically 400-500 cm-' compared to
-
2 000'cm-' for the C - 0 frequency.S. FAIN.
-
If you have island growth, why don't you obtain two-dimensional condensation in the isotherms ?J. G. DASH. - 1) Have you assumed in your J. RUTCHARD. - If the island growth model is analysis that the molecular axes have a definite correct, the absence of two-dimensional orientation with respect to the surface ? Can you condensation presumably implies that the islands measure the orientation ? have defective structures that vary with surface 2) How do the internal frequencies of vibration of coverage and island size. The isotherm would be the molecule compare with the molecule-surface much more sensitive to such variations than would
