High resolution vibrational spectroscopy of CO on Ru(001): The importance of lateral interactions

Surface Science 0 North-Holland

93 (1980) 431-452 Publishing Company

HIGH RESOLUTION VIBRATIONAL SPECTROSCOPY THE IMPORTANCE OF LATERAL INTERACTIONS

OF CO ON Ru(OO1):

H. PFNiiR and D. MENZEL Institut fiir FestkBrperphysik

der TU Miinchen,

D-8046

Garching,

W. Germany

and

F.M. HOFFMANN, A. ORTEGA and A.M. BRADSHAW Fritz-Haber-Institut W. Germany

der Max-Planck-Gesellschaft,

Received

1979; accepted

21 August

Faradayweg

for publication

4-6

28 November

D-l 000 Berlin 33,

1979

The adsorption of CO on Ru(001) has been investigated in the temperature range 80-400 K with IR reflection-absorption spectroscopy and the results correlated with LEED and thermal desorption measurements. The C-O frequency shifts continuously from 1984 cm-l to 2061 cm-’ as a function of increasing coverage, which is attributed mainly to dipole-dipole coupling. No new bands were discovered in the spectrum. The frequency versus coverage relation is also clearly affected by the ordering of the adlayer into the J3 X J3 R 30” and 2J3 X 2J3 R 30” structures. Likewise the shape and half-width of the absorption band depend on the details of the ordering process. A linear relationship between coverage and integrated absorption intensity exists only below 0 = 0.33; thereafter the absorption intensity falls, with the result that at saturation coverage the absorption per adsorbed molecule is only 3540% of the absorption at B = 0.33. This effect is,also ascribed to strong lateral interactions in the adlayer. The intrinsic high resolution of the IR method is necessary for the careful study of these phenomena associated with position, shape and intensity of the absorption band.

1. Introduction Vibrational

spectroscopy

has

developed

into

one

of

the

most

powerful

techn-

iques for the characterisation of the adsorbed state. Of the available approaches IR reflection-absorption spectroscopy offers the possibility of measuring at high resolution over wide pressure ranges. The technique enjoys, however, the particular advantage that it does not influence or disturb the adsorbed layer. In low energy electron diffraction (LEED) investigations this is not the case. It is known for example that, in the case of CO adsorption on single crystal surfaces of the transition metals, electron-induced desorption and dissociation at electron energies above 30 eV often occurs [l-3], a fact which makes the characterisation of the ordered layer with LEED sometimes extremely difficult. On the other hand it is not 431

432

H. Sfniir et al. f Vibrational spectroscopy

of CO on Ru(001)

possible to do without the surface crystallographic information provided by LEED. The method allows us not only to check whether site-specific assignments based on the position of the C-O stretch frequency are indeed compatible with the geometry of the ordered overlayer [4] but also to determine in which coverage ranges ordering processes take place. As we shall see below, the latter have a profound influence on the details of the vibrational spectrum. Our present ideas on the assignment of C-O stretch frequencies are based on the analogy with the metal carbonyls, whereby “terminal” CO absorbs at a frequency above 2000 cm-’ and “bridge-bonded” CO in the range 1800-1950 cm-‘. The chemisorption bond is then thought [5] to consist essentially of the donation of a 5a electron to the metal accompanied by simultaneous back-donation of d electrons into the 2n orbitals. Adsorption in bridge configurations leads to increased metal-carbon bonding, a lower C-O bond order and thus a lower C-O stretch frequency. Previous results for CO adsorption on single crystal planes of palladium [4] confirm this picture at lower coverages, where site specificity plays an important role in the spectra. Under some conditions, however, the lateral interactions between the adsorbed molecules are also important in determining the position of the C-O stretch frequency. The problem of the lateral interactions deserves further attention, therefore. A general feature of the C-O stretch is its shift to higher frequencies as a function of increasing coverage. Several models have been proposed to explain this behaviour. The one which has received most theoretical attention is dipole-dipole coupling [6-91. In a recent paper Scheffler [9] has indicated that shifts of the order of 60 cm-’ in the CO/Pd system can be accounted for with this model. Important is the proper consideration of the image dipoles: relative to the free molecule, a downward shift in frequency occurs for an isolated molecule on a metal surface due to its image dipole. Neighbouring dipoles and their respective images screen this effect resulting in the observed upward shift with increasing coverage. A model of through substrate vibrational coupling has been advanced by Moskovits and Hulse [lo] to explain the observed shifts. Unfortunately, it is experimentally not possible, not even using isotopic mixtures [7], to distinguish between through space and through substrate vibrational coupling. The contribution of chemical effects to the observed shift can, however, be isolated using the method of isotopic mixtures. “Chemical” refers here to the reduction in back-donation into the 2n orbital with increasing coverage as a result of competition for the metal d electrons [5]. Hollins and Pritchard [ll] have shown that the chemical or bonding shift for CO on Cu( 111) is 50-60 cm-‘, albeit in the opposite direction, which appears to be a common feature of the group Ib metals. Co-adsorption experiments with known electron acceptors and donors also indicate that on nickel the bonding shift can be of the order of a few tens of wavenumber [ 12,221. The adsorption system CO/Ru(OOl) has been investigated by several authors [2,13-181. The ESDIAD measurements of Madey [13] indicate that the CO molecular axis is perpendicular to the surface over the whole coverage range. The

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temperature-dependent halfwidth of the emitted ion cone was attributed to the CO bending vibrations. The iatter involve a dipole moment change parallel to the surface and are hence not seen in IR spectroscopy nor, as yet, in EELS. Thomas and Weinberg [ 181 have reported EELS measurements where the C-O stretch frequency moves continuously from 1980 to 2080 cm-’ as a function of increasing coverage. They conclude that the presence of one peak only in the so-called linear region indicates little site specificity and that the surface is both geometrically and electronically homogeneous with respect to CO adsorption. The position of the band would indicate, however, that on-top or near-on-top sites are favoured. This question will be further discussed below. In the present paper we show that the IR method, on account of its high resolution, can deliver much more detailed information not only on the frequency versus coverage relation but also on peak shape and half-width. All three of these parameters depend strongly on the dynamic lateral interactions in the adlayer, which in turn depend on the ordering processes. LEED shows that in this adsorption system a d/3 X d/3 R 30” overlayer occurs until 6 = 0.33 after which disordering takes place [2]. At higher coverages a 2d3 X 243 R 30” structure is formed at T G 200 K, which is then compressed until saturation is reached [ 171. The mutual forces between admolecules are attractive for separations wu, where a is the surface lattice constant, and for smaller separations strongly repulsive [1.5]. The CO/Ru(OOl) adsorption system is thus particularly suitable for demonstrating to what extent high resolution vibrational spectroscopy is affected by mutual separation and ordering of the admolecules.

2. Experimental A detailed description of the LEED/IR system and of the polarisation modulation technique for recording the IR reflection-absorption spectra will be published elsewhere [19]. Polarisation modulation is a pseudo-double beam technique [20], whereby the IR beam from a Sic source is alternately polarised perpendicular (s) and parallel (p) to the plane of incidence by a rotating polariser. Because of the near-perfect screening properties of metals at IR frequencies only vibrational excitations involving a change in dipole moment perpendicular to the surface can occur. Moreover, only p light will be effective in this process. In polarisation modulation the s beam acts as a reference but, because of the differences in reflectivity for p and s light, complete cancellation does not normally occur. Thus the difference in intensity between p and s components is detected with a lock-in amplifier, but a baseline subtraction is also employed. The resolution of the system is 3.5 cm-’ with a noise level corresponding to an absorption of 0.02% at a sweep rate of 1 cm-’ s-l. Thus, as fig. 1 shows, it is possible to observe the C-O stretching band from less than three thousandths of a monolayer of CO on Ru(OO1). The sample was 13 X 6 mm size and about 0.4 mm thick. It was cut by spark erosion from a single crystal sample supplied by Materials Research Corp. Orienta-

434

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spectroscopy

of CO on RufOOlJ

0.33 0.37 0.39 Q43 0,LB 0.56 0.61 0.65 L I * ',

2100

2cE0

0.67 ' I

20.30

* 1 - I

1950

[cm-l]

Wavenumber Fig. 1. The IR absorption band due to the C-O stretch frequency in the system a function of increasing coverage. Temperature of adsorption and measurement:

CO/Ru(OOl) 200 K.

as

tion to within 0.5” with the Laue back reflection method was followed by mechanical polishing with successively finer diamond pastes down to 0.25 pm grain size. Four small Ta wires, about 5 mm long and 0.4 mm in diameter, were spot welded pairwise onto the long edges of the crystal and supported on 2 mm W rods. The latter were clamped to the manipulator. The crystal was heated resistively to 1570 K and could also be cooled down to 80 K with liquid nitrogen. Temperature was measured with a chromel-alumel thermocouple which was spot-welded to the rear of the crystal. An electronic regulating device allowed any desired temperature in this range to be kept constant. Flash desorption was measured by the increase in system pressure on a nude BayarddAlpert ion gauge. It was found possible to use a constant heating current of 15 A, which gave an approximately constant heating rate of 8 K s-r in the CO desorption region (320-550 K). These measurements were used only for the determination of relative coverages and small changes in the heating rate were therefore unimportant. Sample cleaning was carried out by repeated heating to 1500 K and cooling in

H. Pfniir et al. / Vibrational spectroscopy

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5 X lo-’ Torr Oa followed by a flash in vacua [ 151. Sugsequent Ar+ sputtering for 30 min at 300 K and short annealing at 1500 K gave a clean surface. Each spectrum in fig. 1 required a scan of about 3 min, which at a background pressure of 4 X 10-l’ Torr gave negligible contamination during measurement. Reproducibility of IR spectra at fixed coverages proved to be the best test of surface cleanliness. Very small quantities of impurities, not detectable with Auger spectroscopy, as well as possibly surface defects, broadened the IR bands at intermediate coverages or resulted in peak shifting and splitting at other coverages. This was particularly true near 0 = 0.33 and spectra at this coverage were hence used as a test of surface cleanliness and/or quality.

3. Results The measurements of Feulner et al. [16] on this system have shown that the maximum in the intensity of the extra LEED features corresponding to the d/3 structure occurs for adsorption below 350 K at a lower coverage than for desorption from a saturated layer. The‘same was found to be true of the maximum in the ESD current consisting of both CO’ and 0’ ions. Under desorption conditions both maxima coincide with the filling of the strongly bound desorption state. In addition, the 2d3 structure, which is fully developed at 200 K, disappears when the crystal is warmed to 300 K and appears again on re-cooling. Because this behaviour was expected to affect the IR results, sequences of spectra at 80, 200, 300 and 400 K in all accessible coverage regions were recorded. Fig. 1 shows the change in reflectivity (R, - R)/Re, expressed as % absorption [21], as a function of wavenumber for various coverage ranges. The adsorption and measuring temperatures were both 200 K. Despite careful investigation in the wavenumber region 1600.em 2400 cm-’ and in the complete coverage range, it was not possible to detect a second absorption band. The sequences of spectra at 80, 300 and 400 K were similar to that of fig. 1 except that small differences in peak shape, half-width and position were noted. These will be discussed in detail below. The reproducibility of the position of the peak maximum for a particular temperature and coverage was better than *l cm-‘. After each IR spectrum was recorded, the relative coverage was determined from integration of the flash desorption curve. The sequence was then calibrated by setting the coverage at which the strongly bound desorption state was filled to 0 = l/3 [16] ( see above). Saturation coverage at T - 200 K then corresponded to 8 = 0.68, which agrees reasonably well with the value of 0 = 0.63 obtained from desorption measurements via the change in work function [ 151. At 200 K a coverage of 0 = l/3 corresponds to an IR peak maximum at 2021.5 cm-‘. The peak position is slightly dependent on temperature at this coverage: at 400 K the maximum is found at 2018.5 cm-’ and at 80 K at 2025 cm-‘. (The latter value corresponds to a CO layer annealed at 300 K, because at 80 K good ordering does not take place. This is discussed below.) At all temperatures the coverage

436

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sequence is marked by a monotonic shift of the peak position to higher wavenumbers. At saturation coverage and T < 200 K the peak maximum lies at 2061 cm-‘. At the lowest measurable coverage the peak maximum was found at 1984 -‘, which gives a total shift of 77 cm-‘. Shifts of this order of magnitude have zkady been observed for CO adsorption on Pd [4] and Ni [22-241 surfaces. Although Thomas and Weinberg have observed a similar peak shift for CO/Ru(OOl) in EELS [18], their ,peak maximum at saturation was found at 2080 cm-‘. The difference may have been due to uncertainty in the determination of the peak maximum at -100 cm-’ resolution. Fig. 2 shows the dependence of the frequency versus coverage relation on temperature. Although the changes in peak position as a function of temperature are only a few wavenumbers, they do lead to characteristic forms of the curves at different temperatures. At 80 K the absorption band moves almost linearly with small

200K

300K

Ql

0.3

Coverage Fig. 2. Frequency versus coverage (adsorption and measurement).

plots

for

the

a5

Q7

8

temperatures

T= 80 K, 200 K and

300 K

H. Pfniir et al. / Vibrational spectroscopy

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431

deviations at very low and at very high coverages. At 200 K two regions of constant wavenumber, or “plateaus”, appear for 0.2 < 0 < 0.33 and at 6 - 0.5. At 300 K a similar behaviour is observed, but the second plateau is not so sharply defined and the first extends over a shorter coverage range. The curve for 400 K (not shown) is similar to that for 300 K except that the second plateau is not reached: maximum coverage occurs at 0 = 0.35. The two plateaus can be correlated with the formation of the 43 structure between 0.2 and 0.33 coverage and of the 243 structure around 0.5 coverage [2,17,25]. At temperatures above 200 K the extra LEED features from the d/3 structure first appear at higher coverages, which correlates with the shorter regions of constant wavenumber. The plateaus thus correspond to the formation of islands of the ordered overlayer. There are, however, certain differences between the LEED and IR results. The development of the J3 structure in LEED is not shifted to so much higher coverages on going from 200 to 300 K as the change in the plateau would suggest. Furthermore, the LEED intensity maximum at adsorption temperatures below 350 K occurs below 0 = 0.33. The peak shift in IR on the other hand would indicate already at 200 K undisturbed island growth until 8 = 0.33. For the 243 structure the agreement is reasonably good: at 300 K the extra features in LEED can scarcely be seen and the region of constant wavenumber is also very small. A comparison of the half-widths of the absorption band in fig. 1 with the frequency versus coverage relation of fig. 2b shows that these too are closely linked to the region of constant wavenumber. Fig. 3 shows the half-width as a function of coverage for the four adsorption temperatures. The structure in the 80 K curve is less strongly pronounced than at the higher temperatures. On the assumption that the ordering process affects drastically the half-width, we would conclude that the overlayer at 80 K shows little evidence of order. This is confirmed by LEED investigations at 85 K [25], which showed only very weak diffuse extra features from the 43 structure. At the other three temperatures, a pronounced minimum in the half-width occurs between 0 = 0.2 and 0.33. The beginning of the minimum shifts to higher coverages as a function of increasing temperature in the same way as the beginning of the plateau in the frequency versus coverage relation. A meaningful determination of the half-width at the beginning of the minimum is, however, difficult because of the presence of a shoulder (see for example 0 = 0.18 in fig. 1). This shoulder is responsible for the increase in half-width in the coverage region 0 = 0.10-0.18. In fact the absorption band here consists of two smeared-out maxima, which can be clearly separated by warming and re-cooling (see below). The minimum corresponds to a constant half-width of -8 cm-’ for T > 200 K. Between 0.33 and 0.44 coverage the IR band was found to become broader again. In LEED the extra features of the d3 structure disappear in this coverage range and are replaced by a diffuse ring, out of which at higher coverages the spots of the 243 structure develop [ 17,251. The broadening of the IR band as well as the strong shift to higher frequencies in this coverage region can therefore be correlated with the elimination of long-range order and with the increase in cover-

438

H Pfiuir et al. / Vibrational spectroscopy of CO on Ru(OOI)

dl

FWHM 3M)K

57

a3

Fig. 3. Change in half-width various

adsorption

-*

0.5

of the temperatures.

0:3

&

(d)

IR absorption

!

I

a1

band as a function

e

400 K

[cm-l]

a7 e

d7

a3

I

a5

of increasing

z

coverage

for

age of a disordered structure. The second minimum in the half-width between 0 = 0.45 and 0.6 can be correlated with the formation of the 2d3 structure from LEED and with the plateaus of fig. 2 but, in contrast to the LEED results and the on temperature. The frequency versus coverage relation, it is not dependent increase in the half-width near maximum coverage at 200 K (also seen to a small extent at 80 K) is probably connected with the compression of the 243 structure, but an exact coverage determination of the appearance of the latter was not available at the time of the experiments. Although the half-width showed only little evidence for order in a layer produced by adsorption at 80 K, it seemed clear that light annealing should bring about the ordering processes. For this reason the following experiments were performed (fig. 4). CO was adsorbed at 80 K to a particular coverage and the IR spectrum recorded (spectrum (a) in each case). The CO-covered surface was then warmed to 350 K ~- at this temperature desorption does not occur for the coverage range GO.33 -- and re-cooled to 80 K. A second spectrum (b) was then recorded. Between 6’ = 0.1 and 0.3 the annealing process results in a shift of the peak as well as a pronounced splitting. One component remains at a constant wavenumber, 1999 cm-i ; the other component shifts to higher wavenumbers. With increasing coverage the component at 1999 cm-’ becomes successively lower in intensity. For 0 > 0.33 the effect virtually disappears: the shift in the peak and the narrowing effect are of the order of 1 cm-‘. Fig. 5 shows the splitting of the absorption band for 8 < 0.33 presented as a frequency versus coverage plot. Also shown are the curves for the unannealed layers at 80 and 200 K. The fact that a peak splitting is not obtained from adsorption at 200 K indicates that the mobility of the CO mole-

H. Pfniir et al. / Vibrational spectroscopy 260

255

245

250

of CO on Ru(001)

439

[meV]

>....,....,....I

2100

2050

M(x)

1960

Wavenumber (cm-‘) Fig. 4. Effect of annealing the adsorbed CO layer on the IR absorption band: (a) spectrum directly after adsorption at 80 K, (b) spectrum after warming to 350 K and re-cooling to 80 K.

cules is insufficient for real equilibrium to be reached in this temperature and coverage regime. The extent of the peak splitting and the relative intensity of the two components depend not only on the coverage but, for a given coverage, also on the temperature. When an annealed layer, for example at 0 = 0.14 as in fig. 6, is slowly heated from 80 to 300 K, the high frequency component loses intensity and shifts to lower wavenumbers, whereas the low frequency component increases in intensity. At approximately 230 K, the two components can no longer be resolved for this coverage and the whole band then shifts slightly downward in frequency and changes its peak form. This process was found to be fully reversible

440

H. Pfniir et al. / Vibrational spectroscopy

1980

of

CO on Ru(001)

4

-

I

80K

- - 200K

‘;

E Y t z 6 P 2040 +

0.1

0.2

0.3

Coverage

0.4

Q5

>

8

Fig. 5. Frequency versus coverage plots for the curves b of fig. 4: (X) low frequency peak; (0) high frequency peak. The shifts of the unannealed layers for adsorption at 80 K (continuous line) and at 200 K (dashed

line) are indicated

down to 200 K as long as the re-arrangement of the adlayer is after adsorption than to achieve ordering process has taken place. Fig. 7 shows the integrated IR

for comparison.

layer was previously annealed. More extensive clearly required to achieve equilibrium directly equilibrium at a particular temperature once the absorption

intensity

of the CC0 stretch band as

Wavenumber Fig. 6. Variation of peak form as a function 0.14, for a layer pre-annealed at 350 K. Fig. 7. Integrated IR absorption coverage at 80 K and 200 K.

intensity

of increasing

(normalised

temperature

Coverage

8

at constant

coverage,

0 =

as a function

of

to its maximum)

H. Pfniir et al. / Vibrational

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of CO on Ru(001)

441

a function of coverage. The intensities have been normalised to 0 = 0.33, where at 200 K a maximum is also found. After an almost linear rise to the maximum the integrated intensity decreases by about 20-30%, such that at saturation coverage the absorption per adsorbed CO molecule is only 35-40s of the value at 0 = 0.33. This effect does not derive from a change in the overall reflectivity of the sample: adsorption up to maximum coverage changes this quantity by only 0.6% in this frequency range. Annealing the layer was found to have no influence on the integrated intensity of the band, at least within the margins of experimental error.

4. Discussion 4.1. The position of the C-O stretch frequency and site specificity As noted in the Introduction, the position of the C-O stretch frequency in the IR spectrum can be used as an indication of the nature of the surface site. Investigations of the adsorption of CO on the likewise hexagonal faces Pd(ll1) [4] and Ni(lll) [24] show evidence for a strong site specificity. In these two systems initial adsorption on three-fold hollow sites appears to be followed by a change to bridging sites. This is indicated by a marked shift in frequency between 0 = 0.33 and 0.40 over and above the continuous shift due to the lateral interaction. For CO on Pt(l1 l), two bands have been found both in EELS [26] and IR [27] spectra which have been assigned to on-top and bridge-bonded CO, although earlier IR work [28,29] had resulted in the high frequency band only. Thus even on closely packed energetically homogeneous surfaces distinct adsorption sites can exist and the IR method is capable of detecting them. On the Pd(ll1) surface, the situation at higher coverages is even more complex with several peaks appearing in the C-O stretch region, whilst LEED shows that ordered, albeit incommensurate, overlayers are formed [19]. In contrast to this situation we observe for CO/Ru(OOl) essentially only one feature in the C-O stretching region, which shifts continuously from 1984 to 2061 cm-‘. (Although neither the present results nor the EELS results of Thomas and Weinberg [18] show any evidence for a second absorption band, as in CO/Pt(l 11) [26,27], this possibility cannot be completely ruled out. With the sensitivity of the present measuring system a band of lo-15 cm-’ half-width and 0.05% absorption intensity can be identified with reasonable certainty. This would correspond to 8 = 0.002 for a species with the same dynamic dipole moment as the band found. Alternatively, a species with coverage 0 = 0.2 would have to have hundredfold smaller IR cross section to escape detection.) The classical spectral range for on-top CO is generally considered to be above 2000 cm-‘, as opposed to the “bridging” region (sites of two-fold coordination) between 1850 and 1950 cm-‘. We therefore conclude that at low coverages up to 0.33 the CO molecules occupy on-top sites. It is instructive at this point to look at the C-O stretch frequencies in the two

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known binary carbonyls, RUB and Ru(CO)rI [30,31]. Although it might have been expected that the triruthenium cluster compound contains some bridging CO ligands, they are all in fact terminal [32]. In Ru(CO), IR-active vibrations are found at 2035 and 1999 cm-’ ; there appear to be no Raman data in the literature. All the vibrational bands of Rus(C0)r2, however, have been measured and assigned consists of a central triangle of Ru atoms, each having four CO ]311. Ru3(C0),2 ligands, and has D ah symmetry. Six CO ligands are parallel to the Cs axis (i.e. perpendicular to the Rua triangle) and are termed “axial”. The other six are in the ur, symmetry plane of the triangle and termed “radial”. The two sets of ligands do not couple vibrationally. Vibrations resulting from the axial CO ligands are found in the region 2028 to 2127 cm-’ and from the radial sort from 1989 to 2011 cm-‘. It appears that (asymmetric) CO bridges do exist in the carbonyl hydride H2Rud(C0)ra [33] which, apart from bands in the region 2000 to 2090 cm-‘. gives rise to an IR band at 1880 cm-‘, i.e. in the classical range of bridging CO. Generally a comparison between the surface situation and an inorganic analogue is not necessarily straightforward. Firstly, we know that the frequency of the isolated CO molecule on the surface is appreciably lowered due to the interaction with the image dipole and that the depolarisation due to dipole--dipole coupling at higher coverages may not be complete (see section 4.2 below), so that a downward shift remains even at high coverage. Secondly, the presence of more than one CO ligand on the metal atoms in the binary carbonyls leads to strong vibrational coupling not occurring in the surface situation. Nonetheless it is quite striking that the frequency ranges cited above for the linear CO groups almost exactly span the region over which the adsorption band shifts in the present work. Furthermore, the A; vibration of the axial CO ligands at 2062 cm-’ 1sstrongly analogous in character to the A1 mode [34] of CO on the surface. The assignment of the observed band to linearly bound CO appears warranted, therefore. Also, it appears sensible to expect that a bridging surface species would appear in the range around 1850 cm-‘. Given that onrtop sites are occupied up to 0 = 0.33 (fig. 8a) and probably above as extra CO molecules are squeezed at random into vacant on-top sites of the d/3 structure one must ask what happens in the coverage region 0.4 to 0.55 where the 2d3 coincident lattice is formed (a structure model, after Williams and Weinberg [ 171, is shown in fig. 8b). Under these conditions only a small percentage of the CO molecules can be adsorbed in what could be termed crystallographic on-top sites. Despite this only one band is observed in the IR spectrum. This apparent contradiction between crystallographic and spectroscopic data can be resolved if we assume that at these coverages the static lateral interaction between the CO molecules (strikingly visible also in kinetic data [I 51) becomes as important as the periodic potential of the crystal surface. Each CO molecule must then be regarded as roughly equivalent and linearly bound to the surface with an identical RuC separation, so that a uniform C-O stretch frequency is maintained. The mode of bonding is thus the same as in the on-top case where each molecule is associated with a specific surface atom. The term “linear” CO is preferable to “on-top” in this

H. Pfniir et al. / Vibrational spectroscopy

of CO on Ru(001)

443

(a)

(b)

Fig. 8. Surface models for (a) the 43 after Williams and Weinberg [ 171.

X

43 R 30” and (b) the 2J3

X

2J3 R 30” structures

case, since it emphasises the state of hybridisation of the carbon atom over its coordination to the surface metal atoms. This does not make it impossible that at 0 < 0.33, i.e. in the absence of repulsive lateral interactions, the geometric on-top sites would still be energetically most favourable. Also the bonding can still change with coverage, but at a given coverage only small differences exist between the of the metal surface adparticles in the various “sites”. Why the rehybridisation orbitals brought about by the dense CO layer would lead to such a uniform potential in the case of Ru(OOl), but not in the other cases of hexagonally close-packed metal faces (see above) must remain an open question at present. The alternative explanation would be that there is indeed a second species which, however, has an IR cross section smaller by at least two orders of magnitude. This appears to be extremely unlikely. 4.2. The effect of lateral interactions in the coverage region 0 < 0.33 The shift and line shape of the absorption band due to the C-O stretch vibration on metal surfaces has been attributed to lateral interactions. Scheffler [9] has shown that shifts of the order of SO-60 cm-’ can be explained using the dipoledipole coupling model. This does not, however, exclude other mechanisms contributing to the overall shift, as we explained in the Introduction. The presence of the static lateral interaction, which is certainly an electronic effect qualitatively similar to that described by Blyholder [.s], is indicated by the LEED and kinetic

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data [15]. Since the main effects can, however, be explained using Scheffler’s model, we will restrict the discussion to dipole--dipole coupling. Following Scheffler [9] and earlier authors [6-81, we describe first of all the model. The dipole moment, p, of a molecule can be divided into a static kst) and dynamic part: p = Pst + Q(W) EIOC)

(1)

where Q(W) is the dynamic polarizability. The local electric field, E’oc(w) is composed of the incident (Ee) and reflected (r&e) fields of the radiation as well as of the fields of the image dipole (E”“), other dipoles (EdiP) and their respective images @Other im): ~10~ = (1 + yP) me + @m +&in

+ Eother im ,

(4

a(~) is composed of electronic (c+) and vibrational terms (0, is the dynamic polarizability for o + 0, o,, the frequency of the normal mode associated with the CO stretch, and y the damping constant): 0((o) = (Y,+ The dynamic

% 1 - (w/w($

+ iy(w/w())

dipole moment

(3)

.

of a molecule at

Ri can be derived from eqs. (1) and

(2):

a(w)(l + rp) &dRj>a, f> Pdyn(Ria w, t, = 1 + o(w)[s(e)

~ 1/(4d3)]

= p(o) exp [i(wt - kRi)]

(4)

S contains the dipole sum of the direct dipoles and of the image dipoles, with the exception of the image of the dipole being considered. d is the distance of the C atom from the image plane, i.e. a length of about half the Ru-C bond length. With increasing S, which is achieved by increasing the coverage of adsorbed molecules, either by increasing the number of occupied sites in an ordered island or by growth of a perfectly ordered island, the dynamic dipole moment will be reduced. This screening or depolarization also occurs in the case of static dipoles (see, e.g., ref. [35]), but the two effects are physically quite distinct. Because of the long wavelength of the incident radiation phase differences between the dipoles can be neglected and the change in reflectivity is given by AF-

-no2

[Re rp Re p(w) + Im rp Imp(w)]

,

(5)

where n is the number of illuminated dipoles. At near to grazing incidence, rP is imaginary [21], with the result that only the second term is important. If one neglects the damping, the shift in frequency as a

H. Pfniir et al. / Vibrational spectroscopy

function

of increasing coverage will be given by &,(S - 1/(4d3) 1 + (Y&s - 1/(4d3)

1

112 .

of CO on Ru(001)

445

(6)

Strictly speaking, eq. (6) only holds for one type of adsorption site and then only for a perfectly ordered layer. Each dipole must be considered as identical. If the layer consists of small islands of ordered structures, then the dipole sums are different for the molecules at the edge and for those in the middle of the islands. This means that the two sorts of molecules will not have the same resonance frequency. The difference is more important than appears at first sight because at the resonance of an edge molecule, for example, the screening by the other molecules in the middle, which are not at resonance, is not as effective as the screening by other edge molecules. This effect could be treated by introducing effective dipole sums, S eff. In principle, it is possible to treat the problem quantitatively if assumptions are made about the size and shape of islands, the relative position of their constituent molecules and the width of the resonance in (Y(O). Because of the large number of parameters we do not, however, attempt an exact description but rather use eqs. (l))(6) in a semi-quantitative way. For the coverage region up to 0 = l/3, the complete shift can be accounted for with dipole-dipole coupling using a reasonable value for d, of about 1 A. Taking the gas phase values cr, = 0.057 A3 and CX,= 2.6 A3 [8,9], and an approximate value for wo, a Aw of 38 cm-’ IS obtained when we set d = 0.99 8. The measured value up to 0 = l/3 is 34.5-41 cm-’ depending on temperature. The calculation, however, depends critically on the value of d, which is not accessible experimentally: for d = 1.05 8, Aw = 25 cm-’ and for d = 0.95 8, Aw = 60 cm-‘. A value of d = 0.99 A gives an w. value (from eq. (6) at S = 0) of 2029 cm-‘. The temperature dependence of the shift is difficult to explain. Changes in d, 01, or (Y, with temperature would make the effect more noticeable as 0 + 0 than for 8 = 0.33. The only possibility is a temperature dependence in w. which is compensated at low coverages by a change in one of d, ol, or (Y, in the other direction, but is the main contribution at 0 = 0.33. The strong shift of the band at very low coverages is particularly striking. Two possible reasons come to mind. Either adsorption takes place initially at surface defects giving a characteristically low frequency of -1984 cm-’ or, alternatively, the formation of small clusters of molecules takes place. If the former were true, then the peak at 1984 cm-’ should remain at high coverages which is not the case. The occurrence of an aggregation process even at very low coverages is, however, a reasonably safe assumption because there exists an attractive interaction (-3 kJ mol-’ , pairwise) for intermolecular separations above d3a [ 151. This interaction is expected to lead to the formation of small clusters, the size of which will be dictated by the minimum in the free energy of the adsorbate-covered surface. The size of the islands will increase as a function of coverage. Using the dipole-dipole coupling model described above one obtains the following results: For two mole-

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cules separated by flu, which corresponds to the potential minimum in the lateral interaction, one obtains a frequency shift of 6.5 cm-’ relative to the isolated oscillator. For three such molecules the shift is I? cm-‘. In both cases the parameters from the calculation above are used. In addition, a further shift of about 3-5 cm-’ takes place for up to 0 = 0.1 through the interaction between the various clusters. The frequency shift at low coverages can therefore be accounted for semiquantitatively. For coverages between 8 = 0.1 and 0 = 0.33, it is useful to start by considering fig. 4 in more detail. Adsorption at 80 K results always in a single IR absorption band, which above 6’ = 0.1 splits when the adlayer is annealed. This change in peak shape can be explained by assuming that adsorption at 80 K results in a more or less disordered layer, which can be ordered by warming. From a disordered phase one indeed expects to obtain a peak considerably broader than that from a perfectly ordered layer [36]. (Although figs. 2 and 4 give little evidence for any order at all at 80 K, fig. 3 does show some structure in the half-width versus coverage plot at this temperature. The structure is similar to that at 200 and 300 K, only not so pronounced.) The effect of annealing the 80 K layer is to increase the mobility of the CO molecules such that thermal equilibrium corresponding to the configuration of lowest free energy can be established. Experiments at various temperatures showed that at 0 = 0.14 at least 250 K for 100 s was required to reach thermal equilibrium. It is not possible to say for which temperature during cooling (after the anneal) a particular equilibrium distribution becomes “frozen-in”, i.e. to which temperatures the curves b in fig. 4 correspond. The present measurements merely show that they lie somewhere between 80 and 200 K. What then is the origin of the two peak structure in the spectra b of fig. 4 from the ostensibly ordered layer? One possible explanation is that the two peaks arise from the difference in resonance frequency for molecules in the middle and at the edge of ordered islands. The observed changes in peak form are then due to changes in island size distributions with temperature and coverage. Alternatively, the adlayer consists of ordered islands and a disordered phase of lower coverage (i.e. essentially single molecules) between the islands. This could in fact be considered as an extreme case of island size distribution. We have performed some calculations usings eqs. (5) and (6) for islands of various sizes. The values given above for d, oV, CX,and we were used. The clusters of diameter D, where D is in units of fia, were taken as hexagonal in shape and ordered in the 43 structure. The band width was taken to be yw, = 5 cm-’ and yP was set at (0, 0.52). No attempt was made to account for Seff. The results are shown in fig. 9. To facilitate comparison the curves have been normalised to intensity per molecule in the island. The difference in shielding for dipoles at the edge and in the middle results clearly in two bands, whereby the band at lower frequencies due to the edge dipoles is reduced in intensity as the island gets larger. The difference in shielding also results in a higher absorption intensity per molecule for edge dipoles than for those in the middle: the averaged absorption per molecule

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2040

2020

of CO on Ru(001)

2000

441

km’1

Wavenumber Fig. 9. Calculated IR absorption bands for hexagonal islands of various diameter, normalised to absorption intensity per molecule. D is in units of J3a. The parameters used were w. = 2029 cm-‘, d = 0.99 A, av = 0.057 A3, a, = 2.6 A3 and ywo = 5 cm-l.

for D = 2, where almost all dipoles are edge dipoles, is a factor 1.8 larger than for perfectly ordered layers. For clusters of medium size the low frequency peak exhibits a shoulder, because in a hexagon the edge dipoles are not all equivalent. The curves b of fig. 4 can thus be explained qualitatively on the basis of the present results. The splitting of the band in fig. 9 is, however, too small probably due to neglect of Seff. Putting Seff = $Sedge for the edge dipoles, gives the experimentally observed separation (1 S-20 cm-‘). The smaller S value also enhances the absorption intensity of the edge molecules; the peak form at 8 = 0.14 in fig. 4 would correspond to an island size of D = 6 to 8. These simple calculations can only account semi-quantitatively for the results. In fact, a distribution of sizes and shapes of islands no doubt occurs in practice, depending on the minimum in free energy for a particular coverage. The change in distribution with coverage could explain the discrepancy between figs. 4 and 9, namely that in fig. 4 the position of the low frequency peak is independent of coverage. The latter observation is more consistent with the alternative explanation, whereby the low frequency peak is attributed to a disordered, low concentration phase between the islands. A decision between these models is not possible at present. Essentially the same ideas can be used to explain the variation of peak form with temperature at constant coverage (fig. 6). Under conditions of thermal equilibrium

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the free energy is at a minimum. Under the influence of an attractive interaction between admolecules, two competitive processes take place; the aggregation process decreases the energy but at the same time causes a loss of configurational entropy. The minimum in the free energy is temperature-dependent and causes a decrease in average island size with increasing temperature. In the IF? spectrum this leads to a shift of the high frequency peak to lower wavenumbers and a change in the relative intensity of the two peaks. This is seen by comparing the spectra for T = 80 K and 200 K. A further increase in temperature results in a loss of the two-peak structure which, according to the calculation, is to be expected for D < 2. At higher temperatures still the formation of two- and three-molecule clusters probably takes place. resulting in a further shift of the peak maximum to lower frequencies. For this coverage range it remains for us to discuss the correlation between IR results and LEED. The intensity of the extra LEED spots from the d/3 structure gives a maximum before a coverage of 0.33 is reached. This maximum is shifted to higher coverages at higher temperatures, reaching B = 0.30 at T = 350 K. It would appear that, as the growth of islands proceeds, mismatching can occur at mutual boundaries when islands meet. This leads to relaxation of the CO molecules into new equilibrium positions at these grain boundaries, which in turn reduces the intensity in the extra LEED features. In IR, however-, calculation shows that the change in frequency is very small for small changes in intermolecular separation and would probably only be seen as a broadening of the band. A perfectly ordered layer might even result in a half-width somewhat smaller than 8 cm-‘. That the reduction in LEED intensity is a consequence of islands “meeting”is seen in the disappearance in this coverage range of the low frequency peak due to edge molecules. This discussion shows that in this coverage range (0 G 0.33) the observed phenomena can be explained by dipole --dipole coupling. This does not necessarily mean that there can be no contribution of through substrate vibrational coupling. Chemical effects, however, are expected to be small throughout this range, as the bonding situation is virtually unchanged up to 0 = 0.33. 4.3. The coverage region 0 > 0.33 The coverage region 0 > 0.33 is characterized by a strong repulsive interaction between adparticles, which manifests itself in a reduction of adsorption energy [2,15]. This effect should be observable in a reduction in frequency of the metalcarbon vibration. Thomas and Weinberg [18] were unable, however, to see any shift of this band as a function of coverage. It is possible that the low resolution prevents small changes in wavenumber from being observed in EELS. The present measurements also provide some evidence for a change in electronic configuration of the M-C-O adsorbate complex due to the static lateral interaction above 8 = 0.33; the observed shift in frequency of the C-O stretch of 38 cm-’ from 0 = 0.33 until saturation can only be explained if w,, is increased. Since o0 is the C 0 stretch frequency of a (hypothetical) isolated CO admolecule without its image

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dipole, it contains the frequency lowering due to the chemical interaction with the surface. Even if the coverage is increased to 0 = 1 with the admolecules in a 1 X 1 hexagonal array, the calculated shift on the basis of the dipole-dipole model is only 19 cm-’ relative to 8 = 0.33, if the same parameters are used as in section 4.2. In fact saturation appears to be reached at 0 = 0.68, corresponding to a further shift of only 12 cm-‘. This low sensitivity of the dipole shift of frequency to further changes of distance is due to the structure of eq. (6). The difference of 26 cm-’ compared to the observed result is also too large to be explained in the basis of through substrate vibrational coupling [lo]. The conclusion is, then, that in this coverage range chemical effects are important and may even dominate over effects of coupling. This range is also characterized by a gradual disappearance above 8 = 0.33 of the extra LEED feaures associated with the 43 structure, indicating a loss of order in the adlayer; only at temperatures below 200 K, new ordered structures appear at higher coverages [2,15,17]. The situation is significantly different in the case of Pd( 111) [4,37] and Pt( 111) [28,29,38], where order is retained by a unilateral compression of the 43 unit cell. Why the ruthenium surface behaves differently is not known at present. It is useful to consider what happens microscopically when an additional molecule is adsorbed at .O= 0.33. In fig. 8a we note that adsorption in the middle of a triangle of admolecules would give a group of four where the intermolecular separation is 2.71 A. In Rua(CO)iZ, the axial CO ligands are separated by 2.85 A, but it appears that they are not exactly parallel to each other [32]. Because 2.7 1 A is considerably lower than the Van der Waals minimum for two CO molecules with their axes parallel (-3.3 A [35]), this group of four molecules is energetically unfavourable. Were it to exist on the surface, we would also expect to see another band in the IR spectrum, due to the different dipole sum for the molecule in this group. What in fact takes place is a broadening and smooth shift of the IR band to higher frequencies indicating that the ordered layer relaxes to accommodate the additional molecule. This occurs at random over the surface, destroying the order and giving a distribution of configurations, which in turn broadens the IRadsorption band. The molecules remain, however, linearly bound, as discussed in section 4.1. 4.4. The integrated absorption intensity Since dipole--dipole coupling is moderately successful, for 0 < 0.33, in explaining the form and shift of the absorption band, it is interesting to examine its prediction for the intensity versus coverage curve of fig. 7. As explained above, the absorption intensity per molecule is strongly influenced by the surrounding molecules through the dipole sum, S: it is reduced by the screening through other dipoles and their images. The initial high slope at near-to-zero coverage in fig. 7 is thus due to more or less unscreened molecules in very small clusters. Unfortunately the inaccuracy of coverage determination does not allow us to get a reliable value of

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dI/de here. The slope is, however, noticeably greater than in the region 19= 0.1~ 0.3, where it is approximately linear. This latter behaviour is expected for island formation with D > 6. At this point S for the inner molecules of the island is only changing slowly with increasing island diameter. The edge molecules, the number of which is roughly proportional to the diameter of an island, compensate partly for the reduction in absorption of the inner molecules as the island grows. In order to find an explanation for the maximum in integrated absorption intensity at 19= 0.33 and the subsequent reduction at higher coverages, intensities of D = 20 islands were calculated using eq. (5) for a hexagonal structure with lattice constants varying from da (=4.69 A) to &@a (=3.32 A). The latter corresponds to the compression structure at saturation coverage [ 171. Relative to aa separation the absorption intensity per molecule of the island is reduced to 65% at 4 A and to 35% at 3.32 A. The overall reduction in absorption intensity can thus be explained in this way. Assuming dipole --dipole coupling is alone responsible for the reduction in absorption intensity, the sudden decrease per molecule at 0 = 0.33 must then be due to an abrupt change in intermolecular separation at the points at which additional molecules have been adsorbed. In section 4.3, we considered the effect of adsorbing an additional molecule into the middle of a triangle of CO molecules in the 43 structure. The triangle relaxes to accommodate the fourth molecule, but it is reasonable to suppose that the resulting configuration gives rise to a substantially reduced CO--CO separation. (The reason for the fact that this does not show up in the vibration frequency except by a broadening has been discussed above.) If the CO molecules further away do not relax appreciably, then one additional CO molecule at l3 = 0.33 would influence three molecules already present. To arrive at the experimental situation of approximately constant integral intensity just above 0 = 0.33, the compression must decrease the intensity per molecule by 2.5%. If we use this number in our evaluation, also taking into account the decrease of Serf by the loss of order as well as by the geometric non-equivalence of the four molecules, we arrive at the estimate that the distance between the central molecule and the others must be 4 A or less. Higher coverages lead to a further increase in wavenumber of the IR absorption peak and to a concomitant decrease in integrated absorption intensity. A microscopic description of the phenomenon in the very high coverage range cannot be attempted on the basis of the present semi-quantitative model. These considerations show that the characteristic change of integrated intensity versus coverage can also be understood in terms of dipoleedipole coupling. This does not preclude that chemical effects, which have been shown to be important for the frequency shift in the 0 > 0.33 coverage range, play a role too by changing the dynamic polarisability. We can say, however, that it is not necessary to invoke such effects to explain our observations.

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5. Summary (1) In the system CO/Ru(OOl), high resolution IR reflection-absorption spectroscopy reveals only one rather narrow band for the C-O stretch which shifts from 1984 to 2061 cm-’ with increasing coverage and shows a splitting of up to 25 cm-’ for certain coverages and temperatures after annealing. Any additional peak between 1600 and 2400 cm-’ cannot be larger than -1% of this peak at its maximum intensity. (2) The halfwidth and exact frequency of this peak can be correlated with the lateral interactions and ordering processes in this system as known from LEED and kinetic measurements. (3) The integrated absorption versus coverage curve is approximately linear up to coverages of about 0.3, exhibits a maximum at 0 = 0.33, the coverage corresponding to maximum intensity of the d/3 structure, and decays thereafter. At saturation coverage (about 0 = 0.68) the intensity per molecule is only about 30% of that in the linear region or 35 to 40% of that at the maximum. (4) The results are explained by linear CO species which, up to 0 = 0.33, are adsorbed in on-top positions and segregate into d/3 islands of increasing size. In this coverage range, the shift, width, and splitting of the CO stretch can be explained by assuming dipole-dipole coupling to predominate; peak splitting results from island formation. At higher coverages (0 > 0.33), the linear bond of CO to the surface is retained, although strong repulsive interactions shift most molecules away from geometric on-top sites. This is attributed to interaction-induced rehybridisation of surface orbitals. In this range, chemical influences seem to account for most of the frequency shift; the decrease of intensity, however, can again be understood in terms of dipole-dipole coupling.

Acknowledgements The experimental work described in this paper was performed whilst one of us (H.P.) was guest at the Fritz-Haber-Institut. The project was supported by funds from the ERP-Sondervermogen and from Sonderforschungsbereich 128. We acknowledge useful discussions with M. Scheffler and D. Weaire.

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