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Vibrational spectroscopy of CO absorbed on a Pt(111) surface
Applications ofSurface Science 13(1982)171 North-Holland Publishing Company
179
VIBRATIONAL SPECTROSCOPY OF CO ADSORBED ON A Pt(l 11) SURFACE Neil R. AVERY CSIRO Division of Materials Science, University of Melbourne, Parkville, Victoria 3052, Australia Received 23 February 1981
A qualitative comparison between electron energy loss spectroscopy (EELS) and infrared absorption spectroscopy (IRS) for the vibrational analysis of adsorbed molecules is given. The relative merits of the two techniques are demonstrated with a review of CO adsorption on Pt (111). A description of a recently developed EELS apparatus based on concentric hemispherical dispersive elements is also given.
1. Introduction In this paper, the relative merits of electron energy loss spectroscopy (EELS) and infrared spectroscopy (IRS) for analysis of the vibrational modes of molecules adsorbed on metal surfaces will be compared. For work of this kind, the adsorption of CO on Pt(1 11) has been relatively well studied by both methods and will be used as an example to compare and contrast the two approaches. Also, a brief description of a new high resolution EELS instrument built in this laboratory will be given.
2. EELS and IRS comparison For EELS in the non-dispersive limit, the momentum transfer to the scatterer is minimized and the energy transfer occurs via the long-range Coulomb field of the incident electron with the dynamic dipoles of the vibrational modes in a manner similar to infrared photon absorption [11. In this way the two techniques are expected to yield essentially the same information. Indeed, Thach [2] has compared the effective charge for v(CO) on Pt(l 11) as determined by IRS and EELS and found perfect agreement. In practice, the dipole EELS data is obtained by tuning the elastic scattering geometry for strong specular reflection which necessarily requires massive single crystal surfaces. In this way a strong elastic channel is establithed into and out of the crystal so that the dipole EELS bands are expected to be strongly peaked near the specularly reflected elastic peak [21. For IRS of adsorbed molecules, the data is 0 378-5963/81/0000 0000/$ 02.75 © 1982 North-Holland
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normally taken either after grazing incidence reflection from massive surfaces [3,4], which unlike EELS may be polycrystalline, or in transmission through high area small metal particles which are highly dispersed on an inert support [5]. Whereas the reflection IRS and EELS experiments are performed on surfaces which can be well defined in the modern surface physics sense, the dispersed particles of the transmission IRS experiment are much less well defined. Differences do arise, however, from the surface structure of the small particles. For examples, CO adsorption on alumina supported rhodium produces a gem dicarbonyl species with active symmetric (2101 cm—1) and asymmetric (2031 cm 1) modes [6]. Species of this kind were attributed to adsorption on isolated rhodium atoms and therefore would not be expected to form on the massive surfaces used in EELS and reflection IRS. A surface selection rule has been described for both the dipole EELS [7] and the IRS experiment [8]. This arises from strong dielectric coupling of the dynamic dipoles with the metal at vibration frequencies. Therefore, since both spectroscopies interact through their long-range electric fields, excitation can occur only if the dynamic dipoles have components normal to the surface; the horizontal modes being screened by their image dipoles. This surface selection rule for dipole excitations reduces the number of observed modes but can be useful in determining the orientation of admolecules relative to the image plane of the surface. For EELS, a second scattering mechanism has been recognized [9] in which the inelastic bands are more or less evenly distributed through k space and has been attributed to thort-range impact scattering. Impact scattering may lead to modes for. bidden by the dipole surface selection rule; but from the limited data currently available, it appears that for strong impact scattering, a necessary but not sufficient condition, is that hydrogen atoms are involved in the vibrational mode to a significant extent, e.g. W—H [9], ~~‘m N—H [10], C—H and 6 C—H [11]. There is no evidence for impact scattering playing a role in CO EELS and will not be considered further here. A significant difference between the two approaches to vibrational spectroscopy results from the i03—i04 times greater reactivity of a low energy electron with matter. Indeed, it is just this property of an electron which has led to its widespread use as a probe particle in surface spectroscopies. Whereas EELS data is easily obtained from well-defined single crystal surfaces, comparable IRS data can be obtained only with longer data acquisition times and then only with strongly infrared active molecules, like CO. Enhanced IRS sensitivity may be obtained with dispersed particles but difficulties then arise from infrared absorption, particularly at low frequencies, by the support material. Furthermore, some adsorbates may react chemically with the support making it difficult to separate the role of the metal. The lack of definition of these particles has already been referred to. A further consequence of the high reactivity of electrons with matter is that the EELS experiment must be performed in vacua where the electron mean free path is significantly greater than the dimensions of the apparatus. Typically, this limit is about 10~ Torr although the effective ,~‘
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pressure of the crystal surface may be enhanced by up to 1 ~2 with suitably positioned multichannel array molecular beam dosers, or up to 10~with supersonic nozzles. On the other hand, a suitably designed infrared experiment can compensate for a residual gas pressure up to several hundred Torr which approaches more closely the conditions of applied adsorption and catalytic processes. Modern infrared spectrometers are capable of resolutions which far exceed the demands of condensed phase spectroscopies. Thus, sensitivity is often bought at the expense of resolution which is typicafly set at 2—15 cm 1, depending on the demands of the experiment. EELS instrumentation by contrast is stifi in the early stages of development, with the best current instruments operating at a resolution of ‘—40 cm~.However, performance at this level is difficult to achieve routinely and most data is collected at ‘--‘70 cm 1 resolution. Future developments will almost certainly lower this limit.
3. EELS instrumentation Compared with the profusion of electron spectroscopies which have developed over the past decade or so, high resolution EELS has been relatively slow to emerge. With vibrational bands occurring in the < 500 meV (1 meV 8.0658 cm~)region, the need to monochromize the incident electron beam has been the main detertent electron monochromatOr
filament
crystal
~~\\cHA
Fig. 1. Cut away view of an FFLS apparatuc based on hemispherical energy dispersive elements.
174
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to the development of EELS. Although pioneering work by an Urbana group [12] demonstrated the viability of the tecimique it was principally Thach’s group at Julich which routinely obtained resolutions (40 70 cm 1) where useful data could be gathered [13]. Whereas these instruments have used 127°cylindrical condenser energy dispersive elements, the instrument which has been developed in this laboratory has sought to exploit the superior intrinsic resolving power and point focusing properties of the concentric hemispherical design [14] (fig. 1). For the monochromator, the conflicting demands of high take-off kinetic energy (12 eV) from a hot tungsten filament and low pass kinetic energy in the hemispheres (‘--‘300 meV) was met with a tandem pair of independent strong zoom lenses. The second stage imaged the electron beam with controlled divergence on the equatorial plane of the hemispheres. After energy dispersion by the R0 2 field in the hemispheres onto the output aperture, the monochromized electron beam was accelerated to the desired impact energy (1 6 eV) and focused towards the crystal surface with a final zoom lens. All zoom lenses were of the aperture type with a gap-to aperture ratio of 0.5 to allow direct interpolation of the zoom characteristics from published tables [15]. The accelerating positive focus voltage was used in each case. Electrons scattered from the crystal in ±2.5°cone about the specular beam were retarded and focused onto the input aperture of the analyzer hemispheres by a single cylindrical zoom lens. In this lens, focusing of the scattered beam was relatively insensitive to the focus voltage so that the analyzer could be run in either a constant resolving power or a constant resolution mode. Detection was with a channel electron multiplier (CEM) operating in a particle counting mode. Experience in operating this instrument indicated that after initial iteration of the focus voltages and scattering geometry to maximize the elastic count rate, good resolution was mainly a function of beam alignment which was controlled by electrostatic deflection at many points around the electron path. In this way, the fwhni of the specularly reflected beam from a clean Pt(l 11) surface could be easily and routinely tuned to 45 55 cm 1 with a count rate at 2 4 X 10~counts per second in the elastic channel.
4. CO on Pt(111) 4.1. O~0.5 With its higher resolution, the early stage of CO adsorption is better described by the IRS reflection data although only the v(CO) band can be detected by this technique. Two independent studies [3,4] have shown that initial adsorption occurs essentially reversibly into an isolated (singleton) linear-bonded species with a band at 2065cm 1 which shifts to 2070cm at a coverage of 0.3 X 1014 molecules cm 2~ With further adsorption,island formation occurs yielding a new band at 2083 cm 1, which coexists with the random phase. Saturation at 300 K produces a c(4 X 2)
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LEED pattern [161 and only a single ~(CO)band isseenat2l0l cm ~Earliertransmission IRS work with high area dispersed Pt particles [17] showed a similar monotonic thift from 2040 cm 1 to 2067 cm According to the Blyholder model [18], shifts of the v(CO) frequency from the free CO value (2143 cm 1) were attributed to backdonation from the metal into the 27r* orbital, thereby weakening the CO bond. Shifts to higher frequencies with increasing coverage were then attributed to increasing competition for the backdonated electrons. More recently, it has been shown [3] that the magnitude of the shift can be reproduced with increasing dilution of the C120 with C’30 indicating that instead, the effect is due entirely to dipole—dipole interactions within the adlayer. Indeed, it has been estimated that at least half the singleton shift of 78 cm 1 from the free r CO frequency (2143 cm 1) is due to dipole coupling with its own image. This would imply that backdonation plays a smaller role in linear CO bonding than hitherto expected. Detailed information of this kind can be obtained only with the high resolution (~-‘Scm 1) attainable by IRS, although a recent EELS study [19] has claimed to have seen a coverage induced frequency shift of 32 cm 1 in the related (but not identical) adsorption of CO on Pt(001). Comparable EELS frequency shifts on Pt(lll) have not been reported. Reliable observation of shifts of this magnitude are near the limit of the best modern EELS instruments which certainly are unable to reveal the detail seen by IRS. A second band near 1870 cm 1 has also been reported in a reflection IRS experiment [20]. However, this band is more commonly absent and indeed a concerted effort to detect it in a recent reflection experiment failed [3]. This contrasts with EELS where a second ~(CO) band, comparable in intensity to the 2100 cm 1 band is seen to develop at 1870 cm~after initial saturation of the linear species and is easily assigned to bridge-bonded CO. EELS also reveals two companion bands at 384 cm 1 and 465 cm 1 due to the ~(PtC) modes of the bridge- and linearbonded species, respectively. Fig. 2 shows the EELS spectrum corresponding to the half-monolayer c(4 X 2) surface structure [21]. The genesis of these bands with increasing exposure to CO was compared with the LEED patterns seen at the same exposure in the same apparatus. First, the 2100 cm 1 and 465 cm 1 bands of the linear species appear and temporarily saturate with the development of a diffuse (~f3X \/~)R30°pattern [21]. With further adsorption, the bridge-bonded bands developed and saturate with the appearance of the well ordered c(4 X 2) structure. The surface nets associated with these structures is shown in fig. 3. The location of the c(4 X 2) net on the Pt(1 11) surface to yield equal occupation of the bridge- and linear-bonded species becomes obvious in view of the comparable intensities of the bridge- and linear-bonded v(CO) bands [21 23]. In this way, the combined EELS LEED results have shown that the linear species must reside in an on-top configuration and not in the trigonal sites favored by theoretical considerations [24]. At saturation, LEED predicts that the c(4 X 2) structure contains 7.5 X 1014 molecules cm 2 and is in good agreement with experimental estimates of (7.1 X 1014 molecules cm 2 [3] and 7.5 X 1014 molecules cm 2 [261). On the other hand, a satu~.
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CO or Pt (111) EE o 00 o f.0
0
100
200
300
400
energy loss (meV) Fig. 2. EELS spectrum associated with the half-monolayer c(4 x 2) structure of CO on Pt (Ill).
rated (\/~X ~/~)R30° structure should contain 5.0 X 1014 molecules, i.e. more linear-bonded species than in the c(4 X 2) structure. Since neither the IRS or EELS work show an attenuation of the linear v(CO) band as the (\/~X \/~)R3O°structure gives way to the c(4 X 2), it is probable that the (‘./3 X \/3)R30°structure is incompletely saturated and, in view of the isolated and then island-like growth of this phase [31,is consistent with the diffuse nature of the LFED pattern. Instead, it appears that the adsorbate only “locks-in” to a well ordered structure, the c(4 X 2). when there is equal occupation of linear and bridge-bonded species [3], If, as is supposed, IRS and the dipole EELS data result from the same excitation of the surface dipoles and contain essentially the same information, the failure of IRS to reliably detect bridge-bonded CO on Pt(l 11) remains a significant discrepancy between the two techniques.
Fig. 3. Adsorbate nets for CO on Pt (111) adsorbed in the ~
X \/~)R30°(solid line), c(4 X 2) (dashed line) and “compressed” (dot-dashed line) structures. Note that whereas the (~.J3X sf3) R30° and c(4 x 2) structures occupy sites of high symmetry, the “compressed” structure ineludes molecules of lower symmetry and varh~h1ecemibridged bonding to the substrate.
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4.2.e> 0.5
In common with the adsorption of CO on a range of single crystal metal surfaces, adsorption of CO on Pt(l 11) has been shown to undergo a characteristic continuous LEED beam movement as the coverage increases beyond that required for the formation of the relatively simple c(4 X 2) superstructure [16,21]. Following the first description of the effect on Pd(001) by Tracey and Palmberg [26], it has been common to describe effects of this kind as a continuous uniaxial compression of the adlayer. For Pt(1 11), the saturated structure appears after long exposure (‘-‘200 L) at 170 K and corresponds to the compressed structure shown in fig. 2. The existence of these “compressed” structures has been questioned recently [21,27,28] and, instead, structures based on extended line defects have been proposed. An essential difference between the two models is that whereas the defect model results in all adniolecules residing in well defined sites of low symmetry (with perhaps some small relaxation to avoid congestion) the “compression” model demands that the admolecules occupy non-specific sites apparently oblivious to the periodicity of the surface potential. The sensitivity of the vibrational spectroscopies to the mode of bonding is expected to be a decisive test for the two models. To this end, Baro and Thach [23] attempted an experiment of this type and reported a shift in the ~(PtC) band of the bridge-bonded species from 350 cm to 390 cm as this species saturated. Similarly, the concurrent appearance of a weak band at 720 cm 1 was also reported and assigned to the asymmetric stretching mode of a bridge-bonded CO. Since this band is dipole forbidden when the admolecule has C2~symmetry, the 720 cm 1 band was seen as evidence that the species was moving to sites of lower symmetry as required by the “compression” model. However, it is surprising that no change was seen in the companion v CO mode or in either the v(PtC) or v(CO) bands of the linear species. Indeed, it is doubtful that with the exposures used in this work, roughly 1 L, that the high coverage phase was ever attained. More recently, EELS data has been obtained in an apparatus where the existence of the high coverage phase could be confirmed directly with LEED [21]. In this higher resolution work (6 meV), the ~(PtC) bands of the two species were clearly resolved but failed to show the previously reported peak shift of the bridge-bonded band at any stage during the adsorption process. Similarly, the band at 720 cm 1 was not seen. Indeed, no change was seen in either the band frequency or halfwidth at any stage of adsorption and the only effect of increasing adsorption into the 0 >0.5 phase was a reversible 20% increase in the 2100 cm 1 band with a concomitant 50% decrease in the 1870 cm band. Similar changes were also seen for the corresponding e(PtC) bands. This insensitivity of the bands in the 0 > 0.5 phase is seen as evidence that both the linear- and bridge-bonded species remain adsorbed on sites of high symmetry (C,,. and C2~,respectively). Surface structures, incorporating extended line defects,
178
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.,J.I
/
Vibrational spectroscopy
~
0=0.6
~v-~-~ • •
•1•
•
•‘
~
!_~
I ~. S •~s
0=067
~ I•~!.~
~ Pt 1110]
Pt 10011
Fig. 4. Extend defect structures for the high coverage phase (0 > 0.5) of CO on Pt (111) corresponding to 0 — 0.6 and saturation at 0 = 0.67.
which account for the observations are shown in fig. 4. They consist of strips of c(4 X 2) structure which are antiphase with adjacent strips across defect lines of linear rich species. Increasing coverage is accommodated by decreasing the spacing between the defect lines until a natural limit is reached at 0 = 2/3. If the v(CO) intensities reflect the relative surface coverage of the two species, the observed 2.8 increase in the peak height of the linear species relative to the bridged is in tolerable
agreement with that expected from a species-count of 3.0 for the extended defect model. Kinematically, structures of this kind are expected to produce a splitting of selected c(4 X 2) LEED beams which is inversely proportional to the domain width [29]. A structure factor analysis of the structures shown in fig. 4 show that the c(4 X 2) beams do split in the expected manner, but, significantly, the most intense
beam of the split pair is just the beam seen in the LEED pattern [29]. A small relaxation of linear species in the defect line is also expected in order to accommodate the excessively close packing [211. These extended defect structures also account for the work function data in a simple way. If the linear- and bridge-bonded species are assigned positive and negative surface potentials, respectively, the work function data of Ertl et al. [17] is ex-
plained qualitatively. Thus, the initial decrease in work function of 0.16 eV can be attributed to adsorption into the linear-bonded species of the (‘.f~X ..f3)R30° structure. This is countered by subsequent adsorption of the bridge-bonded species until the work function is returned to near its clean surface value with the development of the half monolayer c(4 X 2) phase. Significantly, further adsorption into
the 8 > 0.5 phase leads to a decrease in work function of 0.06 eV as the linearbonded CO increasingly dominate this phase. While there are clearly quantitative inadequacies and differences in detail with this interpretation, the observed trends are
accounted for easily.
N.R. A very / Vibrational spectroscopy
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5. Summary
For the most part, dipole EELS and IRS provide directly comparable information with the greater sensitivity and spectral range of EELS being countered by the superior resolution of IRS. For adsorption of CO on Pt(l 11), IRS has been responsible for revealing the subtle detail of the low coverage adsorption where small changes in the frequency of the linear-bonded ~ CO band have been interpreted as
initial isolated admolecule adsorption followed by island formation [3]. Similarly, quantitative isotope dilution experiments have questioned the high level of backdonation into the 21r* molecular orbital of CO which hitherto has been assumed. Significantly, however, IRS has failed to reliably detect the v(CO) band of the bridge-
bonded species which is detected easily by EELS as the half monolayer c(4 X 2) structure develops. Changes which occur in the EELS bands as the high coverage, 0 > 0.5, phase develops have been regarded as inconsistent with the “compression” model and instead, favor an extended line defect model which allows on-site adsorption in this coverage regime [21,301.
References [1] L.G. Christophorou, Atomic and Molecular Radiation Physics (Wiley, New York, 1970). [2] H. Ibach, Surface Sci. 66 (1977) 56. [3] A. Crossley and D.A. King, Surface Sd. 95 (1980) 131. [4] K. Horn and J. Pritchard, J. Phys. 38 (1977) C4164. [5] L.H. Little, Infrared Spectra of Adsorbed Species (Academic Press, New York, 1966). [6] iT. Yates, T.M. Duncan, S.D. Worley and R.W. Vaughan, J. Chem. Phys. 70 (1979) 1219. [71H. Ibach, H. Hopster and B.A. Sexton, AppI. Surface Sci. 1(1977)1. [8] H.A. Pearce and N. Sheppard, Surface Sci. 59 (1976) 205. [9] W. Ho, R.F. Willis and E.W. Plummer, Phys. Rev. Letters 40 (1978) 1463. [101 B.A. Sexton and C.E. Mitchell, Surface Sci. 99 (1980) 539. [11] N.R. Avery, unpublished results.
[12] F.M. Propst and T.C. Piper, J. Vacuum Sci. Technol. 4(1967)53. [13] H. Fioitzheini, H. Ibach and S. Lehwald, Rev. Sci. Instrum. 46 (1975) 1325. [14] C.E. Kuyatt and iA. Simpson, Rev. Sci. Instrum. 38 (1967) 103. [15] E. Hasting and F.H. Read, Electrostatic Lenses (Elsevier, Amsterdam, 1976). [161G. ErtI, M. Neumann and K.M. Streit, Surface Sci. 64 (1977) 393. [171R.P. Eischens and W.A. Pliskin, Advan. Catalysis 10 (1958) 1.
[181G. Blyholder, J. Phys. Chem. 68 (1964) 2772. [19) [20] [21] [22] [23]
G. Pirug, H. Hopster and H. Ibach, Surface Sci., to be published. H. Krebs and H. Luth, Appl. Phys. 14 (1977) 337. N.R. Avery, J. Chem. Phys. 74 (1981) 4202. H. Froitzheim, H. Hopster, H. Ibach and S. Lehwald, Appi. Phys. 13 (1977) 147. A.M. Baro and H. Ibach, J. Chem. Phys. 71(1979) 4812.
[24] G. Doyen and G. Ertl, Surface Sci. 43 (1974) 197. [25] (26] [27] [28] [29] [30]
R.W. McCabe and L.D. Schmidt, Surface Sci. 66(1977)101. J.C. Tracey and P. Palmberg, J. Chem. Phys. 51(1969)4852. J. Pritchard, Surface Sci. 79 (1979) 231. J.P. Biberian and M.A. Van Hove, Conf. Vibrations at Surfaces, Namur (September 1980). P.J. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1. N.R. Avery, in preparation.
179
VIBRATIONAL SPECTROSCOPY OF CO ADSORBED ON A Pt(l 11) SURFACE Neil R. AVERY CSIRO Division of Materials Science, University of Melbourne, Parkville, Victoria 3052, Australia Received 23 February 1981
A qualitative comparison between electron energy loss spectroscopy (EELS) and infrared absorption spectroscopy (IRS) for the vibrational analysis of adsorbed molecules is given. The relative merits of the two techniques are demonstrated with a review of CO adsorption on Pt (111). A description of a recently developed EELS apparatus based on concentric hemispherical dispersive elements is also given.
1. Introduction In this paper, the relative merits of electron energy loss spectroscopy (EELS) and infrared spectroscopy (IRS) for analysis of the vibrational modes of molecules adsorbed on metal surfaces will be compared. For work of this kind, the adsorption of CO on Pt(1 11) has been relatively well studied by both methods and will be used as an example to compare and contrast the two approaches. Also, a brief description of a new high resolution EELS instrument built in this laboratory will be given.
2. EELS and IRS comparison For EELS in the non-dispersive limit, the momentum transfer to the scatterer is minimized and the energy transfer occurs via the long-range Coulomb field of the incident electron with the dynamic dipoles of the vibrational modes in a manner similar to infrared photon absorption [11. In this way the two techniques are expected to yield essentially the same information. Indeed, Thach [2] has compared the effective charge for v(CO) on Pt(l 11) as determined by IRS and EELS and found perfect agreement. In practice, the dipole EELS data is obtained by tuning the elastic scattering geometry for strong specular reflection which necessarily requires massive single crystal surfaces. In this way a strong elastic channel is establithed into and out of the crystal so that the dipole EELS bands are expected to be strongly peaked near the specularly reflected elastic peak [21. For IRS of adsorbed molecules, the data is 0 378-5963/81/0000 0000/$ 02.75 © 1982 North-Holland
172
N.R. Avery
/
Vibrational spectroscopy
normally taken either after grazing incidence reflection from massive surfaces [3,4], which unlike EELS may be polycrystalline, or in transmission through high area small metal particles which are highly dispersed on an inert support [5]. Whereas the reflection IRS and EELS experiments are performed on surfaces which can be well defined in the modern surface physics sense, the dispersed particles of the transmission IRS experiment are much less well defined. Differences do arise, however, from the surface structure of the small particles. For examples, CO adsorption on alumina supported rhodium produces a gem dicarbonyl species with active symmetric (2101 cm—1) and asymmetric (2031 cm 1) modes [6]. Species of this kind were attributed to adsorption on isolated rhodium atoms and therefore would not be expected to form on the massive surfaces used in EELS and reflection IRS. A surface selection rule has been described for both the dipole EELS [7] and the IRS experiment [8]. This arises from strong dielectric coupling of the dynamic dipoles with the metal at vibration frequencies. Therefore, since both spectroscopies interact through their long-range electric fields, excitation can occur only if the dynamic dipoles have components normal to the surface; the horizontal modes being screened by their image dipoles. This surface selection rule for dipole excitations reduces the number of observed modes but can be useful in determining the orientation of admolecules relative to the image plane of the surface. For EELS, a second scattering mechanism has been recognized [9] in which the inelastic bands are more or less evenly distributed through k space and has been attributed to thort-range impact scattering. Impact scattering may lead to modes for. bidden by the dipole surface selection rule; but from the limited data currently available, it appears that for strong impact scattering, a necessary but not sufficient condition, is that hydrogen atoms are involved in the vibrational mode to a significant extent, e.g. W—H [9], ~~‘m N—H [10], C—H and 6 C—H [11]. There is no evidence for impact scattering playing a role in CO EELS and will not be considered further here. A significant difference between the two approaches to vibrational spectroscopy results from the i03—i04 times greater reactivity of a low energy electron with matter. Indeed, it is just this property of an electron which has led to its widespread use as a probe particle in surface spectroscopies. Whereas EELS data is easily obtained from well-defined single crystal surfaces, comparable IRS data can be obtained only with longer data acquisition times and then only with strongly infrared active molecules, like CO. Enhanced IRS sensitivity may be obtained with dispersed particles but difficulties then arise from infrared absorption, particularly at low frequencies, by the support material. Furthermore, some adsorbates may react chemically with the support making it difficult to separate the role of the metal. The lack of definition of these particles has already been referred to. A further consequence of the high reactivity of electrons with matter is that the EELS experiment must be performed in vacua where the electron mean free path is significantly greater than the dimensions of the apparatus. Typically, this limit is about 10~ Torr although the effective ,~‘
N.R. Avery
/ Vibrational spectroscopy
173
pressure of the crystal surface may be enhanced by up to 1 ~2 with suitably positioned multichannel array molecular beam dosers, or up to 10~with supersonic nozzles. On the other hand, a suitably designed infrared experiment can compensate for a residual gas pressure up to several hundred Torr which approaches more closely the conditions of applied adsorption and catalytic processes. Modern infrared spectrometers are capable of resolutions which far exceed the demands of condensed phase spectroscopies. Thus, sensitivity is often bought at the expense of resolution which is typicafly set at 2—15 cm 1, depending on the demands of the experiment. EELS instrumentation by contrast is stifi in the early stages of development, with the best current instruments operating at a resolution of ‘—40 cm~.However, performance at this level is difficult to achieve routinely and most data is collected at ‘--‘70 cm 1 resolution. Future developments will almost certainly lower this limit.
3. EELS instrumentation Compared with the profusion of electron spectroscopies which have developed over the past decade or so, high resolution EELS has been relatively slow to emerge. With vibrational bands occurring in the < 500 meV (1 meV 8.0658 cm~)region, the need to monochromize the incident electron beam has been the main detertent electron monochromatOr
filament
crystal
~~\\cHA
Fig. 1. Cut away view of an FFLS apparatuc based on hemispherical energy dispersive elements.
174
N.R. A very / Vibrational spectroscopy
to the development of EELS. Although pioneering work by an Urbana group [12] demonstrated the viability of the tecimique it was principally Thach’s group at Julich which routinely obtained resolutions (40 70 cm 1) where useful data could be gathered [13]. Whereas these instruments have used 127°cylindrical condenser energy dispersive elements, the instrument which has been developed in this laboratory has sought to exploit the superior intrinsic resolving power and point focusing properties of the concentric hemispherical design [14] (fig. 1). For the monochromator, the conflicting demands of high take-off kinetic energy (12 eV) from a hot tungsten filament and low pass kinetic energy in the hemispheres (‘--‘300 meV) was met with a tandem pair of independent strong zoom lenses. The second stage imaged the electron beam with controlled divergence on the equatorial plane of the hemispheres. After energy dispersion by the R0 2 field in the hemispheres onto the output aperture, the monochromized electron beam was accelerated to the desired impact energy (1 6 eV) and focused towards the crystal surface with a final zoom lens. All zoom lenses were of the aperture type with a gap-to aperture ratio of 0.5 to allow direct interpolation of the zoom characteristics from published tables [15]. The accelerating positive focus voltage was used in each case. Electrons scattered from the crystal in ±2.5°cone about the specular beam were retarded and focused onto the input aperture of the analyzer hemispheres by a single cylindrical zoom lens. In this lens, focusing of the scattered beam was relatively insensitive to the focus voltage so that the analyzer could be run in either a constant resolving power or a constant resolution mode. Detection was with a channel electron multiplier (CEM) operating in a particle counting mode. Experience in operating this instrument indicated that after initial iteration of the focus voltages and scattering geometry to maximize the elastic count rate, good resolution was mainly a function of beam alignment which was controlled by electrostatic deflection at many points around the electron path. In this way, the fwhni of the specularly reflected beam from a clean Pt(l 11) surface could be easily and routinely tuned to 45 55 cm 1 with a count rate at 2 4 X 10~counts per second in the elastic channel.
4. CO on Pt(111) 4.1. O~0.5 With its higher resolution, the early stage of CO adsorption is better described by the IRS reflection data although only the v(CO) band can be detected by this technique. Two independent studies [3,4] have shown that initial adsorption occurs essentially reversibly into an isolated (singleton) linear-bonded species with a band at 2065cm 1 which shifts to 2070cm at a coverage of 0.3 X 1014 molecules cm 2~ With further adsorption,island formation occurs yielding a new band at 2083 cm 1, which coexists with the random phase. Saturation at 300 K produces a c(4 X 2)
N.R. Avery / Vibrational spectroscopy
175
LEED pattern [161 and only a single ~(CO)band isseenat2l0l cm ~Earliertransmission IRS work with high area dispersed Pt particles [17] showed a similar monotonic thift from 2040 cm 1 to 2067 cm According to the Blyholder model [18], shifts of the v(CO) frequency from the free CO value (2143 cm 1) were attributed to backdonation from the metal into the 27r* orbital, thereby weakening the CO bond. Shifts to higher frequencies with increasing coverage were then attributed to increasing competition for the backdonated electrons. More recently, it has been shown [3] that the magnitude of the shift can be reproduced with increasing dilution of the C120 with C’30 indicating that instead, the effect is due entirely to dipole—dipole interactions within the adlayer. Indeed, it has been estimated that at least half the singleton shift of 78 cm 1 from the free r CO frequency (2143 cm 1) is due to dipole coupling with its own image. This would imply that backdonation plays a smaller role in linear CO bonding than hitherto expected. Detailed information of this kind can be obtained only with the high resolution (~-‘Scm 1) attainable by IRS, although a recent EELS study [19] has claimed to have seen a coverage induced frequency shift of 32 cm 1 in the related (but not identical) adsorption of CO on Pt(001). Comparable EELS frequency shifts on Pt(lll) have not been reported. Reliable observation of shifts of this magnitude are near the limit of the best modern EELS instruments which certainly are unable to reveal the detail seen by IRS. A second band near 1870 cm 1 has also been reported in a reflection IRS experiment [20]. However, this band is more commonly absent and indeed a concerted effort to detect it in a recent reflection experiment failed [3]. This contrasts with EELS where a second ~(CO) band, comparable in intensity to the 2100 cm 1 band is seen to develop at 1870 cm~after initial saturation of the linear species and is easily assigned to bridge-bonded CO. EELS also reveals two companion bands at 384 cm 1 and 465 cm 1 due to the ~(PtC) modes of the bridge- and linearbonded species, respectively. Fig. 2 shows the EELS spectrum corresponding to the half-monolayer c(4 X 2) surface structure [21]. The genesis of these bands with increasing exposure to CO was compared with the LEED patterns seen at the same exposure in the same apparatus. First, the 2100 cm 1 and 465 cm 1 bands of the linear species appear and temporarily saturate with the development of a diffuse (~f3X \/~)R30°pattern [21]. With further adsorption, the bridge-bonded bands developed and saturate with the appearance of the well ordered c(4 X 2) structure. The surface nets associated with these structures is shown in fig. 3. The location of the c(4 X 2) net on the Pt(1 11) surface to yield equal occupation of the bridge- and linear-bonded species becomes obvious in view of the comparable intensities of the bridge- and linear-bonded v(CO) bands [21 23]. In this way, the combined EELS LEED results have shown that the linear species must reside in an on-top configuration and not in the trigonal sites favored by theoretical considerations [24]. At saturation, LEED predicts that the c(4 X 2) structure contains 7.5 X 1014 molecules cm 2 and is in good agreement with experimental estimates of (7.1 X 1014 molecules cm 2 [3] and 7.5 X 1014 molecules cm 2 [261). On the other hand, a satu~.
176
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/ Vibrational spectroscopy
CO or Pt (111) EE o 00 o f.0
0
100
200
300
400
energy loss (meV) Fig. 2. EELS spectrum associated with the half-monolayer c(4 x 2) structure of CO on Pt (Ill).
rated (\/~X ~/~)R30° structure should contain 5.0 X 1014 molecules, i.e. more linear-bonded species than in the c(4 X 2) structure. Since neither the IRS or EELS work show an attenuation of the linear v(CO) band as the (\/~X \/~)R3O°structure gives way to the c(4 X 2), it is probable that the (‘./3 X \/3)R30°structure is incompletely saturated and, in view of the isolated and then island-like growth of this phase [31,is consistent with the diffuse nature of the LFED pattern. Instead, it appears that the adsorbate only “locks-in” to a well ordered structure, the c(4 X 2). when there is equal occupation of linear and bridge-bonded species [3], If, as is supposed, IRS and the dipole EELS data result from the same excitation of the surface dipoles and contain essentially the same information, the failure of IRS to reliably detect bridge-bonded CO on Pt(l 11) remains a significant discrepancy between the two techniques.
Fig. 3. Adsorbate nets for CO on Pt (111) adsorbed in the ~
X \/~)R30°(solid line), c(4 X 2) (dashed line) and “compressed” (dot-dashed line) structures. Note that whereas the (~.J3X sf3) R30° and c(4 x 2) structures occupy sites of high symmetry, the “compressed” structure ineludes molecules of lower symmetry and varh~h1ecemibridged bonding to the substrate.
N.R. A very / Vibrational spectroscopy
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4.2.e> 0.5
In common with the adsorption of CO on a range of single crystal metal surfaces, adsorption of CO on Pt(l 11) has been shown to undergo a characteristic continuous LEED beam movement as the coverage increases beyond that required for the formation of the relatively simple c(4 X 2) superstructure [16,21]. Following the first description of the effect on Pd(001) by Tracey and Palmberg [26], it has been common to describe effects of this kind as a continuous uniaxial compression of the adlayer. For Pt(1 11), the saturated structure appears after long exposure (‘-‘200 L) at 170 K and corresponds to the compressed structure shown in fig. 2. The existence of these “compressed” structures has been questioned recently [21,27,28] and, instead, structures based on extended line defects have been proposed. An essential difference between the two models is that whereas the defect model results in all adniolecules residing in well defined sites of low symmetry (with perhaps some small relaxation to avoid congestion) the “compression” model demands that the admolecules occupy non-specific sites apparently oblivious to the periodicity of the surface potential. The sensitivity of the vibrational spectroscopies to the mode of bonding is expected to be a decisive test for the two models. To this end, Baro and Thach [23] attempted an experiment of this type and reported a shift in the ~(PtC) band of the bridge-bonded species from 350 cm to 390 cm as this species saturated. Similarly, the concurrent appearance of a weak band at 720 cm 1 was also reported and assigned to the asymmetric stretching mode of a bridge-bonded CO. Since this band is dipole forbidden when the admolecule has C2~symmetry, the 720 cm 1 band was seen as evidence that the species was moving to sites of lower symmetry as required by the “compression” model. However, it is surprising that no change was seen in the companion v CO mode or in either the v(PtC) or v(CO) bands of the linear species. Indeed, it is doubtful that with the exposures used in this work, roughly 1 L, that the high coverage phase was ever attained. More recently, EELS data has been obtained in an apparatus where the existence of the high coverage phase could be confirmed directly with LEED [21]. In this higher resolution work (6 meV), the ~(PtC) bands of the two species were clearly resolved but failed to show the previously reported peak shift of the bridge-bonded band at any stage during the adsorption process. Similarly, the band at 720 cm 1 was not seen. Indeed, no change was seen in either the band frequency or halfwidth at any stage of adsorption and the only effect of increasing adsorption into the 0 >0.5 phase was a reversible 20% increase in the 2100 cm 1 band with a concomitant 50% decrease in the 1870 cm band. Similar changes were also seen for the corresponding e(PtC) bands. This insensitivity of the bands in the 0 > 0.5 phase is seen as evidence that both the linear- and bridge-bonded species remain adsorbed on sites of high symmetry (C,,. and C2~,respectively). Surface structures, incorporating extended line defects,
178
N.R. Avery
.,J.I
/
Vibrational spectroscopy
~
0=0.6
~v-~-~ • •
•1•
•
•‘
~
!_~
I ~. S •~s
0=067
~ I•~!.~
~ Pt 1110]
Pt 10011
Fig. 4. Extend defect structures for the high coverage phase (0 > 0.5) of CO on Pt (111) corresponding to 0 — 0.6 and saturation at 0 = 0.67.
which account for the observations are shown in fig. 4. They consist of strips of c(4 X 2) structure which are antiphase with adjacent strips across defect lines of linear rich species. Increasing coverage is accommodated by decreasing the spacing between the defect lines until a natural limit is reached at 0 = 2/3. If the v(CO) intensities reflect the relative surface coverage of the two species, the observed 2.8 increase in the peak height of the linear species relative to the bridged is in tolerable
agreement with that expected from a species-count of 3.0 for the extended defect model. Kinematically, structures of this kind are expected to produce a splitting of selected c(4 X 2) LEED beams which is inversely proportional to the domain width [29]. A structure factor analysis of the structures shown in fig. 4 show that the c(4 X 2) beams do split in the expected manner, but, significantly, the most intense
beam of the split pair is just the beam seen in the LEED pattern [29]. A small relaxation of linear species in the defect line is also expected in order to accommodate the excessively close packing [211. These extended defect structures also account for the work function data in a simple way. If the linear- and bridge-bonded species are assigned positive and negative surface potentials, respectively, the work function data of Ertl et al. [17] is ex-
plained qualitatively. Thus, the initial decrease in work function of 0.16 eV can be attributed to adsorption into the linear-bonded species of the (‘.f~X ..f3)R30° structure. This is countered by subsequent adsorption of the bridge-bonded species until the work function is returned to near its clean surface value with the development of the half monolayer c(4 X 2) phase. Significantly, further adsorption into
the 8 > 0.5 phase leads to a decrease in work function of 0.06 eV as the linearbonded CO increasingly dominate this phase. While there are clearly quantitative inadequacies and differences in detail with this interpretation, the observed trends are
accounted for easily.
N.R. A very / Vibrational spectroscopy
179
5. Summary
For the most part, dipole EELS and IRS provide directly comparable information with the greater sensitivity and spectral range of EELS being countered by the superior resolution of IRS. For adsorption of CO on Pt(l 11), IRS has been responsible for revealing the subtle detail of the low coverage adsorption where small changes in the frequency of the linear-bonded ~ CO band have been interpreted as
initial isolated admolecule adsorption followed by island formation [3]. Similarly, quantitative isotope dilution experiments have questioned the high level of backdonation into the 21r* molecular orbital of CO which hitherto has been assumed. Significantly, however, IRS has failed to reliably detect the v(CO) band of the bridge-
bonded species which is detected easily by EELS as the half monolayer c(4 X 2) structure develops. Changes which occur in the EELS bands as the high coverage, 0 > 0.5, phase develops have been regarded as inconsistent with the “compression” model and instead, favor an extended line defect model which allows on-site adsorption in this coverage regime [21,301.
References [1] L.G. Christophorou, Atomic and Molecular Radiation Physics (Wiley, New York, 1970). [2] H. Ibach, Surface Sci. 66 (1977) 56. [3] A. Crossley and D.A. King, Surface Sd. 95 (1980) 131. [4] K. Horn and J. Pritchard, J. Phys. 38 (1977) C4164. [5] L.H. Little, Infrared Spectra of Adsorbed Species (Academic Press, New York, 1966). [6] iT. Yates, T.M. Duncan, S.D. Worley and R.W. Vaughan, J. Chem. Phys. 70 (1979) 1219. [71H. Ibach, H. Hopster and B.A. Sexton, AppI. Surface Sci. 1(1977)1. [8] H.A. Pearce and N. Sheppard, Surface Sci. 59 (1976) 205. [9] W. Ho, R.F. Willis and E.W. Plummer, Phys. Rev. Letters 40 (1978) 1463. [101 B.A. Sexton and C.E. Mitchell, Surface Sci. 99 (1980) 539. [11] N.R. Avery, unpublished results.
[12] F.M. Propst and T.C. Piper, J. Vacuum Sci. Technol. 4(1967)53. [13] H. Fioitzheini, H. Ibach and S. Lehwald, Rev. Sci. Instrum. 46 (1975) 1325. [14] C.E. Kuyatt and iA. Simpson, Rev. Sci. Instrum. 38 (1967) 103. [15] E. Hasting and F.H. Read, Electrostatic Lenses (Elsevier, Amsterdam, 1976). [161G. ErtI, M. Neumann and K.M. Streit, Surface Sci. 64 (1977) 393. [171R.P. Eischens and W.A. Pliskin, Advan. Catalysis 10 (1958) 1.
[181G. Blyholder, J. Phys. Chem. 68 (1964) 2772. [19) [20] [21] [22] [23]
G. Pirug, H. Hopster and H. Ibach, Surface Sci., to be published. H. Krebs and H. Luth, Appl. Phys. 14 (1977) 337. N.R. Avery, J. Chem. Phys. 74 (1981) 4202. H. Froitzheim, H. Hopster, H. Ibach and S. Lehwald, Appi. Phys. 13 (1977) 147. A.M. Baro and H. Ibach, J. Chem. Phys. 71(1979) 4812.
[24] G. Doyen and G. Ertl, Surface Sci. 43 (1974) 197. [25] (26] [27] [28] [29] [30]
R.W. McCabe and L.D. Schmidt, Surface Sci. 66(1977)101. J.C. Tracey and P. Palmberg, J. Chem. Phys. 51(1969)4852. J. Pritchard, Surface Sci. 79 (1979) 231. J.P. Biberian and M.A. Van Hove, Conf. Vibrations at Surfaces, Namur (September 1980). P.J. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1. N.R. Avery, in preparation.