Langmuir 1994,10, 3649-3657
3649
Symmetry and the Surface Infrared Selection Rule for the Determination of the Structure of Molecules on Metal Surfaces Jingfu Fan and Michael Trenary* Department of Chemistry, M f C 111, University of Illinois at Chicago, 845 W . Taylor Street, Chicago, Illinois 60607 Received March 21, 1994. In Final Form: July 8, 1994@ Reflection absorption infrared spectra for several adsorbates on Pt(ll1) are presented that illustrate the use of symmetry in applyingthe surface IR selection rule. The selection rule states that only vibrations that belong to the representation of the surface normal will be surface IR active. The selection rule does not forbid vibrations that are dominated by the stretching of bonds that are parallel to the surface, and we show two such examples: the 0-0 stretch of adsorbed 02 and the C-C stretch of di-a bonded ethylene. Using multilayer CH3I as an example, we show how IR bands can be split due to formation of structures with symmetry different from the isolated molecule. Spectra of submonolayer coverages of CH3, ICH3, CCH3, and CCD3 demonstrate that, even for adsorbates having the same symmetry and same functional group, the relative intensities of the allowed vibrations can be quite different.
Introduction Symmetry plays a central role in the selection rules governing the spectra of molecules in the gas, liquid, and solid phases. In surface infrared spectroscopythe selection rule is that a vibrational mode will be IR active if there is a non-zero projection of the dynamic dipole moment along the surface normal.' In the language of group theory, this is equivalent to stating that avibrational state can be accessed by an electric dipole transition from the ground vibrational state if the upper state belongs to the same irreducible representation as the surface normal. Most of the infrared spectra of molecular adsorbates that have been studied to date have been simple enough to permit interpretation without the use of molecular symmetry and group theory. As infrared spectroscopybecomes more widely used for polyatomic adsorbates, it will become increasingly important to employ group theory whenever possible to analyze the spectra. In this paper we present IR spectra of several molecules on the Pt(ll1) surface that illustrate the power of using symmetry in the application of the surface infrared selection rule. The examples include 02,CzH4, ICH3, CCH3, and CH3. There have been several studies of 0 2 on the Pt(ll1) ~ u r f a c e . ~These - ~ studies show that at temperatures below 160 K 0 2 adsorbs molecularly to form a peroxo species with a bond order of 1as compared to a gas phase bond order of2. The reduction in bond order is manifested by an 0-0 stretch at 870 cm-l on the surface, as observed with high-resolution electron energy loss spectroscopy (HREELS),2,3as compared to a value of 1550 cm-l for gas phase 02. Identification of the adsorbed 02 as a peroxo species directly implies that the 0-0 bond is parallel to the surface, a conclusion that has been confirmed through N E W S ~ t u d i e s While .~ the observation of a strong 0-0 stretch a t 878 cm-l with the 0-0 bond parallel to the surface might seem at odds with the surface selection Abstract published in Advance A C S Abstracts, September 1, 1994. (l)Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 214. Bradshaw, A. M.; Schweizer, E. In Spectroscopy ofSurfuces; Clark, R. J. H., Hexter, R. E., Eds.; Wiley: London, 1988. (2) Gland, J. L.; Sexton, B. A.; Fisher, G. B. Surf. Sci. 1980,95,587. ( 3 ) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 1. (4) Canning, N. D. S.; Chesters, M. A. J. Electron Spectrosc. Rel. Phenom. 1983,29,69. (5) Outka, D. A.; Stiihr, J.; Jark, W.; Stevens, P.; Solomon,J.;Madix, R. J . Phys. Rev. B . 1987,35, 4119. @
rule, group theory shows that this mode is IR allowed. Although similar IR spectra were reported for 0 2 on a structurally uncharacterized Pt foil by Canning and Chesters: the IR spectrum of 02 on Pt(ll1)has not been reported previously. Similar considerations apply to C2H4 which is di-a bonded to Pt(ll1)with the C-C axis parallel to the surface.6 We observe a band associated with the C-C stretch which, on the basis of the local symmetry of the molecule, is again consistent with the surface infrared selection rule. Although the IR spectrum of di-a bonded ethylene on R(ll1)has not been previously reported, it has been studied with HREELS. The higher resolution of IR permits clarification of some ambiguities in assignment in the HREELS There have been several studies of methyl halides on the Pt(ll1)~urface'~-'~ withmany ofthe studies concerned with the thermal or photodecomposition to form chemisorbed methyl. While we present IR spectra here for CH3 produced from the decomposition of CH31,we are mainly concerned with the relationship between the orientation of the CH3I molecule on the surface and the IR spectra. This is a particularly nice illustration of symmetry and the IR selection rule as the molecule adsorbs in a tilted geometry which permits observation of all the IR-active fundamentals above 800 cm-'. Furthermore, the spectra show changes with coverage due to reorientation of the molecule. We also use this molecule as an example of how quantitative information on molecular orientation can be obtained from the relative intensities of the IR bands. Although similar IR spectra for CH31on Pt(ll1) have been presented by Zaera et al.,13J4we give a more detailed treatment here. (6) Sheppard, N. Annu. Rev. Phys. Chem. 1988,39, 589. (7) Demuth, J. E.; Ibach, H.; Lehwald, S. Phys. Rev. Lett. 1978,40, 1044. (8) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978,15,407. (9) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982,117,685. (10)Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 184, L352. (ll)Radhakrishnan, G.; Stenzel, W.; Hemmen, R.; Conard, H.; Bradshaw, A. M. Appl. Surf. Sci. 1990,46, 36. (12)Radhakrishnan, G.; Stenzel, W.; Hemmen, R.; Conard, H.; Bradshaw, A. M. J. Chem. Phys. 1991,95, 3930. (13) Zaera, F.; Hoffmann, H. J. Phys. Chem. 1991,95,6297. (14) Zaera, F.; Hoffmann, H.; Griffiths, P. R. J. Electron Spectrosc. Rel. Phenom. 1990, 54/55, 705.
0743-746319412410-3649$04.50/00 1994 American Chemical Society
3650 Langmuir, Vol. 10, No. 10, 1994 Finally, we present spectra for CH31 at multilayer coverages and compare the results with vibrational studies of crystalline CH31.15-20 The spectra are interpreted accordingto the symmetry properties of the known crystal structure of CH3LZ1Earlier vibrational studies ofthe solid did not report a few spectral features observed in our study, only some of which are due to the fact that the films are grown on a metal single-crystal surface. Furthermore, the earlier vibrational studies of solid CH3I were analyzed with the assumption that the solid adopts the C~~'~-Cmc21 crystal structure, whereas a later X-ray diffraction study21 showed that the structure is Dzh16-Pnma. This study thus represents the first interpretation of the vibrational spectra of solid CHJ based on the correct crystal structure. More importantly, many of the spectroscopicphenomena we see for the CH31 thin films are observable simply because of the much stronger signal levels associated with multilayer as opposed to submonolayer coverages. With further increases in signal-to-noise ratios in the surface IR experiment, similar phenomena should be observable for monolayers and submonolayers.
Experimental Section The basic experimental setup, except with the modifications noted below, has been described in detail elsewhere.22 In brief, it consists of a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure of -1 x Torr coupled to a commercial FTIR spectrometer (Mattson RS1). The IR beam enters and exists the UHV chamber through differentially pumped O-ringsealed salt (both CsI and KBr have been used) windows and is p-polarized before it reaches the IR detector. We used a liquid nitrogen cooled MCT (mercury-cadmium-telluride) detector with a low wavenumber cutoff near 800 cm-1. For better results above 2000 cm-l, we also used a liquid nitrogen cooled InSb (indium antimonide) detector with a low wavenumber cutoffnear 1950 cm-l. The IR spectra were taken with a resolution of 2 cm-l except where otherwise noted. Most spectra have been baseline corrected. The preparation and cleaning of the Pt(ll1)single crystal have been described previo~sly.~3 The crystal is considered clean when no impurities are observed with Auger electron spectroscopy. The methyl iodide was purchased from Aldrich Chemical Co., Inc. with aquoted purity of 99.95%. The CH3I was purified by transferring it to a glass bulb and subjecting it to five freezepump-thaw cycles using a liquid nitrogen bath. It was then placed in a dry ice-acetone bath and condensed into another glass bulb cooled by liquid nitrogen. The CZ& and 0 2 were purchased from Matheson Gas Products with quoted purities of 99.99%and 99.998%,respectively. The CzD4 was purchased from Cambridge Isotope Laboratories. The CZ&, C2D4, and 0 2 were used without further treatment. Exposures were made by backfilling the chamber to the desired pressure, as measured by an ionization gauge without corrections for gauge sensitivity. Exposures are reported in Langmuir units (1langmuir = 1 x Torrs).
Results and Discussion
Oxygen on Pt(ll1). Figure 1 shows the infrared spectrum taken with a resolution of 4 cm-l of molecular oxygen on Pt(ll1) obtained following a 9 langmuir exposure with the crystal at 82 K. This exposure gives (15)Mador, I. L.; Quinn, R. S. J . Chem. Phys. 1962,20,1837. (16)Dows, D. A.J. Chem. Phys. 1958,29,484. (17)Hexter, R. M. J. Chem. Phys. 1956,25,1286. (18)Jacox, M.E.; Hexter, R. M. J. Chem. Phys. 1961,35,183. (19)Kopelman, R. J. Chem. Phys. 1966,44,3547. (20)Verderame, F.D.; Nixon, E. R. J. Chem. Phys. 1966,45,3476. (21)Kawaguchi, T.;Hijkigawa, M.; Hayafuji, Y.; Ikeda, M.; Fukushima, R.; Tomiie, Y. Bull. Chem. SOC.Jpn. 1973,46,53. (22)Brubaker, M.E.;Trenary, M. J. Chem. Phys. 1986,85,6100. (23)Malik, I.J.; Brubaker, M. E.; Mohsin, S. B.; Trenary, M. J.Chem. Phys. 1987,87,5554.
Fan and Trenary
I 800
900
1000
Wavenumber (cm
-1
1100
)
Figure
1. 0-0 stretch of molecular oxygen taken at a resolution of 4 cm-l following a 9 langmuir 0 2 exposure with the crystal at 82 K.
a saturation coverage which previous studies have shown to be 0.44 m o n ~ l a y e r . ~In. ~the region above 800 cm-I the spectrum consists of a single band at 878.2 cm-l with a FWHM of 23 cm-l. In the previous HREELS studies2s3 the band was reported to be at 870 cm-I with the moleculesubstrate stretch at 380 cm-l. Confirmation of molecular adsorption under these conditions comes from thermal desorption and vibrational studies of isotopic mixture^.^^^ A previous infrared study4 of molecular oxygen on a recrystallized platinum foil reported the band at 875 cm-l with a FWHM of 20-25 cm-'. The fact that the same linewidth is observed on a Pt(ll1)single crystal as on a polycrystalline foil indicates that the FWHM is not dominated by inhomogeneousbroadening associated with a heterogeneous distribution of adsorption sites. The frequency of the 0-0 stretch is in the same range as for dioxygen transition metal complexes such as Pt(PPh3)~02~ and for the ROz species trapped in a rare gas matrix.25 In such systems the 0 2 binds to the metal in a side-on fashion and can be considered a peroxo species with a single 0-0 bond.24-26For these reasons it was proposed that, in analogy to these compounds, 02 bonds to a single surface platinum atom to form a species in the shape of an isosceles triangle.2 In the HREELS studies a second weaker 0-0 stretch was observed at 700 cm-', which Gland et a1.2 attributed to adsorption at defects and Steininger et aL3 assigned to a peroxo species bound to two platinum atoms. Since the detector used in our studies is limited to the region above 800 cm-l, we did not detect the 700 cm-I band. The local symmetry of the 02-Pt species on the surface corresponds to the Czupoint group with the twofold axis alongthe surfacenormal (the z axis). The surface selection rule implies that only normal modes which transform according to the A1 irreducible representation, the representation of z , will have IR active fundamentals. Both the 0-0 stretch and the molecule-substrate stretch are of A1 symmetry, and therefore both motions contribute t o the normal modes with fundamentals at 878 and 380 cm-I. Nakamura et al.24carried out a simple normal mode calculation for a PtOz triangle and concluded that the higher frequency mode was composed of 90%0-0 stretch (24)Nakamura, A.;Tatsuno, Y.; Yamamoto, M.; Otsuka, S. J.Am. Chem. SOC.1971.93.6052. (25)Huber, H.: Klotzbucher, W.; Ozin, G. A.; Vander Voet, A. Can. J . Chem. 1973,51,2722. (26)Jones, R.D.; Summenrille, D. A.; Basolo, F. Chem. Rev. 1979, 79,139.
Langmuir, Vol. 10, No. 10,1994 3651
Symmetry and the Surface Infrared Selection Rule a) 3L CH ,,
2
I
11340
cm-' Res
p.0004
I
1
1000
1200
1400
2400
2600
2800
3000
Wavenumber ( c m - l )
Figure 2. Spectrum of di-a bonded ethylene (C2H4) after a 3 langmuir c2H4 exposure at 100 K, and spectrum of ethylidyne (CCH3)obtained after the sample has been annealed to 300 K and then cooled back to 100 K.
I
800 900 1 0 0 0 1 1 0 0 1 2 0 0 1 3 0 0
Y
1900 2000
Wavenumber ( c m
-1
2100
2200
)
Figure 3. Spectrum of di-u bonded ethylene (C2D4) after a 3 langmuir CzD4 exposure at 81 K, and spectrum of ethylidyne (CCD3) obtained after the sample has been annealed to 300 K and then cooled back t o 81 K. and 10% 02-Pt stretch. This means that as the 0-0 bond stretches, the molecule-surface distance changes simultaneously. Such motion will necessarily give rise t o a dynamic dipole moment along the surface normal. Thus, symmetry considerations show that the observation of the 0-0 stretch with IR for an 0 2 molecule bound with the 0-0 bond parallel to the surface is perfectly consistent with the surface selection rule. Accounting for the intensity of the band in a quantitative fashion requires considerations beyond symmetry. For example, Canning and Chesters4 have suggested that charge oscillations between the metal and the 0 2 molecule may be the source of the strong intensity. Ethylene and Ethylidyne. Figure 2 shows the infrared spectrum of C2H4 obtained after the crystal is exposed a t 100 K to 3 langmuir of ethylene. The crystal was then annealed to 300 K to convert the ethylene to ethylidyne and cooled back to 100 K, and the upper spectrum was obtained. Figure 3 shows the analogous experiment for deuterated ethylene, except the lower temperature was 81 K. As indicated by the different intensity scales in Figures 2 and 3,the most intense bands for the deuterated species are weaker than for the nondeuterated species, resulting in lower SNR's in Figure
3. The C2H4 spectrum shows three major bands a t 2903, 1047,and 993cm-l and a weak band at 1414cm-'. Similar spectra have been obtained with HREELS, and our spectrum is in particularly good agreement with that of Steininger et al.9 The higher resolution of IR gives more precise band positions, and our 993 and 1047 cm-l bands are completely separate whereas the HREELS spectra show a barely resolved shoulder at 980 cm-l on the 1050 cm-' peak. Although these bands are difficult to resolve with HREELS, Felter et aL2' performed a calculation based on a model of di-a bonded ethylene that indicated that a single band at 990cm-l reported in early HREELS spectra was in fact composed of two unresolved peaks. Our assignment of the IR bands is the same as that of the HREELS study of Steininger.g Thus, the bands at 2903, 1414,1047,and 993 cm-l are assigned to the symmetric C-H stretch, v,(C-H), the CH2scissor mode, d,(CHz), the C-C stretch, v,(C-C), and the CH2 symmetric wag, ew(CHz), respectively. In the spectrum of C2D4 we assign the band a t 2138 cm-' to vs(C-D), 1135 cm-l to ds(CD2),and 895 cm-' to v.(C-C). The symmetric CD2 wag, ew(CD2),was assigned to a band observed at 740cm-l in the HREELS ~ p e c t r u m , ~ placing it below our low wavenumber cutoff. The band at 2035 in Figure 3 is due to background CO adsorption. One difference between our spectrum of C2D4 and that of Steininger et aL9is that we resolve a band they reported at 1150 cm-l into two bands at 1135 and 1154 cm-'. It is difficult to assign both of these bands to fundamentals so as to obtain reasonable isotopic shifts. Ifwe assign 1154 cm-' to d,(CDz), 1135 cm-l to vS(C-C), and 895 cm-' to ew(CD2),we would obtain an isotopic shift for the C-C stretch (w$wD) of 0.92 and a shift of 1.11 for the wag. Furthermore, we would fail to account for the 740 cm-l band observed with HREELS. For these reasons, we do not assign the 1154 cm-l band to a fundamental of C2D4. It may be the first overtone of the symmetric CD2 twist which Steiningerg reported at 600 cm-'. A Fermi resonance with the fundamental a t 1135 cm-' might give the overtone sufficient intensity to be observed. Another possibility, given the relatively poor signal-to-noise ratio for the two peaks, is that it is simply a spectral artifact. The spectra for ethylidyne shown in Figures 2 (CCH3) and 3 (CCD3) after a 300 K anneal are similar to spectra reported before under slightly different ~ o n d i t i o n s . ~ ~ ~ ~ * - ~ ~ For CCH3, the three strongbands at 2885,1340,and 1115 cm-l and the weak band at 2794 cm-l are assigned to v,(C-H), d,(CH3), v,(C-C), and the overtone of d,(CH3), respectively. For CCD3, the symmetric C-D stretch is a t 2141 cm-' while the more intense band at 2047 cm-' is due to adsorption of background CO. Distinguishing between the C-D stretch and background CO is based on the fact that the lower frequency band varied in intensity and frequency among the many spectra of CCD3 acquired, whereas the higher frequency band was quite reproducible. Also, this assignment yields a typical isotope shift of 1.35. The band at 1151 cm-' for CCD3 in Figure 3 is assigned to the C-C stretch. Conspicuously absent from the CCDB spectrum is the symmetric CD3 bend which should be at xl000cm-'. Although it has been observed with HREELS for CCD3 on Pt(lll)9 and with IR for an ethylidyne complex,3' it was extremely weak in both cases. This is (27)Felter, T. E.;Weinberg, W. H. Surf. Sci. 1981,103,265. (28) Chesters, M. A.; McCash, E. M. Surf. Sci. 1987,187,L639. (29)Malik, I. J.;Agrawal, V. K.; Trenary, M. J. Chem. Phys. 1988, 89,3861. (30)Mohsin, S.B.;Trenary, M.; Robota, H. J. Chem. Phys. Lett. 1989, 154,511. (31)Skinner, P.;Howard, M. W.; Oxton, I. A.; Kettle, S. F.; Powell, D. B.; Sheppard, N. J. Chem. SOC.Faraday Trans. 2 1981,77,1023.
Fan and Trenary
3652 Langmuir, Vol. 10, No. 10,1994 an example in which an allowed fundamental is intrinsically weak and thus unobservable. It also demonstrates that isotopic substitution can dramatically alter relative intensities; for CCH3 the symmetric CH3 bend is by far the most intense fundamental while for CCD3 the CD3 symmetric bend is unobservably weak. The di-a bonding of CzH4 to the Pt(ll1)surface with the C-C axis parallel to the surface and with a planar CzPtz skeleton gives the adsorbed molecule a local symmetry of Czuwith the twofold rotation axis along the surface normal. Only the normal modes that transform according to the totally symmetric representation, the representation of the surface normal, can be surface IR active. This makes four of the modes of the gas phase molecule active on the surface: the symmetric stretch, the CH2 wag, the CHZ scissor, and the C-C stretch. The di-a bonding configuration implies that the CHZplanes are tilted away from the surface, in which case it is obvious that the stretching of the C-H bond and the motion associated with changing the angle between the CH2 plane and the surface will give rise to a component of the dynamic dipole moment along the surface normal. By symmetry, all four of the totally symmetric modes contain some of these motions, and therefore all four modes have dynamic dipole moments along the surface normal. There is particularly strong mixing of the CHZwag and C-C stretch motions for the modes with fundamentals at 1047 and 993 cm-I such that the observed equal intensity of the two bands could be explained even if stretching the C-C bond alone does not give rise to a component of the dynamic dipole moment along the surface normal. A non-zero component of the dynamic dipole moment associated with stretching the C-C bond could arise due to charge oscillation between the molecule and the substrate. The situation is quite similar to the 0-0 stretch for 0 2 on Pt(ll1) which also has a local symmetry of CzU and where such charge oscillations have been proposed. The ethylidyne (CCHJ) case represents a straightforward example of the use of symmetry. With a local CsU structure, all of the A1 fundamentals are observed with good SNRs while the E fundamentals, whose positions are known from HREELS, are completely absent. While the fundamental of the asymmetric CH3 bend at -1400 cm-' is not observable, its first overtone has an A1 component and is allowed. Furthermore, by symmetry, it can have a Fermi resonance with the A1 symmetric C-H stretch without which it would surely have insufficient intensity to be observed. The CD3 symmetric bend of deuterated ethylidyne provides a contrasting but important example of a symmetry-allowed fundamental with insufficient intensity to be observed. Although we stress that it is the symmetry of the mode and not the orientation of bonds that determines which bands are observed, the fact that the C-C stretch is along the surface normal for ethylidyne but parallel to the surface for ethylene is consistent with the fact that ethylidyne has the more intense C-C stretch. A recent LEED calculation of ethylidyne on Pt(lll)32 indicates that the C-C axis is actually tilted away from the surface normal by -6" so that the local symmetry is not strictly C3,,. It is therefore important to assess whether a 6" tilt angle is consistent with the ethylidyne IR spectrum. The integrated area of infrared band i can be ~~
(32) Stake, U.;Barbieri, A.;Materer, N.;Van Hove, M. A,; Somojai, G.A. Surf. Sei. 1993,286,1.
written as33
*@!vi0G((3)
A. = hc a
where Mi,is the transition dipole moment component in the direction of the surface normal, n is the number of dipoles, a is the surface area, v, is the resonant frequency, and G((3)is a function which is dependent upon the angle of incidence and the dielectric constant of the substrate. All of the symmetric vibrational modes of ethylidynept(111)have their dynamic dipole moments parallel to the C3 axis, and all of the asymmetric vibrational modes have their dynamic dipole moments perpendicular to the axis. If we let /3 equal the angle between the C-C a x i s and the surface normal, then the projectionsof the dipole moments for the symmetric and asymmetric vibrational modes, M,, and Masp,are given by
M,, = M, coa /3
(2)
Mas, = Ma, sin p
(3)
where the subscripts "s" and "as" denote symmetric and asymmetric, respectively. By combining eqs 1-3, we obtain the following expression for the angle, p, between the C3 axis and the surface normal:
G(0)is only weakly frequency dependent, and G(O)JG(O), x 1. The resonant frequency v and the band area A can be measured from the IR spectrum. We use eq 4 to calculate the expected intensity of the asymmetric CH3 bend at 1420 cm-' on the basis of the observed intensity of the symmetric CH3 bend a t 1340 cm-'. Although the asymmetric bend was not observed here, it was observed to have approximatelythe same intensity as the symmetric CH3 bend in the IR spectrum31of CCH~CO~(CO)S. In the complex, the asymmetric bend is degenerate, and both modes of the degenerate pair are IR active. Hence we use MJM, = 4 in eq 4, which is derived for a single mode. Although the linewidth of da,(CH3)on the Pt(ll1)surface is not known, it was found to be twice as large as for d,(CH3) in a transmission IR study of ethylidyne on a W A1203 catalyst.23 If the same width ratio applies on Pt(1111,then for equal integrated areas the peak height of d,,(CH3) would be half that of d,(CH3). The band height of the symmetric CH3 bend in Figure 2 has an absorbance while the peak-to-peak noise is equal to an of 3.4 x in the region around 1450 cm-l. absorbance of ~7 x From eq 4 a tilt angle of 6" would give d,(CH3) a peak absorbance of only 2 x 10-5, placing it below the noise level. Ifthe FWHM ofd,(CHs) were 4-5 times the FWHM of d,(CH3), then it would be even further below the noise level of Figure 2. Because the FWHM of d,(CH3) for ethylidyne onPt(ll1)isunusuallynarrow atonly2.2 cm-', a value of 8-12 cm-I for the FWHM of d,,(CH3) is not unreasonable since linewidths of this size are commonly observed with RAIRS (reflection absorption infrared spectroscopy). Thus a -6" tilt angle for ethylidyne on Pt(ll1) is consistent with the IR spectrum of Figure 2. C& from CHSI Decomposition. Figure 4 shows a set of infrared spectra following a 3 langmuir exposure of CH31with the crystal at 81 K and upon annealing to 200 and 250 K. A MCT detector was used t o obtain these spectra. The spectrum at 81 K is of chemisorbed CH31 ____
(33) Persson, B. N. J.Solid State Commun. 1979, 30, 163.
Symmetry and the Surface Infrared Selection Rule
Langmuir, Vol. 10,No. 10,1994 3653
200K
&
r
n
81K 900
1050
1200
1350
2700
2850
3000
3150
-1
) Figure 4. Spectrum of CH31 obtained after a 3 langmuir exposure at 81 K followed by spectra obtained upon annealing to 200 and 250 K. By 250 K all of the CHBIhas dissociated to chemisorbed CH3. Wavenumber ( c m
2600
2700
2800
2900
Wavenumber (cm
-1
3000
)
Figure 5. Higher sensitivity spectrum obtained with an InSb detector ofthe C-H stretch region of chemisorbed CH3obtained by annealing chemisorbed CH3I to 240 K. with the following peak assignments: 872 cm-l, methyl rock, Q(CH~); 1222 cm-l, symmetric CH3 bend, d,(CH3); 1405 cm-l, asymmetric CH3 bend, da,(CH3); 2903/2911 cm-', symmetric C-H stretch, v,(CH); 3024 cm-l, asymmetric C-H stretch, v,(CH). A more detailed interpretation of the CH31 spectra will be given below. Here we focus on its decomposition. After the surface is annealed to 200 K, a new band a t 2875 cm-' appears, and the symmetric C-H stretch of methyl iodide is shifted to 2921 cm-l. After the surface is further annealed to 250 K, the band at 2875 cm-l increases and shifts to 2879 cm-l, and a new band appears at 2756 cm-l; at the same time, all the bands of methyl iodide disappear. Verification of the 2756 cm-l band is provided by the spectrum shown in Figure 5, which was obtained in the same way as the 250 Kannealed spectrum ofFigure 4 but with an InSb detector, which gives a lower noise level in the region above 1950 cm-l. The line shape of the symmetric C-H stretch at 2880 cm-' in Figure 5 clearly shows some asymmetry. This is in agreement with a prediction by T ~ b i that n ~ the ~ optical properties of the platinum substrate will cause asymmetry for a high-frequency band. On the basis of (34)Tobin, R. G. Phys. Rev. 1992,B45, 110.
the infrared spectra of ethylidyne on Pt(ll1)and of metal complexes containing methyl groups,31,35-39 the band at 2879 cm-l is assigned to the symmetric C-H stretch, and the band at 2756 cm-l to the overtone of the asymmetric CH3 bend of the surface methyl radical. After the surface is annealed to 300 K and above, all the IR bands of CH3(ad) disappear, and no new bands develop. We also measured TPD (temperature programmed desorption) spectra of CH31/Pt(lll)which show CH4 and Hz desorption a t 230-260 K in agreement with previous s t ~ d i e s . ' ~ J ~ Furthermore, our spectrum of chemisorbed CH3 obtained from CH31decomposition is virtually identical to what we obtain40through the direct exposure to gas phase methyl radicals produced from the thermal decomposition of gas phase (CH&Nz. All of these factors provide strong evidence that the species on the surface following a 300 K anneal of CH31is indeed chemisorbed CH3. Our infrared spectrum of chemisorbed CH3 is consistent with a local symmetry of C3" with the threefold axis along the surface normal. Thus, the fundamentals of the E symmetry modes, Q(CH~),da,(CH3), vas(CH), are not observed while the band at 2879 cm-l, v,(CH), is one'of the two symmetry-allowed fundamentals. The other symmetry-allowed fundamental, the symmetric CH3bend, appears to be too weak to observe. The band at 2756 cm-', like the band a t 2794 cm-l that we observe in the ethylidyne spectrum, is assigned to the overtone of das(CH3). The presence of this overtone is a common feature of the vibrational spectra of CH3 groups, including the spectra of metal complexes containing methyl group^.^^-^^ This assignment of the 2756 cm-' peak is different from that of Henderson et al.,1° who attributed it to a "soft)) C-H stretch due to hydrogen interacting with iodine and the platinum surface and concluded that methyl has C, symmetry on Pt(ll1). Our ability to observe the symmetric C-H stretch but not the symmetric CH3 bend, d,(CH3), for CH3 on Pt(ll1) is in contrast with the previous IR study13where a very weak band at 1244 cm-l was tentatively identified due to d,(CH3) although the symmetric C-H stretch was not observed. The symmetric CH3 bend has been observed wtih HREELS1°J2with good SNRs at 1150-1180 cm-l. In a HREELS study of methyl on Ni(lll), d,(CH3) is observed at 1220 cm-l and is the most intense band in the spectrum.41 The weakness of d,(CH3) in our IR spectrum of CH3 is in marked contrast to CCH3 and ICH3on Pt(ll1) where d,(CH3) has the highest intensity. This illustrates the important point that while frequencies are quite characteristic of a given functional group, relative intensities are not. Our observations on the variability of d,(CH3) in surface IR spectra is actually a general feature of CH3Y (Y = I, Br, C1, F, CeH5, etc.) molecules where the IR intensity of d,(CH3)varies strongly with the nature ofthe substituent group, Y.35-39,42-44 For example, d,(CH3) is very weak for (35)McKean, D. C. Spectrochim. Acta A 1973,29,1559. (36)Van De Vondel, D. F.; Van Den Berghe, E. V.; Van Der Kelen, G. P. J . Organomet. Chem. 1970,23,105. (37)Graves, R.; Homan, J. M.; Morgan, G. L.Znorg. Chem. 1970,9, 1592. (38)Clegg, D. E.;Hall, J. R. J. Orgunomet. Chem. 1970,22,491. (39)Higuchi, S.;Kuno, E.; Tanka, S.; H. Kamada, H. Spectrochim. Acta A 1972,28,1335. (40)Fairbrother, H.; Peng, X. D.; Viswanathan, R.; Stair, P. C.; Fan, J.;Trenary, M. Surf. Sci. Lett. 1993,285,L455. (41)Lee, M. B.; Yang, Q.Y.; Tang, S. L.; Ceyer, S.T. J . Chem. Phys. 1986,85,1693. (42)Dickson, A. D.; Mills, I. M.; Crawford, B., Jr. J . Chem. Phys. 1957,27,445. (43)Bishop, D. M.; Cheung, L. M. J. Phys. Chem. Ref. Data 1982, 11,119. (44)Castiglioni, C.; Gussoni, M.; Zerbi, G. J.Mol. Struct. 1986,141, 341.
Fan and Denary
3654 Langmuir, Vol. 10,No.10, 1994 _j/
!
Io.0002
1"'6 0,
S
0 C 0
m
I
N
e
0 In
n
4:
1230 I,
850
1300
1400
1500
2900
3000
-1
Wavenumber ( c m ) Figure 6. Submonolayer spectra at a resolution of 2 cm-' of CH3I at 81 K for exposures of (a)0.5, (b) 1.0, (c) 2.0, and (d)4.0 langmuir. The region from 850 to 1500 cm-l was obtained with a MCT detector while the region above 2850 cm-l was obtained in a separate experiment with an InSb detector.
---m--e4
1228
CH3F but becomes twice as strong as v,(C-H) for CH3I. A similar intensity variation is also found in many organic compounds. Higuchi et a1.39 measured the infrared absorption intensities of the symmetric stretch and bend modes of the methyl group for 13compounds. They found that there is a correlation between the intensity of 6,(CH3)and v,(C-H) and the polarization of CH3+-Y- due to the inductive effect of the substituent, Y. The 6,(CH3) is weak when the polarization is either weak or strong, but v,(C-H) is strong only when the polarization of the CH3+-Y- is weak. On the basis of Higuchi's argument,39 our results would suggest that the carbon-platinum bond of methyl is covalent-like with a weak polarization between platinum and methyl. Submonolayer Coverage of CHsI on F't(ll1). Figure 6 shows the infrared spectra a t a resolution of 2 cm-l of chemisorbed CH31as a function of exposure from 0.5 to 4 langmuirs. The spectra between 850 and 1500 cm-l were obtained with a MCT detector, and the spectra above 2850 cm-' with an InSb detector. Figure 7 shows spectra over a wider exposure range from 2 to 60 langmuirs, which correspond to coverages from submonolayer to many multilayers. At the lowest exposure of 0.5 langmuir, three bands centered a t 878,1230, and 1410 cm-l are observed in Figure 6a. No significant changes in band positions were observed for exposures less than 4 langmuirs. At a 4 langmuir exposure, the band initially at 1230 cm-l has shifted to 1216 cm-l and the intensities of the bands at 1409 and 872 cm-' have decreased markedly. At the same time, the two bands between 2900 and 2950 cm-' merge into a relatively broad band centered at 2909 cm-l. TPD spectra indicate that a 4 langmuir exposure corresponds to about 1 monolayer and that further exposures give multilayers. Most of the assignments for the monolayer and submonolayer spectra are straightforward and are given in Table 1. After a 10 langmuir exposure, a new band appears at 1242 cm-l due to the symmetric CH3 bend, d,(CH3), ofthe multilayer, and two very weak bands at 1400 and 1425 cm-l are observed in the asymmetric CH3 bend, da(CH3), region. At the same time, the symmetric C-H stretch v,(C-H) and CH3 rock e(CH3)of the chemisorbed CH31disappear. The band at 1242 cm-l and the two bands in the da(CH3)region continue to grow after 20 and 40 langmuir exposures. The symmetric C-H stretch and the CH3 rock of the methyl iodide multilayer appear a t -2948 and -885 cm-l, respectively, after a 40
z2905
1409
Symmetry and the Surface Infrared Selection Rule Table 2. Tilt Angle for (2-I
Langmuir, Vol. 10, No. 10, 1994 3655
on Pt(ll1) 10.001
N c
2 8 40
59
57
37 50
31 51
60
50
50
80 100
35 34
31 32
monolayer coverages using 0.5 cm-l resolution, no splitting of the asymmetric CH3 bend or methyl rock bands was observed. Figures 6 and 7 reveal that the relative peak intensities of the symmetric and asymmetric vibrational bands change with the CH3I exposure, indicating that the orientation of the molecule with respect to the surface normal has a strong dependence on coverage. The tilt angle as a function of exposure is given in Table 2 and was calculated from the relative intensities of the asymmetric CH3 bend and the CH3 rock to the symmetric CH3 bend using eq 4. Because there are no data available for the dynamic dipole moments of adsorbed methyl iodide, we use the data from gas phase IR.42 The degeneracy of a mode has to be included in calculating the dynamic dipole moment from the gas phase IR intensity. However, degeneracy is not included here in calculatingthe expected intensity ratios of the surface since in tilting a molecule such as CH31 away from the surface normal only one component of the degenerate pair would become surface IR active. As an internal consistency check, we use two pairs of infrared band areas in the calculation. Band areas rather than peak heights were used for the relative intensities because the d,(CH3) band is much sharper than the da(CH3) and Q(CH~) bands. As Table 2 shows, the molecular axis of methyl iodide is initially tilted by -60" from the surface normal and changes to -30" at about 1 monolayer. Evidently the molecules reorient to achieve a higher packing density as the coverage increases. In the first few multilayers the molecules are randomly oriented as indicated by the broad bands and low intensities in the 10 langmuir spectrum of Figure 7. A calculated tilt angle of 45" is characteristic of a multilayer with randomly oriented molecules. As discussed below, a structure at least locally like that of crystalline methyl iodide is apparently formed for exposures 260 langmuirs. It is found that organic thin films on metal substrates usually form crystalline structures.45 Zaera et al.14studied a series of alkyl halides on Pt(ll1) and found that it was very common €or orientation to change with coverage. Simple symmetry arguments alone cannot account for the presence of the two bands in the symmetric C-H stretch region of 2910-2930 cm-' and the one band in the asymmetric C-H stretch region at 3030 cm-l in Figure 6. There are three normal modes associatedwith the C-H stretch motion for the CH3 group. With CsUsymmetry these are the A1 symmetric stretch and the doubly degenerate E asymmetric stretch. Since the CH31molecule is tilted on the surface, the local symmetry can be no higher than C,. Upon reduction of the symmetry from CsUto C,, the degeneracy of the E vibrations is split into modes of the A and A"representations and the A1 modes become A. Only the A modes are surface IR active. The result of reducing the symmetry from CsUto C,by tilting would still give only two bands in the C-H stretch region, (45) Swalen, J. D.; Rabolt, J. F. Characterization oforientation and Lateral Order in Thin Films by Fourier Transform Spectroscopy. In Fourier Transform Infrared Spectroscopy, Applications to Chemical Systems 14; J. R., Ferraro, Basile, L.J., Eds.; 1985;p 283.
c
n
(D
1
ID
- I
I
1000
1200
1400
2800
2600
Wovenumber ( c m
-1
3000 3050
)
Figure 8. Spectrum of CH3I thin film at 2 cm-I resolution obtained with a MCT detector following a 100langmuirexposure at 81 K.
one associated with the former E modes and one with the former A1mode. If the tilting gave C1 symmetry, all modes would be surface IR active, and we would expect three bands. Although this can account for the number of observed bands, it requires that one of the former E modes be so highly perturbed that it shifts down in frequency to the point that it is very close to the former A1 symmetric stretch, Large decreases in C-H stretching frequencies due to "soft"C-H bonds have been reported p r e v i ~ u s l y . ~ ~ ~ ~ However, an interaction with the surface strong enough to split the degenerate C-H stretch by such a large amount would also be expectedto split the other degenerate bands at 1410 and 877 cm-l. Another explanation is that the bands at 2910 and 2928 cm-l are due to symmetric C-H stretches of molecules with different numbers of neighboring CH31molecules. In a study of PF3 on Pt(lll),we found that the symmetric P-F stretch split due to two distinct local arrangements of the molecules while the symmetric PF3 bend remained a single sharp peak.47In general, stretch vibrations appear to be more sensitive than bending vibrations to intermolecular interaction^.^^ This explanation for CH3I is consistent with the changes with coverage seen in Figures 6 and 7, although the situation is complicated by the reorientation which also accompanies changes in coverage. CHJ Thin Film on Pt(ll1). The infrared spectrum of a CH31thin film, obtained at 2 cm-l resolution, is shown in Figure 8. The surface was prepared by exposure of 100 langmuirs of CH3I to the surface at 81 K. Additional spectral features are revealed in the 0.5 cm-l resolution spectrum of Figure 9 obtained after a 240 langmuir exposure with the crystal at 81 K Similar to the case for the high-resolution infrared spectra of crystalline methyl i~dide,'~-~O we observe that each band of the doublet in the da(CH3)region at -1400 cm-' in Figure 8 further splits into two bands centered at 1426 and 1419 and 1400 and 1395 cm-l, respectively. We also observe that one band of the e(CH3)doublet further splits into two bands centered at 894 and 897 cm-l. The splitting of the e(CH3) doublet was not reported before, possibly due to insufficient resolution of the grating infrared spectrometersused.16~18~19 The 0.5 cm-' spectrum also reveals a shoulder at 1242 cm-l beside the intense band at 1237 cm-l for d,(CH3). (46) Raval, R. S.;Parker, S.F.; Chesters, M. A. Suf. Sci. 1993,289, 227. (47)Agrawal, V. IC;Trenary, M. J . Chem. Phys. 1991,95,6962. (48) Marechal, Y.In Vibrational Spectroscopy ofHydrogen Bonded Complexes in the Gaseous Phase; Molecular Interactions; Ratajczak, H., Orvile-Thomas, W. J., Redshaw, M., Eds.; John Wiley & Sons: Chichester, 1980;Vol. 1.
Fan and Trenary
3656 Langmuir, Vol. 10,No. 10,1994 I -
Figure 9. Spectrum of CH31 thin film at 0.5 cm-I resolution obtained with a MCT detectorfollowing a 240 langmuir exposure at 81 K. Table 3. IR Assignments for CHsI(s) and Thin Films on Pt(111)
thin films on Pt(lllIb solid" 3047,3034 2933 2834 2812,2803 2456 2128,2120 1425,1420 1401,1396 1240,1235 1040,1038 895,887 519,523 a
2 cm-' res
0.5 cm-' res
3047,3035 2934 2833 2803,2776 2456
3034 2933 2832 2802,2776 2455
1427 1403 1236 1040 897,886
1426,1419 1400,1395 1242,1237 1039 897,894,886
Reference 19. This work.
The band at 2455 cm-l is the overtone of the symmetric CH3 bend. The shoulder on v,(C-H) a t 2945 cm-' is assigned to the combination of v,(C-I) and 26,(CH3). This band apparently gains intensity as a result of a Fermi resonance with the v,(C-H) at 2934 cm-'. We observe three bands at around 2800 cm-'. The overtone of 6-, (CH3) would be expected a t -2820 cm-l (twice the fundamental) and the overtone of the combination of e(CH3)and v(C-I) would also be close to 2820 cm-'. These two overtones are so close to each other that a Fermi resonance is likely to occur between them. Splittings due to symmetry properties of the crystal, as discussed below, further complicate the assignments. For these reasons, our assignments of the three bands in the region at -2800 cm-l are somewhat tentative. The bands observed in Figures 8 and 9 are presented along with the values reported for solid CH31in Table 3. It should be noted that the high SNR's that can be achieved with the thin films allow us to see several overtones and combination bands in addition to the fundamentals. Such spectral features are generally difficult to detect at submonolayer coverages with the current sensitivity of RAIRS. Yet such observations of spectral features beyond the fundamentalscan provide important new information on the properties of chemisorbed molecules and are one of several reasons why increased sensitivity is highly desirable. The close agreement in Table 3 between the thin films on Pt(ll1) and the spectra ofthe solid suggests that the film adopts the same crystal structure as the bulk solid. The presence of crystalline order is also suggested by the sharpness of the bands in Figures 8 and 9.
Figure 10. Symmetry correlation diagram showing the relationship among irreducible representations of the C3"(gas phase), C, (site group), and D2h (unit cell group) point groups. Given that our spectra of the CH31thin films indicate the formation of crystalline CH31, a full analysis of the spectra should be based on the symmetry properties of the bulk solid. Solid CH31adopts the D=16-Pnma space group with four molecules per unit cell although earlier analyses of the vibrational spectra were based on the CzVCmc21 structure. The site symmetry for the molecule is C,, and the correlations between the irreducible representations of the C3" (gas), C, (site), and D= (unit cell) point groups are shown in Figure 10. This correlation diagram was used by Shenvood et al.49to interpret the spectra of sodium iodate which has the same crystal structure as methyl iodide. The vibrational bands of the isolated CH31molecule can be split into additional bands by two distinct effects. The lower symmetry of the site removes the degeneracy of the E modes. In addition, since there are four molecules in the unit cell, there are four distinct ways to combine a given mode from each molecule, resulting in a quadrupling of the number of distinct modes. This can give rise to splittingby the correlation-field effect. However, only modes of the crystal with BIU,Bzu,and Bsu symmetries have IR-active fundamentals. Thus, according to Figure 10, the A1 fundamentals of the isolated molecule could be split into two IR-active components, and each E mode could give rise to three IR-active fundamentals. The splitting of the A1 bands would be due entirely to the correlation-field effect and the splitting of the E bands due to both lower site symmetry and to the correlation-field effect. We also note that only the vibrational modes with A' symmetry in the C, group and B1, symmetry in the D z h group have dynamic dipole moments along the c axis of the unit cell. The multiple bands in the region 1390-1430 cm-l are not simply due to splitting of the fundamentals of the isolated moleculebut rather to a Fermi resonance between and Q ( C H ~ ) . ' ~ , ~ ~ 6,(CH3) and the combination of ~~(c-1) This point has been extensively discussed in connection with the vibrational spectra of solid methyl i ~ d i d e . ' ~ , ~ ~ Thus, the two bands separated by %25cm-' are due to the two Fermi resonance states each of which is split by 5-7 cm-l by the correlation-field effect. Splitting of a similar size is also observed for 6,(CH3) and e(CH3). We interpret as due to the liRing the large splitting(-11 cm-l) of~(CH3) of degeneracy due to lowering of the site symmetry and the small splitting (-3 cm-l) as due to the correlationfield effect. There are two other apparent differences between our spectra of the thin film on Pt(ll1) and bulk solid state methyl iodide: (1) on R ( l l 1 ) one component of the 6,(CH3) band was observed to have much larger intensity (49) Shenvood,P.M. A.; Turner, J.J.Spectrochim.Acta A 1970,26,
1975.
Symmetry and the Surface Infrared Selection Rule
1
\
1
'\
-4
0
\
I
\
1
137"
-i\
I
18"
Figure 11. Possible structure for the crystallinemethyl iodide thin film, projected along the b axis. The positions and orientations of the CH3I molecules are based upon the actual crystal structure. For the purpose of discussion, only two of the four molecules inside the unit cell are shown. The two molecules shown represent the two different orientations for the methyl iodide crystal. than the other, while two components of equal intensities were observed for the bulk; and (2) the ratio of the symmetric (A1 of isolated CH31) to the asymmetric (E of isolated CH31)intensities is much larger on F't(ll1) than in the bulk. These differences can be explained by a preferential alignment of the c axis of the CH31unit cell with respect to the Pt(ll1) surface normal. The B1, vibrations of the unit cell have the symmetry of a vector along the c axis while the Bz, and B3, vibrations have the symmetry of vectors perpendicular to the c axis. Thus if the c axis is oriented close to the Pt(ll1) normal, the B1, fundamentals should be much more intense than fundamentals of Bz, and B3, symmetry. In Figure 9, we observe bands corresponding to all three of the infrared-active ~ suggesting that the c representations of the D z group, axis of the crystalline methyl iodide thin film is tilted away from the surface normal. From eq 4, the relative intensity of the two components of d,(CH3) is related to the tilt angle, a,between the c axis of the unit cell and the surface normal by 2
AI242
t a n a=-
A1237
whereA12~~ andAlZ3,are the integrated areas for the two components of d,(CH3) at 1242 and 1237 cm-l. No transition dipole moments appear in eq 5 because they are the same for the two components of 6,(CH3),and the frequency ratio is taken as unity. A similar expression has been used in previous studies using polarized infrared
Langmuir, Vol. 10, No. 10, 1994 3657 radiation to extract information on molecular ~ r i e n t a t i o n . ~ ~ The integrated peak ratio for the bands at 1242 and 1237 cm-l is measured to be -0.08, which gives a = 16"from eq 5; i.e., the c axis of the unit cell is tilted away from the surface normal by -16". The crystal structure of methyl iodidez1 is such that the molecules inside the unit cell have two different orientations: half have their molecular axes -21" away from the c axis,and the axes of the other half are -339" (or --21") away. Figure 11 shows the relationship among the c axis of the unit cell, the surface normal, and the orientation of the CH31molecules. When the c a x i s of the unit cell is tilted by -16", one pair of molecules is tilted away from the surface normal by -37", and the other pair is tilted by -5°.21 Therefore,the methyl iodide in the crystalline thin film has an average tilte angle of -21" relative to the surface normal, based on the structure in Figure 11. This is likely a more reliable value than the -33" given in Table 2 based on the C3"symmetry of an isolated molecule.
Conclusions As is the case in other areas of spectroscopy, symmetry and group theory provide the most reliable guide to the interpretation of surface infrared spectra. For both chemisorbed 0 2 and C2H4 we observe vibrations associated with bonds that are parallel to the surface in agreement with the symmetryof the correspondingmodes. For CCH3, all allowed fundamentals and none of the forbidden fundamentals are observed for a local symmetry of CsU. However, for CCD3 and CH3 the allowed symmetric bend vibration is too weak to observe, demonstrating that, although functional groups can have characteristic frequencies, relative intensities can be highly variable and can change markedly with isotopic substitution. For submonolayer coverages of CH31, all symmetry-allowed bands for a tilted geometry are observed. For CH31thin films we observe splitting of bands due to the formation of a solid structure with reduced site symmetry and to the presence of four molecules per unit cell. The spectra are readily explained by the symmetry properties of the unit cell of crystalline CH31. These examples illustrate general phenomena which can be observed in surface IR spectra. Acknowledgment. This work was supported by a grant from the National Science Foundation (CHE9221687) and by a Teacher-Scholar award to M.T. from the Camille and Henry Dreyfus Foundation. We thank Dr. Sheher B. Mohsin for obtaining the spectra of Figure 2 and Professor Norman Sheppard for advice on the assignments of the CzD4 bands. (50)Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: Boston, 1990; p 99.
