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Infrared reflection-absorption study of the adsorbate-substrate stretch of CO on Pt(111)
Surface Science 214 (1989) L237-L245 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
INFRARED REFLECTION-ABSORPTION STUDY OF THE ADSORBATE-SUBSTRATE STRETCH OF CO ON Pt(ll1) Igor J. MALIK Department Received
and Michael
of ChemistT, 21 December
TRENARY
University of Illinois at Chicago, Chicago, IL 60680, USA 1988; accepted
for publication
14 January
1989
We report an infrared reflection-absorption spectroscopy (IRAS) study of the coverage and temperature dependence of the Pt-CO stretch of CO adsorbed at on-top sites of Pt(ll1). At 80 K we observe the band at 474 cm-’ at the lowest coverage, a maximum frequency of 478 cm-’ at The line widths show a 0.19 monolayer which then decreases to 466 cm-’ at 0.55 monolayer. sudden increase near 0 = 0.3 which is associated with the onset of bridge site occupancy. We also obtained the temperature dependence of the Pt-CO stretch for coverages of 0.19 and 0.50 monolayer. The approximately quadratic dependence of the full width on temperature (FWHM to a vibrational energy varying between 3 cm-’ at 80 K and 11 cm-’ at 350 K) is compared decay mechanism via excitation of three substrate phonons and a reasonably good agreement is found.
1. Introduction Infrared reflection-absorption spectroscopy (IRAS) has been used in a vast number of studies of carbon monoxide adsorption on metal surfaces [l-3]. When CO bonds with the molecular axis perpendicular to the surface (the most common geometry) there are two infrared active modes: the C-O stretch and the CO-substrate stretch. Yet, with one exception [4], all previous IRAS studies have been of the C-O stretch with frequencies in the range of 1800-2100 cm-‘. We present here an IRAS study of the CO-metal stretch. For terminally bound CO on Pt(ll1) this mode has a frequency of - 470 cm-’ (the exact value depending on coverage and temperature), making it the lowest energy mode yet observed with IRAS. This study (together with a recently published independent IRAS work dealing with the same vibrational mode [4]) thus represents an important extension of the low frequency limit of the technique and thereby opens the way for IRAS studies of a class of surface vibration, the substrate-adsorbate modes, which have been largely unexplored with high resolution spectroscopy. These modes, being energetically as well as spatially close to substrate phonon modes, can be expected to interact more strongly with the phonon degrees of freedom than the higher frequency intra-adsorbate modes. Also, the lower electron-hole pair density of states at 0039-6028/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L238
I.J. Malik, M. Trenaq / IRAS of CO on Pt(IIl)
low frequencies makes relaxation via electron-hole pair excitations less likely. The vibrations of CO chemisorbed on Pt(ll1) have been intensely investigated in recent years both experimentally and theoretically. IRAS was used to characterize the C-O stretch of both on-top (2100 cm-‘) and bridge-bonded CO (1850 cm-‘) [5,6]. The Pt-CO stretch of on-top CO was studied by emission infrared spectroscopy [7] and recently also by IRAS [4]. All the dipole-allowed fundamentals (i.e. the C-O and Pt-CO stretches of both on-top and bridge-bonded CO - the last one being at 380 cm-‘) and several overtones and combination bands were measured by electron energy loss spectroscopy (EELS) [8]. Inelastic He scattering spectra of CO on Pt(ll1) [9] show low frequency losses (48, 60, and 141 cm-‘) attributed to frustrated translations and rotations of both on-top and bridge-bonded CO. The assignment is based on a normal mode analysis of adsorbed CO [9,10]. Our Pt-CO stretch data are essentially in agreement with the above cited studies. However, our higher signal-to-noise ratio compared to the previous IR studies [4,7] allowed us to observe several coverage and temperature dependent effects not reported before.
2. Experimental Our apparatus [ll] and crystal preparation method [12] have been described earlier. Briefly, the apparatus consists of a stainless steel ultrahigh vacuum (UHV) chamber with a current base pressure of 8 x lo-” Torr coupled to a commercial Fourier transform (FT) IR spectrometer with a KBr beam splitter. The infrared beam is p-polarized with a wire grid polarizer on a KRS-5 (thallium bromo-iodide) substrate and enters and exits the chamber through differentially pumped KRS-5 windows. A liquid He cooled Si bolometer (Infrared Laboratories, Tucson, AZ) was used. As KRS-5 transmits to - 250 cm-’ and our bolometer can operate well below 100 cm-‘, we are currently limited only by our KBr beam splitter which does not transmit below 400 cm-’ and which strongly attenuates the IR beam even at 450 cm-‘. We will soon be installing a Mylar beam splitter for work between 250 and 550 cm-’ which should permit the study of the Pt-CO stretch of bridge bonded CO at an inter- 380 cm- ‘. Because of the slow response time of the bolometer, ferometer mirror speed of only 0.6 cm/s was used whereas we used a mirror speed of 2.5 cm/s with a photoconductive mercury-cadmium-telluride (MCT) detector for the C-O stretch region. The only contamination on the crystal observed by Auger electron spectroscopy (AES) was carbon. It was removed by exposing the crystal to 5 X 10V9 Torr of 0, from a gas doser at 700 K for approximately one hour. The exposures of the crystal to CO were performed by backfilling the chamber. The relation between coverages and exposures was established from a series of thermal desorption spectra (TDS) where the
I.J. Malik, M. Trenaty / IRAS
of CO on Pt(I I I)
L239
saturation at 300 K served as a reference point (B = 0.50). Thermal desorption spectra were not taken after the IR experiments. The IR spectra of the Pt-CO stretch are ratios of 3000 sample scans to 3000 background scans (61 min each) at 2 cm-’ resolution. The spectra of the C-O stretch region represent a ratio of 256 sample scans to 256 background scans (2 min each) at 2 cm-’ resolution. Although the temperature dependence of the Pt-CO stretch was measured at constant coverage it proved impractical for two reasons to use the same overlayer for more than one set of scans: first, additional CO was adsorbing from the residual background gas which might significantly increase the coverage over extended periods of time; second, it was necessary to minimize the time interval between background and sample scans and to obtain both at the same temperature in order to get good quality spectra. The crystal was heated to 1000 K between different sets of scans. The intrinsic full widths at half maximum (FWHM) were calculated from the observed FWHM’s on the assumption that the intrinsic and instrumental full widths add in quadrature to give the observed full width. We present intrinsic FWHM’s in this paper.
3. Results and discussion Fig. 1 shows the Pt-CO stretch and C-O stretch spectral regions at 80 K as a function of coverage. The C-O stretch region is shown because it is easy to detect the presence of bridge-bonded CO by a band near 1850 cm-’ and much of the change in the Pt-CO stretch with coverage appears to be related to the occupation of bridge sites. The trends in frequency and FWHM of the Pt-CO stretch as a function of coverage at 80 K are summarized in fig. 2. From the lowest detectable coverages up to 13= 0.19 the frequency of the Pt-CO stretch increases linearly with coverage from 474.4 to 477.6 cm-’ while the FWHM remains constant at = 3 cm-‘. At coverages higher than 8 = 0.2 the direction of the frequency shift is reversed (frequency reaching 465.9 cm-’ at saturation) while the FWHM sharply increases at 8 2: 0.3. The increase in FWHM is accompanied by the occurrence of irregularities in the lineshape (splitting, shoulders). The exact shape of these features was not reproducible from case to case and it simply may be due to noise. However, spectra of low coverages (0 = 0.1) with comparable signal-to-noise ratios always showed a smooth lineshape. The dramatic change in behavior around B = 0.2 is associated with the onset of adsorption at the bridge sites as illustrated by the increasing signal at 1850 cm -’ in fig. lb. The spectra in fig. 1 were collected after exposures at 80 K without annealing the adsorbate overlayer. We also obtained spectra at different coverages of an overlayer annealed at 300 K (250 K for 8 = 0.55) that did not differ from those of an unannealed overlayer.
L240
I.J. Malik, M. Trenary / IRAS a)
Pt-CO
stretch.
80 K
of CO on Pt(lll)
b)
C-O
stretch.
80 K
bridge
on-top
0.19 0.06
440
460
480
_+.._A_
500
1800 Wavenumber
0.19
1850
1900
2073
2125
(cm-‘)
Fig. 1. (a) The Pt-CO stretch band for on-top sites as a function of coverage at 80 K. The spectra were obtained with 2 cm-’ resolution. (b) The C-O stretch region showing the onset of bridge site occupancy at 0 = 0.2. Note the expanded absorbance scale for the bridge-bonded CO.
464
8 0 0
0.2
0.4
0.6
0.4
0.6
CCMRlge
0.0
0.2 COVerage
Fig. 2. Plots of the frequency
(a) and intrinsic FWHM (b) of the Pt-CO coverage at 80 K.
band
as a function
of
I.J. Malik, M. Trenary / IRAS of CO on Pt(lIl)
L241
The maximum in frequency of the Pt-CO stretch at 8 = 0.19 occurs at a lower coverage than 8 = 0.37 where Ttishaus et al. [5] observed a maximum in the coupling shift for the C-O stretch of the on-top molecules. In contrast to the results shown in fig. 2, the frequency of the C-O stretch as a function of coverage does not show a maximum. This, together with the insensitivity of the C-O stretch of the on-top bonded CO to occupation of the bridge sites [5], suggests that the nature of the two modes is quite different. This is not very surprising if one takes into account their difference in frequency and in distance from the surface. The increase in line width above 0.3 monolayer is probably related to a change in local environment as the CO overlayer changes from an overlayer structure consisting of regularly spaced islands of (6 x fi)R30 o to the c(4 x 2) structure. Although changes in FWHM are often seen with changes in overlayer structure [l-3], no such change has been reported for the C-O stretch of on-top CO on Pt(ll1) [5,6]. The IR peak areas show a maximum with coverage at - 0.19 monolayer, although the uncertainties are rather high. If we assume that only on-top sites are occupied at B = 0.19 and that 50% of the molecules are at on-top sites at t9 = 0.50, then the IR area per absorber drops roughly by a factor of 2 between 8 = 0.19 and 0.50. Hoge et al. [4] also reported a decrease in intensity per molecule at higher coverages. This effect is well-known in studies of the C-O stretch where it is attributed to screening by neighboring dipoles [13]. The magnitude of this effect should be approximately the same as for the C-O stretch as it depends on the value of LX_the electronic polarizability, which is a frequency independent parameter characteristic of the adsorbed molecule as a whole. In fact, a maximum in intensity with coverage of the C-O stretch of on-top CO on Pt(ll1) has been observed [6]. By contrast, the absolute intensity, which is quite different for the two modes, is determined by (Y”, the vibrational polarizability. The temperature dependence of the Pt-CO stretch at coverages 0 = 0.19 and 8 = 0.50 is presented in figs. 3 and 4. At 8 = 0.19 the number of molecules bonded exclusively at on-top sites is a maximum. The two-dimensional arrangement of CO molecules at this coverage was suggested to have local (0 x fi)R30 o structure with a long range (8 x 8) symmetry [5]. Fig. 3 shows several spectra of 6 = 0.19 between 80 and 350 K. The exposures for 8 = 0.19 were performed with the crystal at 80 K, 8 = 0.50 was obtained by saturating the surface at 300 K (5 L exposure). Annealing of the 8 = 0.19 overlayer at 300 K did not affect the Pt-CO stretch band. The temperature dependence is summarized in fig. 4 where the vibrational frequency and FWHM is plotted as a function of temperature. While the frequency dependence differs for the two coverages, the FWHM’s show a similar behavior. There are several possible contributions to the temperature effects seen in figs. 3 and 4. We cannot conclusively rule out inhomogeneous broadening but the observed temperature dependence indicates that it is not the dominant
L242
I.J. Malik, M. Trenary / IRAS
of CO on Pr(lll)
350 300 250
440
460 Wavenumber
Fig. 3. The Pt-CO
stretch
500
480 (cm-9
as a function of temperature for a coverage sample was redosed for each spectrum.
of 0.19 monolayer.
The
mechanism for these particular coverages. Inhomogeneous broadening is likely responsible for the lineshapes of the disordered overlayers with significant bridge site population (0.25 < 6 < 0.55, 8 # 0.50). Because it is predicted to be temperature independent [14], we rule out vibrational energy decay via electron-hole pair excitation in the substrate as the dominant line broadening mechanism. The interaction with the substrate phonons thus remains to be considered. We used Persson’s model [15] for dephasing through interaction with phonons and for energy decay via a three-phonon excitation (maximum frequency of Pt phonons is - 200 cm-’ [16]) to fit our FWHM temperature dependence. While the dephasing model gives a much stronger temperature dependence than observed, the three-phonon excitation model fits the experimental data quite well (fig. 4b). We therefore suggest that energy decay via excitation of three phonons is the mechanism dominating the lineshape. We assume that for energy decay via the excitation of phonons only processes involving the creation of the minimum number of phonons (i.e. 3) required by energy conservation will be important. We note that the model used is a very simple one and might not reflect the complexities of the system very well. For example. the model uses the Debye
I.J. Malik, hf. Trenary / IF&IS of CO on Pt(lI1)
%++
++ +++
+ ++
0
0
L243
200 IOU Temperature
(K)
100 Tempe%re
(K)
1
300
400
3ccl
400
Fig. 4. Plots of the frequency (a) and intrinsic FWHM (b) of the Pt-CO stretch at a coverage of 0.19 (+ ) and 0.50 (m) monolayer as a function of temperature. The different error bars at low and high temperatures reflect the increasing uncertainty of measurement with increasing temperature. Predictions of a three-phonon excitation mechanism (1) and a model describing dephasing via interaction with substrate phonons (2), both normalized at 180 K, are also shown.
phonon density of states (DOS) which can differ rather dramatically from the real density of states at a metal surface [17]. Tobin and Richards [7] used the same model [15] but with the bulk phonon DOS instead of Debye phonon DOS for platinum (both phonon densities give very similar temperature dependence in this model) and found their experimental full widths less temperature dependent than the model predicts for either three-phonon decay or dephasing mechanisms. They concluded that the lineshape is inhomogeneously broadened. Since our lineshapes are narrower and show a stronger temperature dependence than those of Tobin and Richards [7] the two different conclusions are not necessarily in conflict. The linear dependence of frequency and quadratic dependence of full width on temperature is in accord with a dephasing mechanism in which the mode being studied is anharmonically coupled to a low frequency mode which in turn is damped by resonant energy exchange with the substrate [18]. A likely candidate for the low frequency exchange mode is the frustrated translation with a frequency of 48 cm-’ and FWHM < 8 cm-’ (the FWHM as observed
L244
I.J. Malik, M. Trenary / IRAS of CO on Pt(lI1)
with inelastic helium scattering corresponds to the instrumental resolution [9]). The full width of this mode, if determined by its lifetime (T,), would correspond to the damping parameter TJ in the exchange-coupling model [18]. We obtain from the slopes of the frequency versus T and FWHM versus T2 plots at 8 = 0.19 a value for 77 of 22 cm-‘, which is considerably larger than the FWHM of the frustrated translation obtained with He scattering [9]. This fact and the better agreement with the phonon decay mechanism shown in fig. 4b lead to our conclusion that vibrational energy decay by way of excitation of three substrate phonons is the dominant line broadening mechanism. Our discussion of the mechanism responsible for the Pt-CO lineshape clearly reveals a need for a more detailed theoretical treatment of this problem dealing explicitly with the CO/Pt(lll) system. For example, it would be useful to have a prediction of how decay through excitation of substrate phonons would affect the frequency shift with temperature of the Pt-CO stretch as this is easier to measure precisely than the temperature dependence of the line widths. Such a theoretical treatment should also include the effect of coverage on the temperature dependence. We hope that the new data on the Pt-CO stretch will stimulate renewed activity in this area just as the availability of high quality infrared data has stimulated theoretical work dealing with the C-O stretch.
Acknowledgement This work was supported (CHE-8603891).
by a grant from the National
Science Foundation
References [l] F.M. Hoffmann, Surface Sci. Rept. 3 (1983) 107. [2] Y.J. Chabal, Surface Sci. Rept. 8 (1988) 211. [3] A.M. Bradshaw and E. Schweizer, in: Avarices in Spectroscopy, Ed. R.E. Hester (Wiley, New York, 1988). [4] D. Hoge, M. Hiishaus, E. Schweizer and A.M. Bradshaw, Chem. Phys. Letters 151 (1988) 230. [5] M. Tiishaus, E. Schweizer, P. Hollins and A.M. Bradshaw, J. Electron Spectrosc. Related Phenomena 44 (1987) 305. [6] B.E. Hayden and A.M. Bradshaw, Surface Sci. 125 (1983) 787. [7] R.G. Tobin and P.L. Richards, Surface Sci. 179 (1987) 387. [8] H. Steininger, S. Lehwald and H. Ibach, Surface Sci. 123 (1982) 264. [9] A.M. Lahee, J.P. Toennies and Ch. Wiill, Surface Sci. 177 (1986) 371. [lo] N.V. Richardson and A.M. Bradshaw. Surface Sci. 88 (1979) 255. [ll] M.E. Brubaker and M. Trenary. J. Chem. Phys. 85 (1986) 6100.
I.J. Malik, M. Trenary / IRAS [12] [13] [14] [15] [16] [17] [18]
of CO on Pt(lll)
L245
I.J. Malik, M.E. Brubaker, S.B. Mobsin and M. Trenary, J. Chem. Phys. 87 (1987) 5554. P. HoIIins and J. Pritchard, Progr. Surface Sci. 19 (1985) 275. J.W. Gadzuk and A.C. Luntz, Surface Sci. 144 (1984) 429. B.N.J. Persson, J. Phys. C 17 (1984) 4741. J.E. Black, F.C. Shanes and R.F. Wallis, Surface Sci. 133 (1983) 199. R.E. Allen, G.P. Alldredge and F.W. de Wette, Phys. Rev. B 4 (1971) 1661. B.N.J. Persson, R. Ryberg, Phys. Rev. B 32 (1985) 3586.
SURFACE
SCIENCE
LETTERS
INFRARED REFLECTION-ABSORPTION STUDY OF THE ADSORBATE-SUBSTRATE STRETCH OF CO ON Pt(ll1) Igor J. MALIK Department Received
and Michael
of ChemistT, 21 December
TRENARY
University of Illinois at Chicago, Chicago, IL 60680, USA 1988; accepted
for publication
14 January
1989
We report an infrared reflection-absorption spectroscopy (IRAS) study of the coverage and temperature dependence of the Pt-CO stretch of CO adsorbed at on-top sites of Pt(ll1). At 80 K we observe the band at 474 cm-’ at the lowest coverage, a maximum frequency of 478 cm-’ at The line widths show a 0.19 monolayer which then decreases to 466 cm-’ at 0.55 monolayer. sudden increase near 0 = 0.3 which is associated with the onset of bridge site occupancy. We also obtained the temperature dependence of the Pt-CO stretch for coverages of 0.19 and 0.50 monolayer. The approximately quadratic dependence of the full width on temperature (FWHM to a vibrational energy varying between 3 cm-’ at 80 K and 11 cm-’ at 350 K) is compared decay mechanism via excitation of three substrate phonons and a reasonably good agreement is found.
1. Introduction Infrared reflection-absorption spectroscopy (IRAS) has been used in a vast number of studies of carbon monoxide adsorption on metal surfaces [l-3]. When CO bonds with the molecular axis perpendicular to the surface (the most common geometry) there are two infrared active modes: the C-O stretch and the CO-substrate stretch. Yet, with one exception [4], all previous IRAS studies have been of the C-O stretch with frequencies in the range of 1800-2100 cm-‘. We present here an IRAS study of the CO-metal stretch. For terminally bound CO on Pt(ll1) this mode has a frequency of - 470 cm-’ (the exact value depending on coverage and temperature), making it the lowest energy mode yet observed with IRAS. This study (together with a recently published independent IRAS work dealing with the same vibrational mode [4]) thus represents an important extension of the low frequency limit of the technique and thereby opens the way for IRAS studies of a class of surface vibration, the substrate-adsorbate modes, which have been largely unexplored with high resolution spectroscopy. These modes, being energetically as well as spatially close to substrate phonon modes, can be expected to interact more strongly with the phonon degrees of freedom than the higher frequency intra-adsorbate modes. Also, the lower electron-hole pair density of states at 0039-6028/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L238
I.J. Malik, M. Trenaq / IRAS of CO on Pt(IIl)
low frequencies makes relaxation via electron-hole pair excitations less likely. The vibrations of CO chemisorbed on Pt(ll1) have been intensely investigated in recent years both experimentally and theoretically. IRAS was used to characterize the C-O stretch of both on-top (2100 cm-‘) and bridge-bonded CO (1850 cm-‘) [5,6]. The Pt-CO stretch of on-top CO was studied by emission infrared spectroscopy [7] and recently also by IRAS [4]. All the dipole-allowed fundamentals (i.e. the C-O and Pt-CO stretches of both on-top and bridge-bonded CO - the last one being at 380 cm-‘) and several overtones and combination bands were measured by electron energy loss spectroscopy (EELS) [8]. Inelastic He scattering spectra of CO on Pt(ll1) [9] show low frequency losses (48, 60, and 141 cm-‘) attributed to frustrated translations and rotations of both on-top and bridge-bonded CO. The assignment is based on a normal mode analysis of adsorbed CO [9,10]. Our Pt-CO stretch data are essentially in agreement with the above cited studies. However, our higher signal-to-noise ratio compared to the previous IR studies [4,7] allowed us to observe several coverage and temperature dependent effects not reported before.
2. Experimental Our apparatus [ll] and crystal preparation method [12] have been described earlier. Briefly, the apparatus consists of a stainless steel ultrahigh vacuum (UHV) chamber with a current base pressure of 8 x lo-” Torr coupled to a commercial Fourier transform (FT) IR spectrometer with a KBr beam splitter. The infrared beam is p-polarized with a wire grid polarizer on a KRS-5 (thallium bromo-iodide) substrate and enters and exits the chamber through differentially pumped KRS-5 windows. A liquid He cooled Si bolometer (Infrared Laboratories, Tucson, AZ) was used. As KRS-5 transmits to - 250 cm-’ and our bolometer can operate well below 100 cm-‘, we are currently limited only by our KBr beam splitter which does not transmit below 400 cm-’ and which strongly attenuates the IR beam even at 450 cm-‘. We will soon be installing a Mylar beam splitter for work between 250 and 550 cm-’ which should permit the study of the Pt-CO stretch of bridge bonded CO at an inter- 380 cm- ‘. Because of the slow response time of the bolometer, ferometer mirror speed of only 0.6 cm/s was used whereas we used a mirror speed of 2.5 cm/s with a photoconductive mercury-cadmium-telluride (MCT) detector for the C-O stretch region. The only contamination on the crystal observed by Auger electron spectroscopy (AES) was carbon. It was removed by exposing the crystal to 5 X 10V9 Torr of 0, from a gas doser at 700 K for approximately one hour. The exposures of the crystal to CO were performed by backfilling the chamber. The relation between coverages and exposures was established from a series of thermal desorption spectra (TDS) where the
I.J. Malik, M. Trenaty / IRAS
of CO on Pt(I I I)
L239
saturation at 300 K served as a reference point (B = 0.50). Thermal desorption spectra were not taken after the IR experiments. The IR spectra of the Pt-CO stretch are ratios of 3000 sample scans to 3000 background scans (61 min each) at 2 cm-’ resolution. The spectra of the C-O stretch region represent a ratio of 256 sample scans to 256 background scans (2 min each) at 2 cm-’ resolution. Although the temperature dependence of the Pt-CO stretch was measured at constant coverage it proved impractical for two reasons to use the same overlayer for more than one set of scans: first, additional CO was adsorbing from the residual background gas which might significantly increase the coverage over extended periods of time; second, it was necessary to minimize the time interval between background and sample scans and to obtain both at the same temperature in order to get good quality spectra. The crystal was heated to 1000 K between different sets of scans. The intrinsic full widths at half maximum (FWHM) were calculated from the observed FWHM’s on the assumption that the intrinsic and instrumental full widths add in quadrature to give the observed full width. We present intrinsic FWHM’s in this paper.
3. Results and discussion Fig. 1 shows the Pt-CO stretch and C-O stretch spectral regions at 80 K as a function of coverage. The C-O stretch region is shown because it is easy to detect the presence of bridge-bonded CO by a band near 1850 cm-’ and much of the change in the Pt-CO stretch with coverage appears to be related to the occupation of bridge sites. The trends in frequency and FWHM of the Pt-CO stretch as a function of coverage at 80 K are summarized in fig. 2. From the lowest detectable coverages up to 13= 0.19 the frequency of the Pt-CO stretch increases linearly with coverage from 474.4 to 477.6 cm-’ while the FWHM remains constant at = 3 cm-‘. At coverages higher than 8 = 0.2 the direction of the frequency shift is reversed (frequency reaching 465.9 cm-’ at saturation) while the FWHM sharply increases at 8 2: 0.3. The increase in FWHM is accompanied by the occurrence of irregularities in the lineshape (splitting, shoulders). The exact shape of these features was not reproducible from case to case and it simply may be due to noise. However, spectra of low coverages (0 = 0.1) with comparable signal-to-noise ratios always showed a smooth lineshape. The dramatic change in behavior around B = 0.2 is associated with the onset of adsorption at the bridge sites as illustrated by the increasing signal at 1850 cm -’ in fig. lb. The spectra in fig. 1 were collected after exposures at 80 K without annealing the adsorbate overlayer. We also obtained spectra at different coverages of an overlayer annealed at 300 K (250 K for 8 = 0.55) that did not differ from those of an unannealed overlayer.
L240
I.J. Malik, M. Trenary / IRAS a)
Pt-CO
stretch.
80 K
of CO on Pt(lll)
b)
C-O
stretch.
80 K
bridge
on-top
0.19 0.06
440
460
480
_+.._A_
500
1800 Wavenumber
0.19
1850
1900
2073
2125
(cm-‘)
Fig. 1. (a) The Pt-CO stretch band for on-top sites as a function of coverage at 80 K. The spectra were obtained with 2 cm-’ resolution. (b) The C-O stretch region showing the onset of bridge site occupancy at 0 = 0.2. Note the expanded absorbance scale for the bridge-bonded CO.
464
8 0 0
0.2
0.4
0.6
0.4
0.6
CCMRlge
0.0
0.2 COVerage
Fig. 2. Plots of the frequency
(a) and intrinsic FWHM (b) of the Pt-CO coverage at 80 K.
band
as a function
of
I.J. Malik, M. Trenary / IRAS of CO on Pt(lIl)
L241
The maximum in frequency of the Pt-CO stretch at 8 = 0.19 occurs at a lower coverage than 8 = 0.37 where Ttishaus et al. [5] observed a maximum in the coupling shift for the C-O stretch of the on-top molecules. In contrast to the results shown in fig. 2, the frequency of the C-O stretch as a function of coverage does not show a maximum. This, together with the insensitivity of the C-O stretch of the on-top bonded CO to occupation of the bridge sites [5], suggests that the nature of the two modes is quite different. This is not very surprising if one takes into account their difference in frequency and in distance from the surface. The increase in line width above 0.3 monolayer is probably related to a change in local environment as the CO overlayer changes from an overlayer structure consisting of regularly spaced islands of (6 x fi)R30 o to the c(4 x 2) structure. Although changes in FWHM are often seen with changes in overlayer structure [l-3], no such change has been reported for the C-O stretch of on-top CO on Pt(ll1) [5,6]. The IR peak areas show a maximum with coverage at - 0.19 monolayer, although the uncertainties are rather high. If we assume that only on-top sites are occupied at B = 0.19 and that 50% of the molecules are at on-top sites at t9 = 0.50, then the IR area per absorber drops roughly by a factor of 2 between 8 = 0.19 and 0.50. Hoge et al. [4] also reported a decrease in intensity per molecule at higher coverages. This effect is well-known in studies of the C-O stretch where it is attributed to screening by neighboring dipoles [13]. The magnitude of this effect should be approximately the same as for the C-O stretch as it depends on the value of LX_the electronic polarizability, which is a frequency independent parameter characteristic of the adsorbed molecule as a whole. In fact, a maximum in intensity with coverage of the C-O stretch of on-top CO on Pt(ll1) has been observed [6]. By contrast, the absolute intensity, which is quite different for the two modes, is determined by (Y”, the vibrational polarizability. The temperature dependence of the Pt-CO stretch at coverages 0 = 0.19 and 8 = 0.50 is presented in figs. 3 and 4. At 8 = 0.19 the number of molecules bonded exclusively at on-top sites is a maximum. The two-dimensional arrangement of CO molecules at this coverage was suggested to have local (0 x fi)R30 o structure with a long range (8 x 8) symmetry [5]. Fig. 3 shows several spectra of 6 = 0.19 between 80 and 350 K. The exposures for 8 = 0.19 were performed with the crystal at 80 K, 8 = 0.50 was obtained by saturating the surface at 300 K (5 L exposure). Annealing of the 8 = 0.19 overlayer at 300 K did not affect the Pt-CO stretch band. The temperature dependence is summarized in fig. 4 where the vibrational frequency and FWHM is plotted as a function of temperature. While the frequency dependence differs for the two coverages, the FWHM’s show a similar behavior. There are several possible contributions to the temperature effects seen in figs. 3 and 4. We cannot conclusively rule out inhomogeneous broadening but the observed temperature dependence indicates that it is not the dominant
L242
I.J. Malik, M. Trenary / IRAS
of CO on Pr(lll)
350 300 250
440
460 Wavenumber
Fig. 3. The Pt-CO
stretch
500
480 (cm-9
as a function of temperature for a coverage sample was redosed for each spectrum.
of 0.19 monolayer.
The
mechanism for these particular coverages. Inhomogeneous broadening is likely responsible for the lineshapes of the disordered overlayers with significant bridge site population (0.25 < 6 < 0.55, 8 # 0.50). Because it is predicted to be temperature independent [14], we rule out vibrational energy decay via electron-hole pair excitation in the substrate as the dominant line broadening mechanism. The interaction with the substrate phonons thus remains to be considered. We used Persson’s model [15] for dephasing through interaction with phonons and for energy decay via a three-phonon excitation (maximum frequency of Pt phonons is - 200 cm-’ [16]) to fit our FWHM temperature dependence. While the dephasing model gives a much stronger temperature dependence than observed, the three-phonon excitation model fits the experimental data quite well (fig. 4b). We therefore suggest that energy decay via excitation of three phonons is the mechanism dominating the lineshape. We assume that for energy decay via the excitation of phonons only processes involving the creation of the minimum number of phonons (i.e. 3) required by energy conservation will be important. We note that the model used is a very simple one and might not reflect the complexities of the system very well. For example. the model uses the Debye
I.J. Malik, hf. Trenary / IF&IS of CO on Pt(lI1)
%++
++ +++
+ ++
0
0
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200 IOU Temperature
(K)
100 Tempe%re
(K)
1
300
400
3ccl
400
Fig. 4. Plots of the frequency (a) and intrinsic FWHM (b) of the Pt-CO stretch at a coverage of 0.19 (+ ) and 0.50 (m) monolayer as a function of temperature. The different error bars at low and high temperatures reflect the increasing uncertainty of measurement with increasing temperature. Predictions of a three-phonon excitation mechanism (1) and a model describing dephasing via interaction with substrate phonons (2), both normalized at 180 K, are also shown.
phonon density of states (DOS) which can differ rather dramatically from the real density of states at a metal surface [17]. Tobin and Richards [7] used the same model [15] but with the bulk phonon DOS instead of Debye phonon DOS for platinum (both phonon densities give very similar temperature dependence in this model) and found their experimental full widths less temperature dependent than the model predicts for either three-phonon decay or dephasing mechanisms. They concluded that the lineshape is inhomogeneously broadened. Since our lineshapes are narrower and show a stronger temperature dependence than those of Tobin and Richards [7] the two different conclusions are not necessarily in conflict. The linear dependence of frequency and quadratic dependence of full width on temperature is in accord with a dephasing mechanism in which the mode being studied is anharmonically coupled to a low frequency mode which in turn is damped by resonant energy exchange with the substrate [18]. A likely candidate for the low frequency exchange mode is the frustrated translation with a frequency of 48 cm-’ and FWHM < 8 cm-’ (the FWHM as observed
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I.J. Malik, M. Trenary / IRAS of CO on Pt(lI1)
with inelastic helium scattering corresponds to the instrumental resolution [9]). The full width of this mode, if determined by its lifetime (T,), would correspond to the damping parameter TJ in the exchange-coupling model [18]. We obtain from the slopes of the frequency versus T and FWHM versus T2 plots at 8 = 0.19 a value for 77 of 22 cm-‘, which is considerably larger than the FWHM of the frustrated translation obtained with He scattering [9]. This fact and the better agreement with the phonon decay mechanism shown in fig. 4b lead to our conclusion that vibrational energy decay by way of excitation of three substrate phonons is the dominant line broadening mechanism. Our discussion of the mechanism responsible for the Pt-CO lineshape clearly reveals a need for a more detailed theoretical treatment of this problem dealing explicitly with the CO/Pt(lll) system. For example, it would be useful to have a prediction of how decay through excitation of substrate phonons would affect the frequency shift with temperature of the Pt-CO stretch as this is easier to measure precisely than the temperature dependence of the line widths. Such a theoretical treatment should also include the effect of coverage on the temperature dependence. We hope that the new data on the Pt-CO stretch will stimulate renewed activity in this area just as the availability of high quality infrared data has stimulated theoretical work dealing with the C-O stretch.
Acknowledgement This work was supported (CHE-8603891).
by a grant from the National
Science Foundation
References [l] F.M. Hoffmann, Surface Sci. Rept. 3 (1983) 107. [2] Y.J. Chabal, Surface Sci. Rept. 8 (1988) 211. [3] A.M. Bradshaw and E. Schweizer, in: Avarices in Spectroscopy, Ed. R.E. Hester (Wiley, New York, 1988). [4] D. Hoge, M. Hiishaus, E. Schweizer and A.M. Bradshaw, Chem. Phys. Letters 151 (1988) 230. [5] M. Tiishaus, E. Schweizer, P. Hollins and A.M. Bradshaw, J. Electron Spectrosc. Related Phenomena 44 (1987) 305. [6] B.E. Hayden and A.M. Bradshaw, Surface Sci. 125 (1983) 787. [7] R.G. Tobin and P.L. Richards, Surface Sci. 179 (1987) 387. [8] H. Steininger, S. Lehwald and H. Ibach, Surface Sci. 123 (1982) 264. [9] A.M. Lahee, J.P. Toennies and Ch. Wiill, Surface Sci. 177 (1986) 371. [lo] N.V. Richardson and A.M. Bradshaw. Surface Sci. 88 (1979) 255. [ll] M.E. Brubaker and M. Trenary. J. Chem. Phys. 85 (1986) 6100.
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of CO on Pt(lll)
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