178 Volume 53, Number 2, 1999 APPLIED SPECTROSCOPY0003-7028 / 99 / 5302-0178$2.00 / 0

q 1999 Society for Applied Spectroscopy

Two-Dimensional FT-IR Correlation Analysis of theChemisorption of Nitric Oxide on Pt(100)

NOEL P. MAGTOTO, NELSON L. SEFARA, and HUGH H. RIC HARDSON*Department of Chemistry, Clippinger Laboratories, Ohio University, Athens, Ohio 45701-2979

In addition to the previously reported vibrational frequencies of NOstretch at 1591± 1650 cm 2 1 and 1770± 1800 cm 2 1 on Pt(100) surface

at 300 K, we observed an infrared spectral band centered around

1689± 1692 cm 2 1. With the use of two-dimensional IR correlationanalysis, we were able to distinguish the various adsorption sites of

NO under the present experimental conditions. The infrared band

at 1689± 1692 cm 2 1 developed independently of the band at 1639±1654 cm 2 1. By varying the pressure of NO and the surface temper-

ature, we were able to demonstrate that the 1689± 1692 cm 2 1 band

grows only at higher NO pressure and lower surface temperature.

Index Headings: Chemisorption; Nitric oxide; Pt(100) single crystalsurface; Infrared re¯ ection-absorption spectroscopy; 2D correlation

spectroscopy.

INTRODUCTION

Two-dimensional (2D) IR correlation spectroscopy isan analytical tool capable of simplifying complex spectraconsisting of many overlapped peaks and identifying var-ious inter- and intramolecular interactions.1 Its basic prin-ciples based on time-resolved detection of IR signalswere ® rst laid down by Noda to provide insights into theproperties of polymer systems such as submolecular in-teractions and the mechanism of polymer deformation.2,3

It was subsequently extended into Raman and other typesof spectroscopy with the use of a new formalism de-signed to handle the arbitrary functional dependence ofsignal ¯ uctuations on any physical variable.4 This newformalism does not specify the physical nature of theeffect of the perturbation on the system. Since the effectof each perturbation is uniquely associated with the spe-ci® c stimulus-to-response mechanism, any physical in-formation obtained in the analysis is governed by thechoice of the nature of the perturbation. The general ex-perimental approach in 2D correlation spectroscopy in-volves the application of an external perturbation that canselectively excite various chemical components of a giv-en system. The excitation and subsequent relaxation pro-cesses, which are manifested in the changes in peak in-tensities, shifts in spectral band frequencies, and varia-tions in peak shapes, are monitored by a given spectro-scopic probe. The dynamic spectra are then transformedinto 2D spectra with the use of the correlation method.Since its inception in 1986, 2D IR correlation analysishas been applied to the studies of various systems.5±7

Most recently, we have analyzed changes in the second-ary structure of the protein b -lactoglobulin by chemicallyand thermally inducing spectral variations of the amide Iand II bands.8,9 Because of the success of 2D correlationin understanding various systems, we would like to ex-

Received 27 June 1998; accepted 25 September 1998.* Author to whom correspondence should be sent.

plore its usefulness in analyzing the interaction of gasmolecules with metal surfaces. In this study, we startedwith the interaction between simple molecules and singlecrystal surfaces. In particular, we studied the adsorptionof nitric oxide on Pt(100).

Chemisorption of NO on various platinum surfaces hasbeen investigated extensively in the past not only becauseof its usefulness in industrial and automotive emissioncontrol but also because of its ability to provide infor-mation on molecular processes occurring on the surface.Of particular importance is the Pt(100) single crystal sur-face. This surface has caught the attention of many work-ers because of its ability to show nonlinear behavior suchas multiple steady states, kinetic oscillations, spatiotem-poral patterns, and chaos during a chemical reaction.10,11

With the use of vibrational techniques such as Fouriertransform infrared re¯ ection-absorption spectroscopy(FT-IRAS) and high-resolution electron energy loss spec-troscopy (HREELS), different adsorption sites of NO onthe Pt(100) single crystal surface have been identi® ed at300 K.12 But most of these studies have been carried outby transiently exposing the Pt surface to NO gas. Studieshave shown that under ¯ ow conditions and at relativelyhigher pressure regimes an adsorption site for a givenchemisorbed species can be observed that is otherwiseundetectable during transient exposures. For example,chemisorbed NO on Pt(100)-hex surface has been ob-served under ¯ ow conditions at temperatures as high as470 K.12,13 But under transient exposure up to 2 3 10 2 3

Pa/s, no evidence was found for either molecular or dis-sociative adsorption of NO on the hex at temperaturesabove 380 K.14 In the present study, we have employedin situ FT-IRAS to demonstrate different adsorption sitesfor NO on Pt(100) at 300 K. Under ¯ ow conditions ofconstant NO pressures, we have identi® ed and character-ized with the use of the 2D FT-IR correlation analysis anew infrared spectral band centered around 1689±1692cm 2 1 for the chemisorbed NO on the Pt(100) at 300 K.

EXPERIMENTAL

Experiments were performed in an ultrahigh vacuum(UHV) ¯ ow reactor equipped with an FT-IR spectrometercontaining a nitrogen-cooled MCT narrow-band detector,a quadrupole mass spectrometer, and an Ar 1 sputteringgun. The base pressure in the chambers can reach as lowas 10 2 10 Torr as measured by ion gauges. The reactantgases are introduced into the chamber by leak valves witha minimum controllable leak rate of 10 2 10 TL/s. Detaileddescription of our apparatus can be found in Ref. 15.

We probed the vibrational frequency of the chemi-sorbed NO in situ by continuously leaking NO into the¯ ow reactor while the IR beam was directed onto the

APPLIED SPECTROSCOPY 179

surface at an angle of 80 8 . FT-IRA spectra were collectedby coadding 100 scans at 4 cm 2 1 resolution. The absor-bance spectrum was obtained by taking the ratio betweenthe single-beam spectra collected after (sample) and be-fore (background) the introduction of NO. This spectrumwas then baseline corrected with the use of the GRAMS386 spectral software package (Galactic IndustriesCorp.).

The single crystal Pt(100) sample was a disk measur-ing 10 mm in diameter and 1.2 mm thick. It was preparedprior to running experiments by following an extensivevacuum-cleaning procedure that includes repeated cyclesof argon ion sputtering, annealing, and oxidation. Thereactant gases CO and NO were obtained in 99.99% pu-rity from Matheson Gas Co. Isotopic nitric oxide (99.99%atom 15N, 99.4% atom 18O) was purchased from Isotec,Inc. No further puri® cation procedures were used.

CALCULATIONS OF 2D CORRELATIONSPECTRA

In order to generate 2D correlation spectra, all thespectral intensities from each of the 13 spectra collectedat a particular pressure of NO were assembled in a singledata matrix. Then the spectral intensities were mean-cen-tered by subtracting the average spectrum from each oneof 13 spectra. The resulting mean-centered dynamic spec-tra y(n , P) represent the variations in spectral intensity ateach frequency (n ) as a function of the pressure (P). Thespectral intensity variations at each wavelength were thencorrelated with one another with the use of the general-ized 2D correlation method developed by Noda, whichtransforms the original spectra into two correlation spec-tra commonly known as synchronous and asynchronous2D correlation spectra.4

The synchronous 2D spectrum, F (n 1, n 2), was calcu-lated by using Eq. 1, where n represents the total numberof spectra.

n1F (n , n ) 5 y (n , P ) ´ y (n , P ). (1)O1 2 1 j 2 j

n 2 1 j 5 1

The synchronous 2D correlation spectrum is symmetricwith respect to the diagonal. It is characterized by auto-peaks spread along the diagonal and cross peaks locatedat off-diagonal positions. It provides information on theextent of simultaneous or coincidental variations in thespectral intensities of IR bands.

The numerical computation of the asynchronous spec-trum with the use of the Fourier transform technique canbe complex and time consuming even for a large dataset. Computations become especially dif® cult when thedata along the perturbation axis are somewhat limited.Recently, a new 2D correlation analysis that utilizes thetime-domain Hilbert transform was introduced as an al-ternative to the Fourier transform.16 The asynchronous 2Dcorrelation spectrum, C (n 1, n 2), was computed by usingEq. 2.

n n1C (n , n ) 5 y (n , P ) ´ M ´ y (n , P ). (2)O O1 2 1 j jk 2 jn 2 1 j 5 1 k 5 1

The M jk function de® ned below is known as the Hilberttransform matrix.

0 if j 5 kM 5 (3)jk 5 1/ p (k 2 j ) otherwise.

The asynchronous correlation spectrum consisting purelyof cross peaks is antisymmetric with respect to the di-agonal. It represents variations in spectral intensities thatare completely or partially decoupled. All calculationswere done with the use of Matlab 5.0 (The MathworksInc.).

RESULTS AND DISC USSION

Effects of Pressure on NO Chemisorption. The ad-sorbate on the Pt(100) surface was established by initiallyadmitting NO at 1.80 3 10 2 7 Torr. A series of spectrawas then collected as the pressure of NO was increased(Fig. 1A) and decreased (Fig. 1B). We observe in Fig.1A the changes in the spectra of chemisorbed NO at 300K when the pressure of NO is increased up to 3 3 10 2 3

Torr. In this ® gure, a single band that shifts in frequencyfrom 1639 to 1647 cm 2 1 dominates the spectra below apressure of 9.32 3 10 2 6 Torr. However, at 9.32 3 10 2 6

Torr, a band begins to appear around 1689 cm 2 1. Thisband increased in intensity and shifted to 1692 cm 2 1 at apressure of 3.00 3 10 2 3 Torr. Under the same pressureconditions, the band at 1639 cm 2 1 shifted to a frequencyof 1654 cm 2 1. An additional infrared band is observed ata NO pressure of 2.53 3 10 2 4 Torr. This band initiallyappeared around 1819 cm 2 1, then shifted to 1823 cm 2 1

at a NO pressure of 3.00 3 10 2 3 Torr. When the pressurewas lowered back to 10 2 7 Torr, only the band at ; 1654cm 2 1 remained in the spectrum (Fig. 1B). The 1823 cm 2 1

band was the ® rst to disappear. A small remnant of the1692 cm 2 1 band still manifested itself at pressures up to10 2 6 Torr before it ® nally disappeared at 10 2 7 Torr. It isinteresting to note also that the band at ; 1654 cm 2 1 shift-ed to higher frequencies at a pressure of 10 2 7 Torr, whereit remained until the surface was heated up to 450 K. Theappearance of the 1692 cm 2 1 band at higher pressuresand its disappearance at lower pressures is reversible. Ex-periments carried out on different days showed that thisbehavior is reproducible; i.e., this band begins to appearat relatively higher pressures ( ; 9 3 10 2 6 Torr) and onlywhen the platinum surface is continuously exposed to NOgas.

In order to clarify the origin of the band at 1689±1692cm 2 1, we used isotopic substitution in the form of 15N18Oand performed experiments under conditions identical tothose described above. Figure 2 shows the results of theseexperiments. Again, all three bands manifested them-selves in the spectra collected at 10 2 3 Torr. However, allthe bands shifted in frequencies by as much as 66 cm 2 1.This shift corresponds closely to the square root of theratio of reduced masses, [ m (NO)/ m 15N18O)]1/2, whichstrongly suggests that the species that gives rise to theseIR bands is due to molecular NO adsorbed on the Pt(100)surface. The bands at 1639±1654 cm 2 1 and 1819±1823cm 2 1 on Pt(100) have been identi® ed previously with theuse of HREELS and IRAS as the bent and linear Pt±NOat 300 K, respectively.17,18 The ; 1823 cm 2 1 band is foundmainly on the defect sites created during the hex 5 1 31 phase transition of the Pt(100) surface.18 However, theband at 1689±1692 cm 2 1 has not been reported for the

180 Volume 53, Number 2, 1999

FIG. 1. A series of IR spectra showing the changes in the spectralfeature of adsorbed NO as the pressure of NO is increased from 1.80

3 10 2 7 to 3.00 3 10 2 3 Torr (A) and is brought down from 3.00 3 10 2 3

to 1.80 3 10 2 7 Torr (B) at 300 K. The wavenumber labels point onlyto the major IR peaks. The separation in the pressure is not even. Thearrows indicate the increase (A) and decrease (B) in the pressure.

FIG. 2. IR spectra of 15N 18O at 300 K. The pressure of 15N18O is in-creased from 1.80 3 10 2 7 to 3.00 3 10 2 3 Torr. The wavenumber labelspoint only to the major IR peaks. The separation in the pressure is noteven. The arrow indicates the increase in the pressure.

Pt(100) surface at 300 K. Note that, as the 1689±1692cm 2 1 band grows, the 1639±1654 cm 2 1 band decreasesin intensity (Fig. 1A). Similar spectral behavior was ob-served by Dunn et al. for the Pt(111)/NO system whena band at 1631 cm 2 1 was replaced by a band near 1700cm 2 1 with increasing NO exposure.19 One way to inter-pret this observation is that the chemisorbed species thatgives rise to the 1654 cm 2 1 band is structurally trans-forming into an adsorbed species whose vibrational fre-quency is centered at 1692 cm 2 1. An alternative interpre-tation is that the infrared bands arise from two distinctadsorption sites for NO. The decrease in the intensity ofthe 1639±1654 cm 2 1 band may simply be due to intensity

borrowing by the higher spectral frequency.20,21 Noticealso that the 1639 cm 2 1 band shifted to 1654 cm 2 1 as thepressure was increased. It has been shown that this typeof shift correlates well with the increase in the adsorbatecoverage, which in this case strongly suggests an increasein the concentration of the NO adsorbed on the site thatgives rise to the 1639±1654 cm 2 1 band.22 If the NO cov-erage for this site is increasing, it is possible that NO isadsorbing on a different site that is associated with theband at 1689±1692 cm 2 1.

To resolve the issues raised above, we performed a 2Dcorrelation analysis on the spectra shown in Fig. 1A. Theresults of the synchronous (A) and asynchronous (B) an-alyses are displayed in Fig. 3. The contour map of thesynchronous 2D spectrum is characterized by three au-topeaks at 1640, 1656, and 1690 cm 2 1 located along thediagonal and cross peaks located off-diagonal. All theautopeak frequencies agree well with those identi® edfrom the conventional absorbance spectra in Fig. 1A. Thespectrum is truncated from 1600 to 1750 cm 2 1 since therewas no peak observed at ; 1823 cm 2 1. The absence ofthis peak is due to its very weak correlation intensity. Aclose inspection of the 3D mesh of the synchronous spec-trum reveals an extremely small peak in this region thatcannot be captured in the contour map. Two of the au-topeaks correspond to the IR bands at ; 1654 cm 2 1 and; 1692 cm 2 1 observed in Fig. 1A. The autopeak at 1640cm 2 1 represents the spectral variations measured whenthe pressure was changed from 1.8 3 10 2 7 Torr to 9.253 10 2 5 Torr. Autopeaks, which are always positive, rep-resent the overall effect of a given perturbation on thedynamic variations of IR signals. In this case the externalperturbation is the variations in pressure. Cross peaks de-velop in the synchronous spectrum when the dynamicresponses of two spectral signals to the perturbation arecoordinated. Positive cross peaks indicate that the spec-

APPLIED SPECTROSCOPY 181

FIG. 3. (A) Synchronous and (B) asynchronous 2D FT-IR correlation contour plots of chemisorption of NO on Pt(100) calculated from the spectrain Fig. 1A. Dotted lines represent regions of negative correlation.

tral intensities are either increasing or decreasing in thesame direction, while the negative cross peaks show thatone spectral intensity is increasing while the other is de-creasing. These responses suggest strong interactions be-tween the chemical species that give rise to cross peaksin the synchronous spectrum.4 The negative cross peaksin the coordinates 1640/1656 cm 2 1 and 1640/1690 cm 2 1

suggest that the 1690 and 1656 cm 2 1 bands appear at theexpense of the 1640 cm 2 1 band. The positive cross peakat 1656 and 1690 cm 2 1 indicates that the changes in thesetwo bands occur in the same direction. The former ob-servation indicates that a structural transformation of theadsorbate did occur, while the latter suggests that the1656 and the 1690 cm 2 1 bands are formed at the sametime.

In the asynchronous spectrum a cross peak betweentwo dynamic spectral signals can develop only if theirintensities vary out of phase with each other. Cross peaksusually indicate that interactions between chemical spe-cies are decoupled and uncoordinated. This feature hasbeen used to discriminate chemical species that exist indifferent local molecular environments.1,8,9 Figure 3B dis-plays the contour map of the asynchronous correlationspectrum, which shows cross peaks at the following co-ordinates (cm 2 1): 1648/1655, 1640/1695, 1655/1695, and1686/1695. The asynchronous spectrum is markedly dif-ferent from the synchronous spectrum in that two addi-tional IR peaks are observed in the former. It appears thatthe ; 1654 cm 2 1 band is split into 1648 and 1655 cm 2 1

bands, while the ; 1692 cm 2 1 is resolved into 1686 and1695 cm 2 1 bands. The appearance of cross peaks in thesecoordinates indicates that the response patterns of thechemical species that give rise to these bands are inde-pendent of each other. This could only mean that the NOmolecules associated with these bands exist in differentmolecular environments; that is, the adsorption sites arenot identical. It must be pointed out that correlation crosspeaks for both synchronous and asynchronous spectra arealso observed for the IR signals at 1640, 1656, and 1690cm 2 1 (Fig. 3A). But the synchronous correlation arises

only because the variations in the spectral signals are notabsolutely orthogonal. Whereas synchronous cross peaksmerely suggest possible coordination in the responses be-tween spectral bands, asynchronous cross peaks provideunequivocal information that the responses are uncoor-dinated. As long as cross peaks develop in the asynchro-nous spectrum, bands are deconvoluted with certainty.1

Thus, the band at ; 1695 cm 2 1 ( ; 1692 cm 2 1 in Fig. 1A),which has not been reported previously, represents a dif-ferent adsorption site for NO. The bands at 1648 and1686 cm 2 1 resulting from the deconvolution of the ; 1654cm 2 1 band and the ; 1692 cm 2 1 band can be attributedto island formation. NO is known to form islands or clus-ters of adsorbed molecules.18 The formation of these is-lands is traditionally characterized by the broadening andthe splitting of IR bands, which are due to the differencein the intermolecular attractions experienced by the mol-ecules located near the center and the border of the is-lands. Since the border molecules experienced fewer in-teractions, their response to an external perturbation isdecoupled from that of the center molecules. Thus, theIR signals originating from these molecules could be dis-tinguished from each other in the asynchronous correla-tion spectrum, as shown in Fig. 3B.

It has always been dif® cult to assign spectral frequen-cies of NO adsorbed on speci® c Pt surface adsorptionsites because of its amphoteric character. Since the 2 p *orbital of NO is singly occupied, it can either accept ordonate electrons. This characteristic leads to a variety ofPt-NO bonding, which in turn leads to a wide range ofNO stretching frequencies. While a considerable contro-versy as to NO stretching frequency assignment still ex-ists, the present consensus on the Pt(100) surface gravi-tates towards the following: the lower spectral feature(i.e., 1591±1655 cm 2 1) is assigned to a bent terminal ad-sorption site, while the higher frequency band (1770±1800 cm 2 1) is assigned to a linear terminal NO chemi-sorbed on defect sites. These assignments relied mainlyon the NO stretching frequencies found in nitrosyl com-plexes; i.e., twofold bridging nitrosyls have NO stretch-

182 Volume 53, Number 2, 1999

FIG. 4. Changes in the IR spectra of NO as the surface temperature israised from 300 K to 480 K (A) and cooled down to 300 K (B) whilethe NO pressure is held at 3.00 3 10 2 3 Torr. Temperature is changedin steps of 20 K unless indicated otherwise. The wavenumber labelspoint only to the major IR peaks. The arrows indicate the increase (A)and decrease (B) in the temperature.

ing frequencies from 1480 to 1545 cm 2 1, a linear terminalfrom 1650 to 2000 cm 2 1, and a bent terminal from 1525to 1700 cm 2 1.23 With the same guidelines, the band ob-served at 1689±1692 cm 2 1 may be assigned to either abent or a linear top site. However, recent detailed quan-titative structural analysis that includes the effects of vi-brational and dipole±dipole coupling has raised seriousconcerns about the general applicability of assigning ad-sorption sites for NO via infrared stretching frequenciesalone.24±27 Because two-dimensional correlation analysisonly indicates the possible existence of two distinct ad-sorption sites for NO at high pressure, assignments ofadsorbate systems based purely on vibrational spectros-copy are avoided. However, we shall continue to respec-tively refer to the bands at 1639±1654 cm 2 1 and 1689±1692 cm 2 1 as occurring in the bent top site and linear topsite only to facilitate discussion. The difference in be-havior between these bands during thermal perturbation,which will be discussed shortly, justi® es this identi® ca-tion.

Effects of Temperature on NO Chemisorption. Wealso investigated the changes in the spectra of adsorbedNO on Pt(100) with varying surface temperature, asshown in Fig. 4. The sample was heated from 300 to 500K (Fig. 4A), then cooled down to 300 K (Fig. 4B) whilethe pressure of NO was held at 3.00 3 10 2 3 Torr. Unlessotherwise indicated, the temperature was incremented by20 K. The behavior of the three bands in the frequencyregion between 1639 and 1692 cm 2 1 was markedly dif-ferent during both the heating and the cooling stages. Theband at 1823 cm 2 1 was hardly distinguishable from thenoise at a surface temperature of 360 K during the heat-ing stage, and it did not appear when the surface wascooled down to 300 K. The early disappearance of theNO band at 1823 cm 2 1 during the heating stage may beattributed to the dissociation of NO on the defect sites.Earlier reports have shown that defect sites are quite ac-tive for the dissociation of NO even at temperatures aslow as 300 K.28±30 The absence of a band due to chemi-sorbed NO on defect sites during the cooling stage waspossibly caused by the blocking of these sites by oxygenatoms, which are known to preferentially adsorb on thesesites.31 Since it is a well-established fact that oxygen be-gins to desorb beyond 600 K, the oxygen atoms whichcame from the dissociation of NO remained adsorbed onthe surface even during the heating stage.32

We now turn our attention to the behavior of the bandsat ; 1692 and 1654 cm 2 1 during the heating and coolingstages. The band at 1692 cm 2 1 disappeared at 400 K,while the one at 1654 cm 2 1 vanished from the spectrumonly when the surface was heated to 450 K. Because thedissociation of NO on the 1 3 1 phase can take placeonly at temperatures above 380 K, we may assume thatthe decrease in the intensity of the 1692 cm 2 1 band isdue to the desorption of the linearly bound NO from thetop site, while that of the 1654 cm 2 1 band may be causedboth by desorption and dissociation of NO.33 During thecooling stage, both the bent and the linear terminal NObands are observed, although at temperatures lower thanthe temperatures at which they were last seen during theheating stage. The ; 1654 cm 2 1 band begins to show it-self at a relatively higher temperature of 430 K; the; 1692 cm 2 1 band begins to appear prominently in the

spectrum only at 360 K. Figure 4 suggests that the linearterminal NO is weakly bound to the surface as opposedto the strongly bound bent terminal, since the latter re-mained on the surface up to 440 K. This ® nding is con-sistent with the theoretical calculations performed bySmith and Carter on the interaction of NO with Pt atomsin which only the bent terminal was predicted to bebound with a bond energy of 20.4 kcal/mol, in excellentagreement with the binding energy of 19 6 2 kcal/molobtained from TPD studies.34,35

CONCLUSION

In this study, two-dimensional correlation analysis ofthe variations in the IR spectral intensities has allowed

APPLIED SPECTROSCOPY 183

us to characterize the adsorption of NO on the Pt(100)surface in a continuous-¯ ow system at 300 K. Asynchro-nous correlation analysis was able to distinguish two dif-ferent adsorption sites for NO as shown by the indepen-dent development of infrared bands at 1689±1692 cm 2 1

and 1639±1654 cm 2 1. The evolution of the infrared N±O stretching bands is affected by both the NO pressureand the surface temperature. We found that the band at1689±1692 cm 2 1 appears only at higher NO pressure andlower surface temperature, which indicates that the spe-cies associated with this band is weakly bound to theplatinum surface. The results obtained in this work dem-onstrate the possible advantage of 2D correlation IR spec-troscopy in surface analysis particularly in the adsorp-tions and reactions of complex molecules, which poten-tially give rise to highly overlapped bands.

1. I. Noda, Appl. Spectrosc. 44, 550 (1990).2. I. Noda, Bull. Am. Phys. Soc. 31, 520 (1986).3. I. Noda, J. Am. Chem. Soc. 111, 811 (1989).4. I. Noda, Appl. Spectrosc. 47, 1329 (1993).5. I. Noda, A. E. Dowrey, and C. Marcott, Appl. Spectrosc. 47, 1317

(1993).6. C. Roselli, J. Burie, T. Mattioli, and A. Boussac, Biospectroscopy

1, 329 (1995).7. Y. Ozaki, Y. Liu, and I. Noda, Appl. Spectrosc. 51, 526 (1997).8. N. L. Sefara, N. P. Magtoto, and H. H. Richardson, Appl. Spectrosc.

51, 536 (1997).9. N. L. Sefara, T. K. Green, and H. H. Richardson, paper submitted

to Biospectroscopy (1998).10. For a recent review, see R. Imbihl and G. Ertl, Chem. Rev. 95, 697

(1995).11. W. F. Egelhoff, in The Chemical Physics of Solid Surfaces and

Heterogeneous Catalysis, Vol. 4, D. A. King and D. P. Woodruff,Eds. (North-Holland, Amsterdam, 1982), Chap. 9.

12. N. P. Magtoto and H. H. Richardson, J. Phys. Chem. 100, 8482(1996).

13. N. P. Magtoto and H. H. Richardson, Surf. Interface Anal. 25, 81(1997).

14. H. Bonzel, G. Borden, and G. Pirug, J. Catal. 53, 96 (1978).15. S. Hong and H. H. Richardson, J. Vac. Sci. Technol. A. 11, 1951

(1993).16. I. Noda, Presented at the 2nd International Symposium on Ad-

vanced Infrared Spectroscopy, Durham, North Carolina (1996), Pa-per A-16.

17. G. Pirug, H. P. Bonzel, H. Hopster, and H. Ibach, J. Chem. Phys.71, 593 (1979).

18. P. Gardner, R. Martin, M. Tushaus, and A. M. Bradshaw, Surf. Sci.240, 112 (1990).

19. D. S. Dunn, M. W. Mark, J. L. Hayden, and J. Overend, J. Catal.78, 225 (1982).

20. A. S. Wexler, Spectrochim. Acta, Part A 23 1319 (1967).21. M. W. Severson, C. Stuhlmann, I. Villegas, and M. J. Weaver, J.

Chem. Phys. 101, 1648 (1994).22. A. Crossley and D. A. King, Surf. Sci. 95, 131 (1980).23. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (Wi-

ley, New York, 1972), 3rd ed.24. S. Aminpirooz, A. Schmalz, L. Becker, and J. Haase, Phys. Rev.

B. 45, 6337 (1992).25. D. K. Paul, T. P. Beebe, Jr., K. J. Uram, and J. T. Yates, J. Am.

Chem. Soc. 114, 1949 (1992).26. M. C. Asensio, D. P. Woodruff, A. W. Robinson, K.-M. Schindler,

P. Gardner, D. Ricken, A. M. Bradshaw, J. C. Conesa, and A. R.Gonzalez-Elipe, Chem. Phys. Lett. 192, 259 (1992).

27. P. Gardner, R. Martin, R. Nalezinski, C. L. A. Lamont, M. J. Weav-er, and A. M. Bradshaw, J. Chem. Soc. Faraday Trans. 91, 3575(1995).

28. H. D. Schmick and H. W. Wassmuth, Surf. Sci. 123, 472 (1982).29. S. W. Jorgensen, N. D. S. Canning, and R. J. Madir, Surf. Sci. 179,

28 (1987).30. Th. Fink, J. Dath, R. Imbihl, and G. Ertl, J. Chem. Phys. 95, 2109

(1991).31. J. L. Gland and V. N. Korchak, Surf. Sci. 75, 733 (1978).32. K. Grif® ths, T. E. Jackman, J. A. Davies, and P. R. Norton, Surf.

Sci. 138, 113 (1984).33. Th. Fink, J. Dath, M. Bassett, R. Imbihl, and G. Ertl, Surf. Sci.

245, 96 (1991).34. G. W. Smith and E. A. Carter, J. Phys. Chem. 95, 2327 (1991).35. J. L. Gland and B. A. Sexton, Surf. Sci. 94, 355 (1980).