The Journal of
Physical Chemistry
0 Copyright, 1987, by the American Chemical Society
VOLUME 91, NUMBER 23 NOVEMBER 5,1987
LETTERS Infrared Spectrum of CO on NaCI( 100) Hugh H. Richardsont and George E. Ewing* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 (Received: July 16, 1987)
Adsorbed molecules on alkali metal halide surfaces serve as model systems for the study of physisorption, two-dimensional phase transitions, vibrational relaxation, and surface chemistry. However, all previous optical spectroscopy of these substrates has used films rather than well-defined crystal faces. We report here, for the first time, the infrared spectrum of CO on the (100)face of NaCl. While the vibrational frequency matches that of CO on NaCl films, the bandwidth is much narrower. Polarization measurements show that CO is aligned perpendicular to the (100) face. The narrow and easily observed absorption features will make the infrared spectroscopy of adsorbed molecules a sensitive probe of alkali metal halide surface interactions.
Introduction Infrared spectroscopy coupled with thermodynamic measurements has provided a valuable probe for molecular adsorption on alkali metal halide films. Adsorption isotherms, heat of adsorption measurements, vibrational frequencies of adsorbates on different sites, frequency shifts as a function of surface coverage, and particle size estimates are a few quantities which have been measured for small molecules on alkali metal halide films.14 In addition, various theoretical models have been postulated to explaii the adsorption process. In particular for C O adsorbed on NaCl surfaces, Gevirzman et al.' and Gready et al.5 analyzed the physisorption process in terms of dispersion, electrostatic, induction, and repulsion interactions and concluded that the CO molecules bind perpendicular to the (100) face directly above the sodium ions. These salt film studies, which span over 50 years,6 are complemented by more recent ultrahigh-vacuum (UHV) investigations of molecular adsorption on well-defined crystal surface^.'.^ Carefully prepared salt films consist of microscopic cubic crystallites with smooth faces closely resembling macroscopic crys'Present address: Department of Chemistry, Ohio University, Athens, OH 45701. 'To whom correspondence should be addressed.
tal^.^,^ However, we know of no experiment in which the same technique has been applied to molecular absorption processes on both film and well-defined single-crystal substrates-with a single exception. This exception is the measurement of the heat of adsorption of CO on NaCl film's4 and NaCl( 100) single crystal.' We provide here for the first time a direct spectroscopic comparison between molecular absorption on a film and a well-defined crystal surface.
Experimental Section Single crystals, 25 mm X 25 mm X -3 mm with (100) faces exposed, were obtained by cleaving in air high-purity cleavage (1) Gevirzman, R.; Kozirovski, Y.;Folman, M. Tram. Faraday Soc. 1%9, 65, 2206. (2) Heidberg, J.; Stein, H.; Hussla, I. Surf. Sci. 1985, 262, 470. (3) Zecchina, A.; Scarano, D.; Gamone, E. Surf.Sci. 1985, 160, 492. (4) Richardson, H. H.; Baumann, C.; Ewing, G. E. Surf.Sci. 1987, 185,
15. (5) Gready, J. E.; Bacskay, G. B.; Hush, N. S. Chem. Phys. 1978,32,375. (6) De Boer, J. H. Electron Emission and Adsorption Phenomena; University Press: Cambridge, 1935. (7) Hardy, J. P.; Ewing, G. E.; Stables, R.; Simpson, C. J. S. M. Surf. Sci. 1985, 159, L474. ( 8 ) Hardy, J. P.; Cox, P.A.; Ewing, G. E.; Simpson, C. J. S . M. SurJ Sci. 1986, 175, 24 1. (9) Schulz, L. G. J . Chem. Phys. 1949, 17, 1153.
0022-3654/87/2091-5833$01.50/0 0 1987 American Chemical Society
Letters
5834 The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 1
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Sample holder Infrared source Spherical mirror Infrared polarizer UHV chamber Nude ion gouge Quadruple mass spectrometer O f f -axis parabolic mirror F l a t mirror Beam splitter Interferometer He -Ne reference laser Laser detector Infrared detector NaCl cleaved sample Moving mirror Radiation shield
Figure 1. Schematic of the U H V chamber and optical components. A bird's-eye view of the U H V chamber (E) and light throughput is shown in the upper right. An enlarged view of the sample holder (A) and crystals is shown with respect to the interogating light polarization in the lower right. The optical layout of the FTIR interferometer is shown in the upper left.
grade cubes of NaCl (Harshaw). Visual inspection of the crystals revealed smooth faces with occasional terracing. Two NaCl crystals were then mounted on a Cu sample holder (A) in thermal contact with a cryostat work surface inside a Huntington UHV chamber (E) as shown in Figure 1. The crystals were tilted 30' with respect to the interogating infrared propagation k vector. The UHV system was evacuated with a sorption pump (Huntington) to a pressure near mbar where pumping into the UHV region was accomplished with an ion pump (Varian 60 L/s). Baking the UHV system at 150 OC for several days and subsequent cooling to room temperature produced a vacuum of 1 X lo4 mbar. The major residual gases were H2,He, and Ne as revealed by a Baltzers Q M G l l 2 quadrupole mass spectrometer (G). The remaining gases, CO, CH4, and H 2 0 , were less than 5 X lo-" mbar. The NaCl crystals were kept at 100 OC until cooling with an open cycle liquid He cryostat (Janos Research) to a temperature of 40 K. Carbon monoxide (Matheson research purity) was then admitted into the UHV chamber by a variable leak valve (Vacuum Generators). At the same time, the pumping speed of the ion pump was regulated with the main gate valve (Huntington) so that a constant pressure of CO in the sample chamber was maintained. The pressure was determined by a Vacuum Generators nude ion gauge (F) placed in the sample chamber. Spectra were recorded with a Digilab 15-C FTIR interferometer as a function of time until the band area of the adsorbate was constant. The leveling off of the band area with time, its magnitude related to the pressure of CO, determined the approach to equilibrium between the adsorbate gas and the surface. This process took =30 min for a pressure of CO in the mbar range. The polarization of the infrared light was selected with a Molectron wire grid polarizer (D) between the source and the sample chamber. Light from the source, a water-cooled globar, was focused in the middle of the sample chamber, passing through two single crystals (four surfaces) of NaCl, and collected with an off-axis parabolic mirror (H). The collected light was processed into the interferometer and detected with a Hg-Cd-Te detector (N). Spectral resolution
was 0.6 cm-' after triangular apodization of the interferograms. Absorbance, A = log (Zo/Z), was obtained with 1, the reference light intensity through the adsorbance-free NaCl and Z the intensity with CO added.
Results and Discussion Cleavage of NaCl crystals in air has been previously shown to produce (100) faces with occasional s t e p of atomic dimensions.'"-" Water which adsorbs on the freshly cleaved surface is pumped away under mild bakeout conditions.12 Thus, we believe that our experimental procedure produces a 2153-cm-' (100) surface with only a small amount of surface imperfections. Figure 2 shows the polarization spectra for CO on NaCl(100) at 40 K with a CO pressure of 5 X lo4 mbar. It is apparent that only the E p polarization of light results in any absorption. Reducing the CO pressure reduces the absorbance of the 2155-cm-' feature reversibly. However, increasing the pressure beyond 1O-* mbar by an order of magnitude causes no change in the absorbance so we take the coverage for Figure 2 to be saturated monolayer. Pressure increases by 2 orders of magnitude result in the growth of features at 2138 and 2142 cm-' which we assign to CO multilayers since they lie close to the absorption of neat solid CO.I3 The Eppolarization only probes molecules with the component of their transition dipoles, M ~ perpendicular , to the surface while the E, polarization can only excite transition dipoles, pI1,parallel to the surface as shown in Figure 1. The presence of absorption only with E polarization thus confirms the theoretical predicti~n'.~ that the C 6 molecules bind perpendicular to the surface. The E,-polanzed spectrum of CO on NaCl( 100) is compared with adsorption on films* in Figure 3. The band of CO adsorbed ~~
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(10) Basset, G. A. Philos. Mag. 1958, 3, 1042. (11) Bardi, V.; Glachant, A.; Bienfait, M.Surf. Sci. 1980, 97 137. (12) Estel, J.; Hoinkes, H.; Kaarman, H.; Nahr, N.; Wilsh, H. SurJ. Sci. 1976, 54, 393. (13) Ewing, G. E. J . Chem. Phys. 1962, 37, 2250.
Letters
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 5835
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v' (cm-l) Figure 2. Polarized infrared spectra of CO on NaCl(100). The crystals were at 40 K with a CO pressure of 5 X mbar.
on an unannealed NaCl film at 0 = 1.O is broad (fwhh = 12 cm-') and featureless. The band diffuseness is attributed to the heterogeneous adsorption sites presented by the irregularly shaped crystallites making up the film.4 Annealing the NaCl film causes the NaCl particulates to grow from 3-10 to 100 nm along an edge.4,sJ4 The spectrum of CO adsorbed on an annealed NaCl film at e = 0.8 presents a narrowed bandwidth (fwhh = 4 cm-') and reveals structure not seen on unannealed films. The shoulder at 2160 cm-I has been assigned to CO molecules adsorbed on binding sites along steps on the crystal surface, whereas the band at 2 154 cm-I has been assigned to CO molecules adsorbed on smooth face binding sitesS4Lowering the film temperature to 20 K sharpens all features considerably, and a fwhh = 2 cm-I is observe4i4 However, the effects of heterogeneities are still evident since features attributed to CO adsorption on steps, pores, and other sites remain. The absorption frequency for C O on NaCl(100) is 1 cm-I from the film results, but the bandwidth is sharper with fwhh I 0.6 cm-'. These data are consistent with electron microscopy of annealed films which reveal an assortment of regular cubes with smooth faces3 that we can now identify as (100) faces. The absorbance of CO on NaCl film is easy to observe since the number of sites (i.e., Na+) is large on the multiple crystallites making up the film. Depending on the film thickness and degree of annealing, this site density can easily fall in the range 10l6to 10l8 cm-2. By contrast, the site density on NaCl(100) is only 5 X l o i 4cm-2 for each faces4 Nevertheless, absorbance of CO on NaCl( 100) in Figures 2 and 3 is remarkably strong. Unfortu(14) Smart, R. St. C. Trans. Faraday SOC.1971, 67, 1183.
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ij (crn-l) Figure 3. Infrared spectra of CO on NaCl( 100) and NaCl films. The upper trace is taken from ref 4 for the unannealed film at 8 5 K with B = 1.0. The annealed film spectrum of 8 3 K is for 0 = 0.8 from ref 4. The lower trace is the E, polarization spectrum of Figure 2. The absorbance scale factors, a reflection of the available number of adsorption sites of the substrates, are indicated.
nately, superimposed on these spectra are the fringe patterns from the UHV chamber sapphire windows and the polarizer which act as etalons. This fringe pattern, with ii: 1-cm-' oscillations along the base line, has been reduced by removing its signature from the interferogram. This reduction is more successful for the E, than the E, polarization. When the windows and polarizer are wedged or antireflection coated, the fringes will disappear and we anticipate a signal-to-noise ratio of >lo0 for the coverages represented in our experiments. Thus, submonolayer coverages can be easily studied. Since the absorption features of molecules on well-defined alkali metal halide crystal faces can be sharp, their vibrational frequencies and bandshapes can be accurately measured. The vibrational shifts and band profiles, sensitive to the nature of the then provide a rich source of information on adsorption surface interactions. We are now in the process of exploring these molecule-alkali metal halide systems.
Acknowledgment. We thank Professor Charles T. Campbell for many helpful suggestions during the course of this work. The financial support of the National Science Foundation is gratefully acknowledged.
