J. Phys. Chem. B 2000, 104, 3343-3348
3343
Desorption and Reaction of Water on MgO(100) Studied as a Function of Surface Preparation† S. Imad-Uddin Ahmed,‡ Scott S. Perry,* and Oussama El-Bjeirami Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: September 24, 1999; In Final Form: January 26, 2000
Temperature-programmed desorption (TPD) is used in conjunction with X-ray photoelectron spectroscopy (XPS) to probe the surface reactivity of water with MgO surfaces prepared by mechanical polishing/acid etching, vacuum annealing in partial pressures of oxygen, high-temperature annealing in an oxygen ambient, and ion sputtering. The structure and morphology of surfaces prepared by these procedures have been previously characterized on the atomic scale with atomic force microscopy. Surfaces prepared using etching procedures and etch and annealing in partial pressures of oxygen demonstrate an atomically rough surface morphology and broad desorption features indicative of an inhomogeneous surface. Surfaces annealed in an oxygen ambient exhibit atomically flat terraces and sharp desorption features. Ar+ bombardment at 500 eV of atomically flat MgO surfaces results in broad features appearing in the water desorption spectra. These are attributed to surface vacancies created by the sputtering process, which act as adsorption sites. XPS measurements indicate preferential removal of oxygen during the Ar+ bombardment process. Water adsorption or vacuum annealing in partial pressures of O2 results in the removal of vacancies by dissociative adsorption at defect sites. Subsequent TPD measurements of water result in a desorption spectrum that is identical with that obtained from the initially well-ordered surface. This study demonstrates that the thermal desorption of water from MgO surfaces is indicative of local surface order and provides atomic level insight into the methods of preparing stoichiometric MgO(100) surfaces.
Introduction Magnesium oxide with its simple rocksalt structure is a model oxide for fundamental scientific investigations.1 On the technological level, its chemical stability and structural simplicity have made it an ideal candidate as a substrate for the study of gas adsorption,1 metal deposition,1,2 or for the epitaxial growth of various oxides,1-7 particularly oxide superconductors.3 To meet the objectives of both fundamental and technological studies, it is necessary to first prepare and characterize stoichiometric surfaces and to understand the reactivity of these surfaces. There are various methods in the literature for preparing MgO surfaces. Many vacuum studies of crystalline MgO surfaces have employed acid etching as a means of ex situ cleaning. Other methods consist of cleaving a single crystal in ultrahigh vacuum (UHV) and then performing studies on the freshly cleaved surface.1,2,9-15 Single crystals have also been cleaved in air and then subsequently treated by sputering and/ or annealing cycles in UHV. Other methods have involved first mechanically polishing the surfaces and then subjecting them to sputtering and/or annealing cycles.1,2,16-21 An alternative approach is to form MgO films by oxidizing the metallic Mg surface22 or by depositing thin MgO films on Mo(100),1,2,23-29 on single-crystal MgO,30 or on lattice matched substrates.7,31 It has been proposed that the UHV cleaved surface is the most ideal and stoichiometric surface.1,2 However, for most studies involving epitaxial growth or irreversible surface chemistry, preparation by cleaving in UHV is not feasible. As a result, †
Part of the special issue “Gabor Somorjai Festschrift”. * Corresponding author. E-mail:
[email protected]. Fax: (713) 743-2709. ‡Present address: Institut fu ¨ r Physik, Technische Universita¨t-Ilmenau, PF 100565, D-98684 Ilmenau, Germany.
suitable methods of generating atomically flat MgO surfaces where the prepared surface may also be regenerated are needed. Many surface studies in the past have not characterized the surface on an atomic scale because of the unavailability of scanning probe methods.1 This added detail is believed to account for discrepancies in the observed behavior of identical systems.1 The routine incorporation of scanning probe methods in surface preparation is still not fully widespread, but is presently increasing and will eventually contribute to a meaningful comparison between experimental results. A preparation procedure developed by this group provides stoichiometric, atomically flat MgO surfaces.32 The surfaces were prepared by mechanical polishing followed by etching in phosphoric acid and annealing at 1273 K in an oxygen ambient. In previous reports, these surfaces have been characterized by X-ray photoelectron spectroscopy (XPS), low-energy and reflection high-energy electron diffraction methods (LEED and RHEED), and atomic force microscopy (AFM).32,33 The surface reactivity of this oxygen-annealed surface had, as yet, not been determined in our previous studies. Such an examination is necessary to compare the reactivity of this surface with surfaces prepared using other methods, and to further explore the adsorption and reaction of adsorbates with this model and technologically relevant system. Of the reactions studied on MgO, the reaction of water with MgO(100) has been studied extensively. Numerous experimental investigations have been performed using low-energy electron diffraction (LEED),14,15,26 helium atom scattering (HAS),14 Auger electron spectroscopy (AES),21,22 photoemission,3-5,22,34 Fourier transform infrared (FTIR) spectroscopy,13 reflection-absorption infrared spectroscopy (RAIRS),26 high-resolution electron loss spectroscopy
10.1021/jp9934275 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/18/2000
3344 J. Phys. Chem. B, Vol. 104, No. 14, 2000 (HREELS),24,25 secondary ion mass spectrometry (SIMS),17 electron stimulated desorption (ESD),19,21 metastable impact electron spectroscopy (MEIS),27,28 and temperature-programmed desorption (TPD).18,25,26 In addition, many theoretical methods have been applied to explain the interaction between water and the MgO(100) surface.1,35-46 These studies have important implications for the continued use of MgO(100) in areas such as catalysis, chemical sensors, and epitaxial growth. Experimental studies continue to debate the nature of the adsorbed state of water on crystalline MgO surfaces. They have concluded either that water chemisorbs on MgO dissociatively3-5,15,17,18,22,23,25,28 or that molecular adsorption is the predominant mechanism.13,14,18,26 There is no clear consensus on the theoretical front regarding the process of water adsorption on MgO surfaces. A number of theoretical studies support the view that dissociative adsorption on the ideal MgO(100) surface is energetically unfavorable,35,36,39,42,43 but occurs at step edges and corner sites (defect sites).36,38,39,42 However, two recent theoretical studies concluded that on the perfect (100) surface a mixture of dissociative and molecular adsorption occurs.44,45 In this study, we report on the desorption of water from MgO(100) surfaces prepared by a number of different methods. The primary motivation is to present results concerning the interaction of water with a variety of MgO(100) surfaces of known surface morphology and, thereby, to demonstrate the viability of using TPD as a sensitive tool for ascertaining the quality (defect activity) of a surface. A prior study by Striniman et al. has reported on the desorption of water from MgO surfaces as a function of surface preparation, however it was restricted to UHV preparation methods.18 The present study provides direct correlation between surface morphology and water desorption characteristics and explores both ambient and UHV preparation methods. Experimental Section Single crystal MgO(100) samples, mechanically polished on one side and measuring 1 cm × 1 cm, were obtained from a commercial vendor (Alpha Aesar). For all studies described here, the surfaces were first treated by etching for 30 s in 14.6 M (85% w/v) solution of H3PO4 (Aldrich) and then thoroughly rinsed in deionized water. Each sample was then halved (in a direction normal to the surface plane), retaining one piece for control measurements of surface morphology. In all cases, the etching procedure was observed by AFM to produce identical surface morphology.33 Two of the preparation procedures, described as acid-etched and vacuum-annealed in O2, involved the introduction of the MgO substrate into UHV at this point. For the other procedures, acid etched substrates were placed on an alumina boat that was inserted in a 1.0 in. OD alumina tube. The tube was adjusted so that the sample was in the center region of the tube furnace. Sample temperature was monitored using a type K thermocouple which was situated in an enclosed alumina tube that was positioned in the central heating region of the furnace. The sample temperature was then increased to 1273 K and maintained at that temperature for 2 h under an oxygen (99.6% purity) flow rate of 110 standard cubic cm/min (1 atm). Sample cooldown was performed at a rate of 5 K/min. In most cases, the annealed samples were first characterized using atomic force microscopy and then immediately transferred to the UHV system. In some cases, the samples were stored in a vacuum desiccator prior to use. The surface morphology of these samples was measured with AFM prior to introduction to the vacuum system to ensure that brief exposures to ambient conditions had not altered the surface morphology through reactions with ambient water.
Imad-Uddin Ahmed et al. Temperature-programmed desorption and X-ray photoelectron spectroscopy were performed in a stainless steel chamber with a base pressure of less that 2 × 10-10 Torr and equipped with load lock/sample transfer capabilities. The adsorption of water (D2O, Aldrich, 99.96 atom % D) was performed by using a pinhole doser.10,11 This technique limits the dose to the crystal face and eliminates any unwanted background effects. The doser consists of a 2 µm pinhole and a drift tube (5 mm ID) that is attached to a bellows assembly. During dosing, the end of the drift tube was positioned within 0.5 mm of the sample face. In this configuration, all the gas effusing from the tube impinges onto the sample face. The diffusion rate through the pinhole was fixed by maintaining a constant pressure (typically 1 Torr) on the gas handling side. Various gas doses (resulting in various coverages) were achieved by varying the time the sample was exposed to the effusing gas. This technique provides excellent coverage reproducibility. Substrate heating, cooling, and positioning was carried out in UHV using a custom sample manipulator (Thermionics Laboratory) fitted with a transferable thermocouple contact. With this experimental configuration, gas adsorption studies could be performed under UHV conditions without the need for bakeout procedures, which would certainly alter the surface composition. The MgO samples were fixed onto a Mo plate using Ta wires and foils. The plate was then inserted into the sample holder and was held firmly in position using Mo clips. The sample holder was inserted into UHV through a load lock and locked into position on a copper N2 Dewar. The crystal could be heated radiatively using a W filament located behind the sample. Sample cooling was achieved by the flow of N2 gas, first passed through a coil immersed in liquid nitrogen, through the copper Dewar. This setup enabled rapid cooling of the sample to 90 K and heating to 1400 K. Temperature was monitored using a type K thermocouple that was firmly held to the crystal face using thin Ta strips. TPD experiments were performed using a quadrupole mass spectrometer (Fisons/VG, Quartz 200). The sample was positioned approximately 2 cm from the ionizer of the QMS and a linear increase in temperature (∼4 K/s) was applied by radiative heating of the sample from behind. The mass-to-charge ratios (m/q) monitored in these experiments were 2(H2+,D+), 4(D2+), 18(H2O+, OD+), 20(D2O+), 32(O2+). All data were collected using the same mass spectral sensitivities. An arbitrary y-offset has been utilized in some of the desorption spectra in this paper to display some of the multiple sets of data. XPS studies were carried out with an Omicron EA 125 energy analyzer that was situated 30° from the surface normal. Al KR radiation (1486.6 eV) from a VG dual anode source was used as the excitation radiation. In this study, Mg 1s, Mg 2s, O 1s, Mg KLL, and O KVV spectra were measured with a pass energy of 25 eV. Stoichiometric ratios of the MgO surface were calculated using atomic sensitivity factors of 0.66 (O 1s) and 0.15 (Mg 2s). The ratio of these values is in close agreement with the values employed by Peng and Barteau22 and provides a 1:1 (O:Mg) stoichiometric ratio for the most ideal surface identified through our previous studies.32 Results and Discussion The thermal desorption of D2O was measured as a function of surface coverage from MgO(100) substrates prepared by four different procedures according to the details outlined in the previous section. The first set of measurements involved desorption from an acid-etched MgO surface. This procedure has been used by researchers in the past as a method of cleaning
Desorption and Reaction of Water on MgO(100)
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Figure 1. The thermal desorption spectra of water adsorbed on an acid-etched MgO(100) surface are displayed as a function of coverage. The (A) m/q ) 20 amu (D2O) channel is a measure of both molecular and recombinative desorption, while the (B) m/q ) 4 amu (D2) channel reflects the presence of dissociated water on the surface. Arbitrary offsets have been applied to the D2 spectra for presentation purposes.
Figure 2. The thermal desorption spectra of water adsorbed on a MgO(100) surface that has been acid-etched and then annealed (773 K) in vacuum in a 1 × 10-6 Torr partial pressure of O2: (A) m/q ) 20 amu; (B) m/q ) 4 amu. Arbitrary offsets have been applied to the D2 spectra for presentation purposes.
MgO surfaces before insertion to UHV. Prior AFM studies have demonstrated that the morphology of a surface prepared in this way consists of large features that are ∼100 Å high and 100300 Å wide.32 The generation of these surface features may result from the anisotropic etching of the crystalline material; however, convolution of the tip shape limits the certainty with which this can be concluded. The overall rms roughness of this surface was determined to be 36.7 ( 2.2 Å.32 The desorption spectra of molecular water (D2O) from the acid-etched surface include four distinguishable states (Figure 1a). At the lowest coverage, two states centered at 287 and 538 K are populated. With increasing coverage the peak desorption temperature of these states shifts downward to 253 and 513 K, respectively, indicative of a second-order process leading to desorption. (The existence of two states in the 253-287 K range is difficult to determine due to the broad nature of the peak.) From both the temperature range and reaction order of these features,49 we assign these features to the recombinative desorption of molecular water and therefore conclude that water adsorbs dissociatively on this surface. Prior surface vibrational measurements of water adsorbed on MgO have detected the presence of surface hydroxyl species and further support this assignment.25 This study also recorded TPD spectra of water with a dominant peak in this temperature range.25 As coverage increases, two additional states centered at 185 and 153 K are populated, which we assign through comparison to previous studies as water desorbing from the second and multilayers,
respectively.49 We note that all desorption features, except the multilayer peak, are markedly broad, indicating the presence of a large number of adsorption sites. The breadths of the desorption features are consistent with the highly stepped and irregular surface morphology as revealed by AFM.32,33 The coverage-dependent evolution of hydrogen (D2) as shown in Figure 1b is further indication of a defective surface consuming oxygen in the reaction process. The second preparation procedure explored in this study involved annealing the same acid-etched surface, which yielded the results above, in 1 × 10-6 Torr of oxygen at 773 K for 10 min. Annealing metal oxide surfaces in partial pressures of oxygen has been used in many surface science studies as a method of preparing or restoring oxide composition and structure. Our prior studies with AFM indicate that such procedures do induce a change in surface morphology (growth and flattening of the previous features), however topographic features on the order of 50 Å in height still exist.32,33 Water desorption experiments performed on this surface clearly demonstrate that this morphological change is associated with a change in surface reactivity (Figure 2). As coverage increases, a low-intensity, high-temperature feature centered at 450 K, a low-temperature state centered at 226 K, and a multilayer feature centered at 149 K are populated. We again assign the hightemperature state at 450 K as a recombinative desorption feature; however, the reduced temperature of the primary monolayer desorption state is now more indicative of desorption of intact
3346 J. Phys. Chem. B, Vol. 104, No. 14, 2000
Imad-Uddin Ahmed et al.
Figure 3. The thermal desorption spectra of water adsorbed on a MgO(100) surface prepared by annealing (1273 K) for 2 h in an ambient pressure of O2 before introduction to the vacuum system: (A) m/q ) 20 amu; (B) m/q ) 4 amu. Arbitrary offsets have been applied to the D2 spectra for presentation purposes.
water molecules from the surface. This assignment is supported by the observed decrease in hydrogen evolution (Figure 2b). From these changes in the TPD spectra, we conclude that annealing defective surfaces in 1 × 10-6 partial pressures of oxygen effectively weakens the interaction of water with the “rough” MgO surface. The exact manner in which this occurs is not directly evident; however, the additional reaction and uptake of oxygen on the surface is likely. The breadth of the monolayer desorption features together with the surface morphology measured with AFM indicate that the surfaces produced by this procedure are still relatively heterogeneous in terms of adsorption sites. The third preparation procedure explored in this study involved MgO surfaces that had been annealed to 1273 K for 2 h in ambient pressures of O2 before being transferred into the vacuum chamber. This surface was also annealed briefly in a vacuum to 1273 K upon introduction to the system. AFM images of this oxygen-annealed surface show a well-ordered surface consisting of atomically flat terraces that are approximately 500 Å wide with single and double step heights of 2.5 and 4.5 Å ((0.5 Å). The rms roughness calculated over the entire region (1 µm2) was 2.6 ( 0.9 Å, while that of an individual terrace was 1.3 ( 0.1 Å.32 Figure 3a shows the desorption traces obtained by monitoring m/z ) 20 for various D2O doses at 90 K. Four desorption peaks can be identified.50 At lowest coverages, a peak centered at ∼260 K is populated however quickly saturates as coverage is increased. This state appears in the same temperature range as the predominant monolayer feature observed for the acid-etched surface. The thermal desorption spectrum of water from the monolayer also includes a narrow feature centered at 232 K. The peak desorption temperature of this feature is nearly independent of coverage and indicative of a first-order desorption process. Estimations of the integrated areas of these features reveal that the two states, with significantly different fwhm, have approximately equal intensities. Beyond the monolayer, two states are populated. A small state at ∼200 K, which saturates with coverage is assigned to water in the second layer, experiencing a greater interaction with species in the monolayer than the molecules in the multilayer experience. At highest coverages, the spectra are dominated by the multilayer desorption feature which is centered at 152 K. Little hydrogen (D2) is observed to desorb from this well-ordered surface (Figure 3b). While TPD does not provide a direct measure of surface species, we believe that a comparison of prior spectroscopic
studies (HREELS23 and RAIRS26) measured together with TPD spectra, provide insight into the nature of water adsorption to this highly ordered surface. Those studies, performed on epitaxial thin films of MgO, detected surface hyroxyl vibrations with HREELS when significant intensity was present in the TPD spectra in the range of 250-300 K. However, when films prepared at a different time exhibited only a dominant monolayer feature in the range of 230 K, RAIRS measurements detected little evidence for dissociated water in the form of surface hydroxyls. On the basis of those results and our comparison of desorption from disordered and ordered surfaces, we believe that water adsorbs both moleculary and dissociatively to the highly ordered MgO(100) surface prepared by annealing in ambient pressures of oxygen, thus giving rise to the two states observed in the TPD spectra (Figure 3a). The question that immediately arises is at which specific sites on the Mg(100) surface does molecular and dissociative adsorption occur. Because of the fact that our prior AFM studies indicate a low step density32,33 and TPD spectra show relatively equal integrated intensities of the two monolayer states, we are able to conclude that dissociation must be occurring on terrace sites as well as step sites. Recent theoretical studies support the assertion that both molecular and dissociative adsorption occur on wellordered MgO(100) terraces.44,45 Finally, the influence of low-energy Ar+ bombardment of the oxygen-annealed MgO surface on the desorption properties of water was investigated. The preparation of oxide surfaces with ion sputtering procedures is desirable because it would allow the cleaning and regeneration of surfaces for chemical studies. Sputtering treatments (500 eV Ar+ ions, 1-4 min) of the oxygen-annealed MgO surface resulted in noticeable changes in the desorption spectra for both the m/z ) 4 and m/z ) 20 channels. The desorption spectrum of a multilayer coverage of water following a 2 min sputtering cycle is shown in the upper trace of Figure 4a. A slight broadening of the monolayer state centered at ∼260 K is observed in agreement with prior reports of MgO surfaces treated with an unspecified Ar ion treatment.18 However, in contrast, a broad desorption feature is also observed at high temperatures (400-800 K). This desorption state is assigned to the recombination of surface hydroxyls at surface defects which have been generated in the surface by the sputtering treatment. The appearance of this state is closely correlated with the appearance of a D2 desorption state (Figure 4b, upper trace) over the similar temperature range (400-800 K).
Desorption and Reaction of Water on MgO(100)
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Figure 4. The thermal desorption spectra of water are displayed as a function of surface treatment involving ion sputtering. The upper traces in both the (A) m/q ) 20 and (B) m/q ) 4 spectra represent desorption from an atomically flat MgO(100) surface sputtered with 500 eV Ar+ for 2 min. The lower traces represent subsequent desorption measurements following the water exposure of the experiment represented in the upper traces. An offset has been applied to the D2O desorption spectrum collected from the ion sputtered surface for clarity. The D2O spectra have been arbitrarily normalized to have equal intensity in the desorption state centered near 230 K.
TABLE 1: Effect of Ar+ Bombardment and Dosing with D2O on the Mg 2s and O 1s Peak Positions and O 1S/Mg 2s Area Intensity Ratiosa surface preparation of MgO(100)
Mg 2s (eV)
O 1s (eV)
IO1s/IMg2s
etched and annealed in ambient O2 at 1273 K after Ar+ bombardment for 2 min at 500 eV after dosing with D2O and then annealing to 773 K in UHV
89.4 89.4 89.3
529.6 529.5 529.6
1.00 0.93 0.98
a The error in the ratios is (0.02. Because of charging, all spectra were charge referenced to align the Mg 2p core level peak to a binding energy of 50.8 eV in each case.
The chemical identity and evolution of surface defects generated by this ion sputtering was further investigated through subsequent TPD and XPS measurements. Following the temperature ramp to 800 K used to collect the data described above, the MgO substrate was recooled to 90 K and dosed again with an equivalent multilayer coverage of water. The desorption spectra of D2O and D2 from this surface, exposed to water and then annealed, are shown in the lower traces of parts a and b of Figure 4. The high-temperature features are now absent and the spectra are identical to the well-ordered oxygen-annealed surface (Figure 3). An identical outcome was obtained by annealing the ion-sputtered surface in 1 × 10-6 Torr of O2 for 10 min at 773 K before repeating the TPD measurement. This result is in contrast to the results of Stirniman et al. who found that water TPD spectra could not be fully recovered following sputtering and annealing in oxygen partial pressures.18 This discrepancy may arise from differences in sputtering conditions. XPS measurements of the O 1s and Mg 2s core levels of the oxygen-annealed and Ar+ bombarded surface further reveal the effect of sputtering the magnesium oxide surface. The integrated intensities of the O 1s and Mg 2s core level spectra recorded from these surfaces are shown in Table 1. In addition, the stoichiometric ratio of oxygen to magnesium has been calculated for each surface using identical sensitivity factors. These results clearly indicate that the Ar+ bombardment process involves the preferential removal of oxygen and creation of surface vacancies. Preferential removal of surface oxygen upon Ar+ bombardment is frequently observed in metal oxides.1 However, to the best of our knowledge, this is first indication of preferential sputtering in MgO. Subsequent adsorption of D2O or annealing in O2 partial pressures results in dissociative adsorption at these vacancy sites. The evolution of significant amounts of D2 (Figure 4b) following water exposures is consistent with a surface reaction mechanism in which oxygen is consumed and the return to a higher stoichiometric ratio (Table 1). These results indicate
that ion sputtering under mild conditions followed by treatment with water or molecular oxygen can be used a means of cleaning a well-ordered MgO surface in a vacuum. Conclusions This study demonstrates the high sensitivity of thermal desorption measurements of water to variations in the preparation of single-crystal MgO surfaces. Through correlated measurements of surface morphology with AFM (performed in our previous work), we have related the specific character (peak temperature and width) of thermal desorption spectra to the local topography of the oxide surface. In general, these studies find that annealing in ambient pressures of O2, which results in an atomically flat and terraced surface, gives rise to sharp and welldefined desorption features characteristic of a homogeneous array of adsorption sites. For the terraced surface, two monolayer desorption states are observed and are consistent with both molecularly and dissociatively adsorbed water. Acid-etching and etching and annealing in partial pressures of oxygen resulted in broad desorption features representative of a significant distribution of adsorption sites. These studies have further demonstrated that mild sputtering and annealing treatments of a well-ordered MgO surface can be employed as a cleaning procedure if annealing steps are carried out in partial pressures of oxygen. However, this work also clearly indicates that sputtering and annealing (in partial pressures of oxygen) an initially rough surface cannot be used to generate a well-ordered MgO surface. Acknowledgment. This research was supported in part by the Welch Foundation under Grant E-1328 and in part by the MRSEC Program of the National Science Foundation under Award DMR-99632667. We thank Dr. Allan Jacobson for making available the high-temperature furnaces used in these
3348 J. Phys. Chem. B, Vol. 104, No. 14, 2000 studies. We also acknowledge Dr. Hyun Kim and Dr. Phil Merrill for assistance in preparing the oxygen annealed samples and for insightful discussions of the results presented here. References and Notes (1) Henrich, V. E.; Cox P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994 and references therein. (2) Druiez, C.; Chapon, C.; Henry, C. R.; Rickard, J. M. Surf Sci. 1992, 230, 123 and references therein. (3) McKee, R. A.; Walker, F. J.; Specht, E. D.; Jellison, G. E., Jr.; Boatner, L. A. Phys. ReV. Lett. 1994, 72, 2741. (4) Burke, M. L.; Goodman, D. W. Surf. Sci. 1994, 311, 17. (5) Peacor, S. D.; Hibma, T. Surf. Sci. 1994, 301, 11. (6) Tran, T. T.; Chambers, S. A. Appl. Surf. Sci. 1994, 81, 161. (7) Chambers, S. A.; Gao, Y.; Liang, Y. Surf. Sci. 1995, 339, 297. (8) Norton, M. G.; Tietz, L. A.; Summerfelt, S. R.; Carter, C. B. Appl. Phys. Lett. 1989, 55, 2348. (9) Liu, P.; Kendelewicz, T.; Brown, G. E., Jr.; Parks, G. A. Surf. Sci. 1998, 412/413, 287. (10) Liu, P.; Kendelewicz, T.; Brown, G. E., Jr. Surf. Sci. 1998, 412/ 413, 315. (11) Liu, P.; Kendelewicz, T.; Nelson, E. J.; Brown, G. E., Jr. Surf. Sci. 1998, 415, 156. (12) Janssen. A. P.; Schoonmaker, R. C.; Chambers, A. Surf. Sci. 1975, 49, 143. (13) Heidberg, J.; Redlich, B.; Wetter, D. Ber Bunsen-Ges. Phys. Chem. 1995, 99, 1333. (14) Ferry, D.; Glebov, A.; Senz, V.; Suzanne, J.; Toennies, J. P.; Weiss, H. J. Chem. Phys. 1996, 105, 1697. (15) Ferry, D.; Picaud, S.; Hoang, P. N. M.; Girardet, C.; Giordano, L.; Demirdjian, B.; Suzanne, J. Surf. Sci. 1998, 409, 101. (16) Onishi, H.; Egawa, C.; Aruga, T.; Iwasawa, Y. Surf. Sci. 1987, 191, 479. (17) Karolewski, M. A.; Cavell, R. G. Surf. Sci. 1992, 271, 128. (18) Striniman, M. J.; Huang, C.; Smith, R. S.; Joyce, S. A.; Kay, B. D. J. Chem. Phys. 1996, 105, 1295. (19) Soria, E.; de Segovia, J. L.; Colera, I.; Gonza´lez, R. Surf. Sci. 1997, 390, 140. (20) Colera, I.; Gonza´lez, R.; Soria, E.; de Segovia, J. L.; Roma´n, E. L.; Chen, Y. J. Vac. Sci. Technol. A 1997, 15, 1698. Colera, I.; Soria, E.; Segovia, J. L.; Gonza´lez, R. Vacuum 1999, 52, 103. (21) Colera, I.; Soria, E.; Segovia, J. L.; Gonza´lez, R. Vacuum 1999, 52, 103. (22) Peng, X. D.; Barteau, M. A. Surf. Sci. 1990, 233, 283. (23) Wu, M. C.; Corneille, J. S.; Estrada, C. A.; He, J.-W.; Goodman, D. W. Chem. Phys. Lett. 1991, 182, 472. (24) Wu, M.-C.; Estrada, C. A.; Goodman, D. W. Phys. ReV. Lett. 1991, 67, 2910.
Imad-Uddin Ahmed et al. (25) Wu, M. C.; Estrada, C. A.; Corneille, J. S.; Goodman, D. W. J. Chem. Phys. 1992, 96, 3892. (26) Xu, C.; Goodman, D. W. Chem. Phys. Lett. 1997, 265, 341. (27) Gu¨nster, J.; Liu, G.; Kempter, V.; Goodman, D. W. J. Vac. Sci. Technol. A 1998, 16, 996. (28) Johnson, M. A.; Stefanovich, E. V.; Truong, T. N.; Gu¨nster, J.; Goodman, D. W. J. Phy. Chem. B 1999, 103, 3391. (29) Zhou, X.-L.; Cowin, J. P. J. Phys. Chem. 1996, 100, 1055. (30) Yadavalli, S.; Yang, M. H.; Flynn, C. P. Phys. ReV. B 1990, 41, 7961. (31) Tran, T. T.; Chambers, S. A. Appl. Surf. Sci. 1994, 81, 161. (32) Perry, S. S.; Merrill, P. B. Surf. Sci. 1997, 383, 268. (33) Perry, S. S.; Kim, H. I.; Imaduddin, S.; Lee, S. M.; Merrill, P. B. J. Vac. Sci. Technol. A 1998, 16, 3402. (34) Huang, H. H.; Jiang, X.; Zou, Z.; Chin, W. S.; Xu, G. Q.; Dai, W. L.; Fan, K. N.; Deng, J. F. Surf. Sci. 1998, 412/413, 555. (35) Scamehorn, C. A.; Hess, A. C.; McCarthy, M. I. J. Chem. Phys. 1993, 99, 2786. (36) Langel, W.; Parrinello, M. Phys. ReV. Lett. 1994, 73, 504. (37) Sawabe, K.; Morokuma, K.; Iwasawa, Y. J. Chem. Phys. 1994, 101, 7065. (38) Scamehorn, C. A.; Harrison, N. M.; McCarthy, M. I. J. Chem. Phys. 1994, 101, 1547. (39) De Leeuw, N. H.; Watson, G. W.; Parker, S. C. J. Phys. Chem. 1995, 99, 17219. (40) Refson, K.; Wogelius, R. A.; Fraser, D. G.; Payne, M. C.; Lee, M. H.; Milman, V. Phys. ReV. B 1995, 52, 10823. (41) Chacon-Taylor, M. R.; McCarthy, M. I. J. Phys. Chem. 1996, 100, 7610. (42) Tikhomirov, V. A.; Geudtner, G.; Jug, K. J. Phy. Chem. B 1997, 101, 10398. (43) Giradet, C.; Hoang, P. N. M.; Marmier, A.; Picaud, S. Phys. ReV. B 1998, 57, 11931. (44) Giordano, L.; Goniakowski, J.; Suzanne, J. Phys. ReV. Lett. 1998, 81, 1271. (45) Odelius, M. Phys. ReV. Lett. 1999, 82, 3919. (46) Engkvist, O.; Stone, A. J. Surf. Sci. 1999, 437, 239. (47) Olle, L.; Salmeron, M.; Baro, A. M. J. Vacuum Sci. Technol. A 1985, 3, 1866. (48) Bozack, M. J.; Muehlhoff, L.; Russell, J. N., Jr.; Choyke, W. J.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1987, 5, 1. (49) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (50) In previous reports (refs 18 and 26), very similar spectra have been assigned as including only three desorption states. We believe the large shift with increasing coverage in peak desorption temperature near 0.5 ML is indicative of two discreet states. XPS measurements of a 1 ML coverage of D2O on this surface further indicate nonlattice oxygen in two distinct chemical environments.
