J. Phys. Chem. B 2001, 105, 3747-3751
3747
Physisorption of CO on the MgO(100) Surface† Z. Dohna´ lek, Greg A. Kimmel, S. A. Joyce, Patrick Ayotte, R. Scott Smith, and Bruce D. Kay* EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle BouleVard, P.O. Box 999, MSN K8-88, Richland, Washington 99352 ReceiVed: September 5, 2000; In Final Form: NoVember 28, 2000
The ability to grow thin MgO(100) films of quality approaching that of vacuum-cleaved MgO(100) is demonstrated using low-energy electron diffraction and temperature-programmed desorption (TPD) of CO. Highly ordered MgO(100) surfaces are used to study the adsorption and desorption of CO. A linearly increasing sticking coefficient from 0.47 ( 0.03 to 0.90 is observed for relative CO coverages, θ, less than 0.8 monolayer (ML). For this coverage range, the total sticking coefficient is given by SMgO(1 - PCO) + SCOPCO, where SMgO (SCO) is the sticking on the bare (CO-covered) MgO and PCO is the probability of striking the CO-covered surface. In TPD, the desorption of CO is dominated at very low coverages by desorption from sites influenced by defects. At intermediate coverages (0.25-0.8 ML) the CO desorbs via first-order desorption. At 0.8 ML where the monolayer peak saturates, the desorption energy is 17 ( 2 kJ/mol and the preexponential factor is 1 × 1015(2 s-1. The desorption energy increases linearly as coverage decreases due to repulsive interactions between adsorbed CO molecules. Above θ ) 0.8 ML the adsorption occurs on fully-CO-covered MgO(100) surfaces, and further increases in θ are achieved by compression of the CO layer. The compression results in a sharp decrease in desorption energy, which, upon saturation of the first CO layer (θ ) 1 ML) and the formation of a c(4×2) ordered phase, has a value of ∼9 kJ/mol.
I. Introduction CO on MgO(100) is considered a prototypical system for adsorption studies on ionic surfaces. A number of theoretical studies have shown that CO is an excellent candidate for the characterization of MgO surface binding sites with different coordination such as terraces, steps, and kinks.1-5 There are two firm conclusions that can be drawn from these studies: (a) CO is bound to magnesium cation sites with the carbon atom pointing to the surface, and (b) the binding energy of CO increases significantly with decreasing coordination of the Mg cation.1-5 Despite significant theoretical effort, a wide range of binding energies for CO on MgO have been calculated and agreement with experiment is still lacking.1-9 From an experimental point of view, the ability of preparing nearly ideal (100) surfaces on MgO microcrystals (MgO smoke) as well as on MgO monocrystals (cleaved in a vacuum) makes MgO very attractive for experimental studies. The results from the adsorption studies on MgO smoke10-12 and monocrystalline MgO13-17 can be compared with confidence that the observed surface-related phenomena occur on the same types of sites. Since the experimental procedures involving in situ cleavage of the monocrystals are technically challenging, they complicate the use of cleaved monocrystals in a wide range of experimental investigations. Procedures for the epitaxial growth of thin monocrystalline MgO(100) films have been developed18-20 to circumvent problems such as the insulating nature of MgO and cooling to very low temperatures and to have the ability to control the defect density. Unfortunately the results from the thin MgO film studies19 do not always compare favorably with the studies conducted on vacuum-cleaved MgO(100),17 sug†
Part of the special issue “John T. Yates, Jr. Festschrift”. * To whom correspondence should be addressed.
gesting that, under some circumstances, high defect densities may be present on epitaxially grown films. In this study, we demonstrate the capability to grow very high quality MgO(100) films on a Mo(100) substrate with defect densities comparable to that of vacuum-cleaved single crystals of MgO(100) as shown by CO temperature-programmed desorption (TPD).17 Very high quality TPD and sticking measurements allow us to extract kinetic parameters of CO adsorption and desorption with high accuracy and correlate them with previous structural studies.13,15,16 II. Experimental Section A. General Procedures. The experiments were conducted in an ultrahigh vacuum (UHV) chamber with a base pressure of ∼1 × 10-10 Torr. The Mo(100) substrate was cleaned using a standard procedure involving a sequence of O2 annealing at 1500 K (5 min, 1 × 10-6 Torr of O2) to remove mainly carbon contamination, and subsequent e-beam heating in UHV to 2100 K.21 The temperature of the substrate was measured using a W-5%Re/W-26%Re thermocouple. An absolute temperature calibration was performed using the multilayer desorption of various gases (N2, Ar, O2, and H2O) from the sample surface.22 The resulting uncertainty in the absolute temperature reading is estimated to be (2 K. The surface purity and order of the Mo(100) substrate and the MgO(100) thin films were checked using Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). Thin MgO(100) films were grown epitaxially on the Mo(100) substrate at 600 K by evaporation of Mg metal in an O2 atmosphere (PO2 ) 1 × 10-6 Torr).18,23 Complete oxidation of the Mg in the film was confirmed by the absence of the metallic Mg0(LVV) feature in the AES spectrum at 44 eV.18,23 The Mg ribbon (Aldrich, 99%) was placed in an Al2O3 tube that was closed on one side. A tungsten wire was tightly wrapped around
10.1021/jp003174b CCC: $20.00 © 2001 American Chemical Society Published on Web 02/08/2001
3748 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Dohna´lek et al.
Figure 1. Low-energy electron diffraction from the (100) surface of thin MgO films as a function of increasing film thickness. The films were deposited at 600 K with a deposition rate of 0.5 MgO ML/min. All diffraction patterns were acquired with a primary electron energy of 144 eV. The intensity scale of pattern a is 1/5 of the intensity scale of patterns b and c.
the tube to allow for heating of the whole assembly. The temperature of the tube was measured using a type K thermocouple cemented onto the closed side of the tube. The whole doser assembly was placed inside a Ta shield to achieve directional Mg dosing. The film thickness was calibrated using TPD of the Mg monolayer from clean Mo(100). This calibration assumes that the sticking of Mg on the Mo(100) substrate and the MgO(100) films is the same. On the basis of this calibration, the deposition rate used to grow the MgO films in this study was ∼0.5 monolayer (ML)/min. We have found that the quality of our MgO(100) surfaces depends dramatically on the MgO film thickness as illustrated by the (1×1) MgO(100) LEED patterns shown in Figure 1. Corresponding changes are also seen in the TPD experiments (data not shown). The intensity and sharpness of the LEED pattern increase with increasing film thickness up to ∼30 MLs, beyond which no further improvement is observed. Consequently, all experiments in this study were performed on 38 ML thick MgO films that had the highest quality. Repeated annealing of the MgO(100) films up to 1100 K did not cause any changes in the LEED pattern for the clean surface or in the TPD spectra of various adsorbates, indicating that the films are thermally stable. CO gas (Air Liquide, 99.99%) was dosed using a neat, 300 K, supersonic molecular beam directed normal to the MgO(100) surface. The adsorption kinetics were monitored using the beam reflection technique of King and Wells.24 The details of the evaluation procedure are described further in the following section. The desorption kinetics of CO were measured using TPD in a line-of-sight arrangement with the quadrupole mass spectrometer. The dynamic range resulting from this detection configuration, when used in conjunction with molecular beam dosing, is greater than four decades, allowing for the reliable detection of desorption rates as low as 10-4 ML/s. All TPD spectra were acquired using a linear temperature ramp rate of 0.6 K/s. The CO coverages are defined relative to the area under the desorption curve of a saturated monolayer state as labeled in Figures 2 and 3. III. Results and Discussion A. Temperature-Programmed Desorption. A set of CO TPD spectra obtained from the (100) surface of a 38 ML thick MgO film as a function of initial CO coverage is shown in Figures 2 and 3. In Figure 2 the CO desorption rate is plotted on a logarithmic scale to cover the wide dynamic range in the CO desorption rate. In Figure 3, the desorption rate is plotted on a linear scale for coverages of e1 ML. The CO coverages
Figure 2. CO TPD spectra vs CO coverage obtained with a ramp rate of 0.6 K/s after exposure of MgO(100) to CO at 22 K. The following coverages are shown: 0.07, 0.13, 0.18, 0.25, 0.33, 0.38, 0.46, 0.56, 0.71, 0.79, 0.90, 1.00, 1.84, 2.30. A logarithmic scale for the CO desorption rate is chosen to emphasize a wide dynamic range in the desorption rate.
Figure 3. CO TPD spectra for monolayer and submonolayer coverages of CO/MgO(100) (θ ) 0.07, 0.13, 0.18, 0.25, 0.33, 0.38, 0.46, 0.56, 0.71, 0.79, 0.90, 1.00) plotted on a linear scale. All conditions are as in Figure 2.
are obtained by normalizing the area under the desorption curves relative to the area under the desorption spectrum, exhibiting a saturation of states corresponding to the first CO layer (labeled 1ML in Figure 2). For coverages below 0.25 ML, CO desorption at high temperatures (>70 K) is observed and is assigned to desorption from sites effected by the presence of defects. Other adsorbates such as Ar, N2, CH4, H2O, and NH3 (data not shown) had similar
Physisorption of CO on the MgO(100) Surface high-temperature desorption features of comparable intensities, suggesting that the feature is related to defects. To confirm that these states are defect-related, we have intentionally grown films of poorer quality (e.g., smaller film thickness, lower deposition temperature, etc.), and as expected, the percentage of these sites increased. The relatively high desorption energy of CO from defect-related sites, compared to the terrace sites, is in agreement with theoretical calculations of adsorption at steps and kinks.1-5 The minimum defect coverage that we have been able to achieve is ∼0.25 ML as shown in Figures 2 and 3. This can be compared with the defect density of ∼0.15 ML estimated from the high-temperature desorption tails in CO TPD from a vacuum-cleaved MgO(100) surface.17 Clearly, these are relatively high defect densities on surfaces that can be considered the best MgO(100) surfaces currently experimentally achievable. This leads us to believe that the amount of CO desorbing at high temperatures from the defect-related states cannot be directly converted to the concentration of defects on the MgO(100) surface. For example, it is possible that the binding of CO in the vicinity of the CO bound to a defect is also influenced. In this case one defect would affect the desorption energy of more than one CO molecule. Currently we are not able to address this issue in more detail, but the characterization and understanding of the defects on the MgO(100) surfaces prepared under controlled experimental conditions is a subject of our ongoing investigations. From a theoretical point of view it would also be very useful to address issues such as (1) binding of adsorbates on sites in the vicinity of defects and (2) adsorption on defects, such as O and Mg vacancies, which are not related to steps. For increasing CO coverages in the 0.25-0.80 ML range, a first-order desorption feature from MgO(100) terrace sites appears and saturates. To extract the kinetic parameters of desorption, we use a direct inversion of the Polanyi-Wigner equation:25
-
dθ ) ν(θ)θn exp[-E(θ)/RT] dt
in which θ is the adsorbate coverage, t the time, ν the preexponential factor of desorption, n the order of desorption, E the activation energy of desorption, R the gas constant, and T the temperature. T and t are related by dT/dt ) β, where β is the heating rate. To invert the Polanyi-Wigner equation, we assume ν is independent of θ and, for CO physisorbed on MgO(100), n ) 1. In that case, the desorption energy, E(θ), can be calculated for each TPD spectrum as follows:
[
E(θ) ) -RT ln -
dθ/dT βνθ
]
The preexponential factor, ν, is varied until the E(θ) curves extracted from TPD spectra with different initial CO coverages retrace each other as shown in Figure 4. The desorption parameters, ν and E(θ), can then be used to simulate the complete set of desorption spectra by numerical integration of the Polanyi-Wigner equation. In Figure 5, a single E(θ) dependence (see the inset) extracted from the TPD of the saturated monolayer is used to simulate the whole set of desorption curves for different initial coverages. Within the assumption of a constant preexponential factor, the excellent agreement between the simulated and experimental results (Figure 3) provides some confidence in the calculated E(θ).
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3749
Figure 4. Desorption energy vs relative CO coverage obtained from the TPD spectra shown in Figure 3 by inversion of the Polanyi-Wigner equation. First-order desorption and a constant preexponential factor of 1 × 1015 s-1 are used in the inversion procedure.
Figure 5. Simulated CO TPD spectra for the set of initial CO coverages shown in Figure 3. The coverage-dependent desorption energy and preexponential factor used in the simulation are shown in the inset.
In Figure 4 we see that the desorption energy decreases linearly in the coverage range of 0.25-0.80 ML in agreement with theoretical calculations that predict repulsive interactions among the neighboring CO molecules.26 When the first-order desorption peak saturates at 0.8 ML, the CO desorption energy is 17 ( 2 kJ/mol. The preexponential factor is estimated to be ν ) 1 × 1015(2 s-1. The uncertainty is related to the strong correlation between ν and E. In particular, a range of choices of ν and E(θ) can result in comparable fits to the TPD spectra. More generally, if ν depends on coverage (ν ) ν(θ)), then E(θ) would be different from the function shown in Figure 5. The desorption energy of 17 kJ/mol for 0.8 ML of CO/MgO(100) is in excellent agreement with previously determined values of 17 kJ/mol from a CO adsorption isotherm study on microcrystalline MgO(100),10 and 14 kJ/mol obtained from a CO desorption study of a vacuum-cleaved monocrystalline MgO(100) surface.17 The second study used a Redhead analysis27 and assumed ν ) 1013 s-1. (If we use this value of ν in our analysis, we find a desorption energy of 15 kJ/mol.) The agreement with earlier experiments supports our conclusion that the thin MgO(100) films of relatively large thickness (>30 MLs) can reproduce the results from vacuum-cleaved MgO(100) surfaces. In the limit of zero CO coverage, the repulsive interaction between CO molecules is negligible, and therefore one would expect an increase in the CO desorption energy. In our case, we cannot directly determine the value in the zero coverage
3750 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Figure 6. Transient CO partial pressure increase after introduction of the CO beam into the UHV chamber. The pressure increase with the MgO(100) in and out of the CO beam is shown with filled and empty circles, respectively. An exponential fit to the pressure increase with the MgO(100) out of the beam is depicted by the solid line. The dotted line represents the pressure increase due to the nondirectional (effusive) component of the CO beam. The MgO(100) surface temperature during the CO dose was 22 K. The CO flux in the beam was fixed at 9 × 1013 CO/cm2 s.
limit due to the presence of defects in our thin films. If we extrapolate the linearly varying CO desorption energy from the coverage range of 0.3-0.8 ML (corresponding to desorption from terrace sites) to zero coverage (dashed line, Figure 4), the value obtained is E(0) ) 20 kJ/mol. In the 0.9-1.0 ML coverage range, a small desorption feature in the TPD spectra is observed at ∼45 K (Figure 2). This feature can be correlated with desorption from the c(4×2) phase of CO/ MgO(100), which has been observed previously by LEED13,14 and helium atom scattering (HAS)16 at temperatures below ∼41 K. On the basis of the packing density of CO in the solid, it was proposed that there are 6 CO molecules present in the c(4×2) unit cell corresponding to a saturation coverage of 0.75 CO/Mg site. The CO desorption energy from this c(4×2) phase is determined to be 9 ( 2 kJ/mol (see Figure 4), and its value is significantly lower than the CO desorption energy at lower coverages. This should be emphasized since the calculations of the binding energy in the CO c(4×2) phase were compared with the heat of adsorption measured at lower coverages.4,8 Using a fractional occupancy of 0.75 CO/Mg site in the c(4×2) phase that was determined in previous LEED and HAS studies,13,14,16 we can correlate relative coverages as defined here to the fractional occupancies at different coverages. For different desorption features we obtain the following fractional occupancies: defect states, 0.2; first-order desorption feature, 0.6; first monolayer, 0.75; second monolayer, 1.5. For CO coverages higher than 1 ML we can clearly distinguish zero-order desorption from the second and third CO layers. A simple leading edge analysis of the desorption rate from the multilayer peak yields a CO heat of sublimation of 8.0 kJ/mol in good agreement with the literature value of 8.5 kJ/mol.22 B. Sticking Coefficient Measurements. We used the beam reflection technique of King and Wells24 to measure the sticking of CO on MgO(100) as a function of the CO coverage. Figure 6 shows the background CO pressure as a function of time after the CO molecular beam is introduced into the UHV chamber, ∆Pin(t). Figure 6 also shows the chamber pumping response, ∆Pout(t) (measured by raising the sample out of the CO beam), and an exponential fit, f(t), of ∆Pout(t). For these measurements, the mass spectrometer is positioned such that only the back-
Dohna´lek et al.
Figure 7. Sticking coefficient vs relative CO coverage and CO exposure on MgO(100). The sample temperature was 22 K. Data for two different experiments are shown (open and closed symbols).
ground pressure increase is measured. To minimize the propagation of noise from the reference signal into the sticking coefficient measurement, we use the exponential fit, f(t), in place of ∆Pout(t). The CO pressure increase with the sample intercepting the CO beam is caused by the fraction of the molecules that are reflected from the MgO(100) surface. The sample temperature is held at 22 K, which is well below the CO condensation temperature for our fluxes. At short dose times, the CO pressure increases to approximately half the pressure increase with the sample out of the beam. As the dose time increases, the CO pressure decreases, signaling a decrease in the reflected fraction of the CO beam. Even at the longest dose times (>30 s) where the CO adsorption occurs on multilayers of CO, a small pressure increase above the background is observed. This is caused by the presence of a small effusive (nondirectional) component in the molecular beam. The fraction of this effusive component, X (here ∼10%), is a characteristic of the beam source. Knowing the diameter of the aperture through which the beam is entering the UHV chamber and measuring the CO pressure in the pumping region immediately preceding the chamber allow the effusive component to be calculated. The calculation agrees with the pressure rise observed at long dose times (Figure 6). The directional component in the ∆Pin(t) is obtained by subtracting the effusive part (dotted line) from the total pressure increase with the sample in the beam, (1 - X)∆Pin(t). The sticking coefficient, S(t), as a function of dose time is calculated using the following expression:
S(t) ) 1 -
(
)
∆Pin(t) - Xf(t) ∆Pin(t) 1 1) 1-X (1 - X)f(t) f(t)
The resulting sticking coefficient from two different runs (empty and filled symbols) is plotted vs relative CO coverage and CO exposure in Figure 7. The initial sticking coefficient, S0, at the zero CO coverage limit is determined to be 0.47 ( 0.03. For this weakly bound system, the most likely reason for the low value of S0 is poor momentum transfer from CO to the substrate due to their molecular mass difference. As the relative CO coverage increases up to 0.8 ML, the sticking coefficient increases approximately linearly from 0.47 to 0.90. As already discussed, the first-order desorption peak at ∼60 K in the CO TPD saturates at 0.8 ML (see Figures 2 and 3). The linear increase of the sticking coefficient in this
Physisorption of CO on the MgO(100) Surface coverage range can be quantitatively understood by invoking a constant sticking coefficient on clean MgO(100) (SMgO ) S0 ) 0.47) and on CO-covered MgO(100) (SCO ) 0.90). The overall sticking coefficient, for coverages of less than 0.8 ML, follows a simple linear dependence on coverage: S(θ) ) SMgOPMgO + SCO(1 - PMgO), where PMgO is the probability of hitting a clean MgO(100) surface. Since the MgO(100) surface is fully covered by CO at a coverage of 0.8 ML, the probabilities in the abovestated equation are not equal to coverages. At higher CO coverages (0.8-1.0 ML), the sticking coefficient remains constant until saturation of the first monolayer is completed. In this coverage range, the MgO(100) surface is completely covered with CO molecules and the further increase in coverage is achieved through compression of the CO layer. This is accompanied by a dramatic decrease of the desorption energy for 0.8 < θ < 1.0 as seen in Figure 4. The fact that the sticking coefficient on a complete CO monolayer has not reached unity is most probably caused by imperfect momentum transfer on the MgO-bound CO layer. This argument is further supported by the observed unity sticking on the multilayers of CO. Within the second CO layer the sticking coefficient increases linearly with coverage analogously to the mechanism of the sticking in the first CO layer. Upon saturation of the second layer the sticking coefficient approaches unity (within experimental uncertainty) and remains constant for CO condensation on CO multilayers. IV. Conclusions The ability to grow thin MgO(100) films whose quality approaches that of vacuum-cleaved MgO(100) is demonstrated using LEED and TPD of CO. We have found that the order of the MgO(100) surfaces depends dramatically on the MgO film thickness as illustrated by LEED. The intensity and sharpness of the LEED pattern increase with increasing film thickness up to ∼30 MLs, beyond which no further improvement is observed. CO on MgO(100), which has been widely studied, is used as a model system to examine physisorption on MgO(100). A linearly increasing sticking coefficient from 0.47 ( 0.03 to 0.90 is observed for relative CO coverages, θ, of less than 0.8 ML. This is a consequence of constant but different sticking coefficients on bare (0.47 ( 0.03) and CO-covered (0.90) MgO(100) surfaces. In TPD, the desorption of CO is dominated by desorption from the defects at coverages of less than 0.25 ML. At higher coverages (θ < 0.8 ML) the first-order desorption of CO from terrace sites is observed, and a desorption energy of 17 ( 2 kJ/mol and a preexponential factor of 1 × 1015(2 s-1 are extracted for θ ) 0.8 ML. The desorption energy increases linearly as θ decreases due to repulsive interactions between adsorbed CO molecules. At zero coverage the desorption energy on nondefective MgO(100) is extrapolated to be ∼20 kJ/mol. Above θ ) 0.8 ML the adsorption occurs on fully-CO-covered MgO(100) surfaces, and further increases in θ are achieved by compression of the CO layer. This results in a sharp decrease in desorption energy, which upon saturation of the first CO layer
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3751 (θ ) 1 ML) and formation of the c(4×2) ordered phase13,14,16 has a value of ∼9 kJ/mol. Acknowledgment. We would like to greatly acknowledge Prof. John T. Yates, Jr. for his immense contribution in the area of surface science, and for leading numerous graduate students (including Z.D.) and postdoctoral fellows to their scientific maturity. This work was supported by the U.S. Department of Energy Office of Basic Energy Sciences, Chemical Sciences (Z.D., G.A.K., R.S.S., and B.D.K.) and Materials Sciences (S.A.J. and B.D.K.) Divisions, and it was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract No. DE-AC0676RLO 1830. P.A. gratefully acknowledges support from the National Science and Engineering Research Council of Canada (NSERC) for a postdoctoral fellowship. References and Notes (1) Pacchioni, G.; Minerva T.; Bagus, P. S. Surf. Sci. 1992, 275, 450. (2) Neyman, K. M.; Ro¨sch, N. Surf. Sci. 1993, 297, 223. (3) Briquez, S.; Girardet, C.; Goniakowski, J.; Noguera, C. J. Chem. Phys. 1996, 105, 678. (4) Nygren, M. A.; Pettersson, L. G. M. J. Chem. Phys. 1996, 105, 9339. (5) Pelmenschikov, A. G.; Morosi, G.; Gamba, A.; Coluccia, S. J. Phys. Chem. B 1998, 102, 2226. (6) Nygren, M. A.; Pettersson, L. G. M. J. Chem. Phys. 1994, 100, 2010. (7) Neyman, K. M, Ruzankin, S. P, and Ro¨sch, N. Chem. Phys. Lett. 1995, 246, 546. (8) Hoang, P. N. M, Picaud, S, and Girardet, C. Surf. Sci. 1996, 360, 261. (9) Jug, K and Geudtner, G. J. Chem. Phys. 1996, 105, 5285. (10) Furuyama, S.; Fujii, H.; Kawamura, M.; Morimoto, T. J. Phys. Chem. 1978, 82, 1028. (11) Paukshtis, E. A.; Soltanov, R. I.; Yurchenko, N. E. React. Kinet. Catal. Lett. 1981, 16, 93. (12) Marchese, L.; Coluccia, S.; Martra, G.; Zecchina, A. Surf. Sci. 1992, 269/270, 135. (13) Audibert, P.; Sidoumou, M.; Suzanne J. Surf. Sci. 1992, 273, L467. (14) Panella, V, Suzanne, J, Hoang, P. N. M., and Girardet, C. J. Phys. I 1994, 4, 905. (15) Heidberg, J.; Kandel, M.; Meine, D.; Wildt, U. Surf. Sci. 1995, 331-333, 1467. (16) Gerlach, R.; Glebov, A.; Lange, G.; Toennies, J. P.; Weiss, H. Surf. Sci. 1995, 331-333, 1490. (17) Wichtendahl, R.; Rodriguez-Rodrigo, M.; Hartel, U.; Kuhlenbeck, H.; Freund, H.-J. Phys. Status Solidi A 1999, 173, 93. (18) Wu, M.-C.; Corneille, J. S.; Estrada, C. A.; He, J.-W.; Goodman, D. W. Chem. Phys. Lett. 1991, 182, 472. (19) He, J.-W.; Corneille, J. S.; Estrada, C. A.; Wu, M.-C.; Goodman, D. W. J. Vac. Sci. Technol., A 1992, 10, 2248. (20) Wollschla¨ger, J.; Viernow, J.; Tegenkamp, C.; Erdo¨s, D.; Schro¨der, K. M.; Pfnu¨r, H. Appl. Surf. Sci. 1999, 142, 129. (21) Grunze, M.; Ruppender, H.; Elshazly, O. J. Vac. Sci. Technol., A 1988, 6, 1266. (22) Schlichting, H.; Menzel, D. ReV. Sci. Instrum. 1993, 64, 2013. (23) Wu, M.-C.; Corneille, J. S.; He, J.-W.; Estrada, C. A.; Goodman, D. W. J. Vac. Sci. Technol., A 1992, 10, 1467. (24) King, D. A.; Wells, M. G. Surf. Sci. 1972, 29, 454. (25) de Jong, A. M.; Niemantsverdriet, J. W. Surf. Sci. 1990, 233, 355. (26) Sallabi, A. K.; Jack, D. B. J. Chem. Phys. 2000, 12, 5133. (27) Redhead, P. A. Vacuum 1962, 12, 203.
