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Langmuir 1998, 14, 1348-1354
The Chemistry of CH3Cl and CH3Br on Ru(001) T. Livneh and M. Asscher* Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew University, Jerusalem 91904, Israel Received July 3, 1997. In Final Form: September 24, 1997 The interactions of methyl chloride and methyl bromide on Ru(001) have been studied and compared using work function measurements in a temperature-programmed desorption (∆φ-TPD) mode and normal temperature-programmed desorption (∆p-TPD). Both molecules adsorb molecularly at temperatures below 100 K with the halide facing down, forming direct bonding to the metal and reducing the work function by 1.9 and 2.1 V, for CH3Cl and CH3Br, respectively. Dipole moments of the isolated molecules and their polarizabilities were extracted by using electrostatic models for the work function as a function of coverage. While the crystal is heated, methyl chloride desorbs molecularly with no trace of dissociation. This is concluded from the similarity of the differential work function change (DWFC) and the ∆p-TPD spectra. In contrast, 50% of an initial one monolayer of methyl bromide dissociates to form adsorbed methyl and bromide. Significant differences between the DWFC and the normal desorption spectra are discussed in terms of the sensitivity of work function to rearrangement of the molecular species at low temperatures and to dissociation of the parent molecule and its methyl fragments at higher temperatures.
1. Introduction The thermal and photoinduced chemisties of alkyl halides on metal and semiconductor surfaces have been studied extensively in recent years. The importance of these molecules as precursors in many processes in the chemical industry but at the same time their damaging role for our ecology have motivated these studies.1-12 The large permanent dipole moments of methyl halides, typically larger than 1.6 D in the gas phase, have been frequently used to investigate the role of lateral interactions (mostly repulsion) on kinetic processes of adsorbates on solid13 surfaces.11-14 Much of the current understanding of the adsorption and bonding of methyl halides on metal surfaces has been gained from IR absorption measurements (mostly highresolution electron energy loss spectroscopy (HREELS) and reflection-absorption infrared spectroscopy (RAIRS) experiments).15,16 On nonmetal surfaces, X-ray and neuton diffraction data enabled better structural information to be obtained, which have indicated the coverage de(1) Bent, B. E. Chem. Rev. 1996, 96, 1361. (2) Chen, J. C.; Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Am. Chem. Soc. 1987, 109, 1726. (3) Dubois, L. H.; Bent, B. E.; Nuzzo, R. G. Surface Reactions; Springer Series in Surface Science; Madix, R. J., Ed.; Springer Verlag: Berlin, 1994; Vol. 34, Chapter 5, p 135. (4) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. Lett. 1987, 184, L325. (5) Liu, Z.-M.; Costello, S. A.; Roop, B.; Coon, S. R.; Akhter, S.; White, J. M. J. Phys. Chem. 1989, 93, 7681. (6) Zhou, Y.; Henderson, M. A.; Feng, W. M.; White, J. M Surf. Sci. 1989, 224, 386. (7) Zaera, F.; Hoffmann, H. J. Phys. Chem. 1991, 95, 6297. (8) Zhou, X.-L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf. Sci. 1989, 219, 294. (9) French, C.; Harrison, I. Surf. Sci. 1995, 342, 85. (10) French, C.; Harrison, I. Surf. Sci. 1997, 387, 11. (11) Lu, P.-H.; Lasky, P. J.; Yang, Q.-Y.; Wang, Y.; Osgood, R. M., Jr. J. Chem. Phys. 1994, 101 (11), 10145. (12) Berko, A.; Erley, W.; Sander, D. J. Chem. Phys. 1990, 93, 8300. (13) Maschhoff, B. L.; Ledema, M. J.; Cowin, J. Surf. Sci. 1996, 359, 253. (14) Maschhoff, B. L.; Cowin, J. J. Chem. Phys. 1994, 101 (9), 8138. (15) Lin, J.; Bent, B. E. J. Vac. Sci. Technol. 1992, A10 2202. (16) Zaera, F.; Hoffmann, H.; Griffiths, P. R. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 705.
pendent adsorption geometry of CH3Cl on graphite.17 Using He diffraction methods, CH3Br was shown to form also two phases on NaCl(001) but only one phase on graphite and LiF.18 Multilayers of methyl halides were studied on several metal surfaces, in most of them a normal lower temperature multilayer peak has been reported.4,6,8,12 The unique lower desorption temperature of a destabilized second layer compared to the multilayer has been reported so far in the desorption of CD3I from TiO219 and CH3I from Au(100).20 Similar observation was recently reported for CH3Br and CH3I desorption from Ru(001), with somewhat different interpretation regarding the origin of the lower temperature desorption peak.21 In this paper we compare the effect of dipole-dipole interactions on the work function change as a function of coverage for two alkyl halidessmethyl chloride and methyl bromide on Ru(001). These two molecules differ in their reactivity on ruthenium in a significant way, which will be the focus of our comparison: While 50% of the first monolayer of CH3Br decomposes to CH3(ad) and Br(ad) upon heating the substrate above 150 K, CH3Cl desorbs intact without any competing dissociation channel. The influence of the coadsorbed fragments on the desorption rate in the case of CH3Br and their absence in the system of CH3Cl will be discussed. We emphasize the utilization of work function change measurements in a TPD mode (∆φ-TPD) and compare these results to ordinary TPD spectra (∆p-TPD). 2. Experimental Section The experimental part of this study has been described in detail elsewhere.21 Briefly, an ultrahigh vacuum (UHV) chamber with a base pressure of 3 × 10-10 Torr was used for this study, obtained by a turbomolecular and titanium sublimation pumps. (17) Morishige, K.; Tajima, Y.; Kittaka, S.; Clarke, S. M.; Thomas, R. K. Mol. Phys. 1991, 72, 395. (18) Robinson, G. N.; Camilone, N., III; Rowntree, P. A.; Liu, G.; Wang, J.; Scoles, G. J. Chem. Phys. 1992, 96, 9212. (19) Garret, S. J.; Holbert, V. P.; Stair, P. C.; Weitz E. J. Chem. Phys. 1994, 100, 4615. (20) Yang, M. X.; Jo, S. K.; Paul, A.; Avila, L.; Nishikida, K.; Bent, B. E. Surf. Sci. 1995, 325, 102. (21) Livneh, T.; Asscher, M. J. Phys. Chem. 1997, B101, 5705.
S0743-7463(97)00712-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/22/1998
CH3Cl and CH3Br on Ru(001)
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Standard cleaning (sputter gun) and surface characterization tools (Auger and low-energy electron diffraction (LEED)) were used to define the status of the sample. A quadrupole mass spectrometer covered by a Pyrex shield and attached to a translation stage was used for ∆p-TPD measurements. A Kelvin probe (Besocke Type S) was employed to monitor work function change (∆φ) . Both ∆φ-TPD and ∆p-TPD spectra were measured as a function of crystal temperature using the same ac resistive heating routine, which is fully computer controlled, defining the sample heating rate or temperature stabilization to within (0.2 K. At a typical heating rate of 2 K/s, up to seven masses could be monitored simultaneously using the ∆p-TPD routine. The quadrupole sensitivity to different masses and the cracking pattern in the ionizer have been taken into consideration when calibrating relative signal intensities at different masses. The Ru(001) sample (a square piece, 6 × 7 mm, 1 mm thick) was prepared by standard metallurgical methods to within (1° of the (001) crystallographic orientation. A standard sample cleaning procedure has been used which includes cycles of Ar+ ion sputter at 600 V, annealing at 1600 K for 3 min, and oxygen treatment.21 LEED from the clean and annealed surface showed very sharp hexagonal patterns. The sample was spot welded between two 0.5 mm diameter tantalum wires and was attached to a liquid nitrogen reservoir via copper feedthroughs directly welded to the bottom of the Dewar. By pumping over the liquid nitrogen, a surface temperature of 78 K could be obtained. The temperature was monitored by a W5%Re-W26%Re thermocouple spot welded to the edge of the ruthenium sample. Finally, CH3Br and CH3Cl (99.5% pure) were further purified by a few freeze-pump-thaw cycles to eliminate any noncondensable residual gases. Exposure was done by filling the chamber through a leak valve to the desired pressure, the ion gauge signal was then transmitted to a computer and converted to Langmuir units (1 langmuir ) 10-6 Torr·s).
3. Results and Discussion 3.1. Work Function upon Adsorption. Methyl halides adsorb molecularly on most transition metals at surface temperatures below 100 K, except for CH3I, which usually dissociates, as it does on Ru(001).6 This is clearly seen in the case of methyl chloride and methyl bromide when the change in work function (∆φ) is continuously monitored during the adsorption of these molecules on Ru(001); see Figure 1. The large work function change recorded upon adsorption at temperatures below 100 K, is considered as supporting evidence for our claim that these molecules do not dissociate at this stage. The magnitude of ∆φ observed in the case of both molecules is difficult to explain unless the full dipole moment of the molecular species would contribute to the observed work function decrease. We shall substantiate this statement below while discussing work function measurements during sample heating. In the case of CH3Br, the change in work function goes through a minimum at -2.15 V while for CH3Cl the minimum is at -1.88 V. In both cases, therefore, we conclude that the molecules adsorb with the halides in direct contact with the metal and the methyl groups pointing toward the vacuum side. The work function change shown in Figure 1 goes through a minimum at exposures which are equivalent to one full monolayer for both methyl halides. An unusual behavior is then observed at higher exposures, where an increase in the work function is measured once the first monolayer is completed and the second starts to populate. This is a similar trend to the well-known work function behavior of alkali adsorbates on metals.22 The major difference, however, is that the minimum is reached at a coverage of about 0.5 monolayer in the case of alkalis on metals, (22) (a) Bonzel, H. P., Bradshaw, A. M., Ertl, G., Eds. Physics and Chemistry of Alkali Metal Adsorption; Elsevier: Amsterdam, 1989; pp 25-44. (b) Verhoef, R. W.; Zhao, W.; Asscher, M. J. Chem. Phys. 1997, 106, 9353.
Figure 1. Work function measurement during adsorption of (a) CH3Br at 82 K and (b) CH3Cl at 97 K on Ru(001). In the inserts the fits of the data (solid lines) to the Maschhoff-Cowin’s model (dotted lines) are shown . The initial dipole moment and polarizability are derived from this model (see text).
while for the methyl halides the minimum in the work function change is obtained at the completion of the first monolayer. The quantitative definition of the density of the first monolayer of the methyl halides has been described elsewhere for the case of CH3Br21 and is based on deuterium precoverage. We have found a density of 3.6 × 1014 CH3Br molecules/cm2 within the first monolayer, which is defined as θ ) CH3Br/Ru ) 0.22. We assume the same density for CH3Cl as well. It is interesting to note that while the gas phase dipole moment of methyl bromide is slightly smaller than that of methyl chloride, 1.82 and 1.89D, respectively,23 on the surface CH3Br causes a larger work function decrease, which means that its surface dipole moment is larger; see discussion below. This is understood in terms of the stronger chemical interaction of the methyl bromide, as demonstrated by the higher molecular desorption temperature and of course the fact that the methyl bromide partially dissociates. From work function measurements alone one cannot unambiguously conclude that both CH3Br and CH3Cl adsorb with their molecular axis parallel to the surface normal. We shall elaborate now on an electrostatic model, based on dipole-dipole repulsion among neighbor adsorbates, which is based on the above assumption. The quality of the fit of this model to the (23) Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC: Roca Raton, FL, 1995.
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experimental data (insert of Figure 1) supports the vertical adsorption geometry, at least up to 0.9 monolayer. In a recent detailed treatment of the physics of a permanent dipole adsorbed on a metal surface, Maschhoff and Cowin13,14 have extended the expression given by Topping24 for the changing dipole as a function of coverage due to lateral interactions, to include the effect of image dipole, the distance of the dipole from the surface plane, and the distance between the charges. The modified dipole was then calculated, and its effect on the changing activation energy for desorption has been demonstrated. We have utilized the expressions of Maschhoff and Cowin for the dipole moment in order to evaluate the work function as a function of coverage for both CH3Br and CH3Cl. Briefly, the relevant expressions are as follows: The work function change ∆φ is given by13,21
∆φ ) -
(
4πµ0n
1 + R F(n) -
where
F(n) )
4π
x3CRs
(
3
)
1 3 4β - βd2
1+
1 2β 1+ CRs
[ (
µ0 > 0
)] ) 2 3/2
(1)
(2)
d is the separation between charges of the dipole, β is the distance of the dipole center from the metal surface, n is the surface density of methyl bromide molecules, µ0 is the dipole moment of an isolated adsorbate in the absence of image dipole stabilization,13 and R is its polarizability. F(n) depends on the surface geometry and describes the characteristics of the electric field due to neighbor dipoles. RS ) 1.07457/xn and C ) 0.658 are parameters which arise from the hexagonal symmetry and the assumption of homogeneous distribution of parallel dipoles. The isolated adsorbate’s dipole moment µ0 and the (coverage independent) polarizability R are the fitting parameters to the experimental data. Using the full monolayer density described above, the calculated work function as a function of coverage is shown in the insert of Figure 1 for both methyl bromide and methyl chloride. The dipole moment of the isolated molecular adsorbates obtained from the best fit are 2.57 ( 0.05 and 2.12 ( 0.05 D for CH3Br and CH3Cl, respectively. These are larger than the gas phase values, as stated above, reflecting the significant charge exchange with the metal resulting in the increased dipole moment. The polarizability of the adsorbates obtained this way is R ) (8.6 ( 0.5) × 10-24 and (9.2 ( 0.5) × 10-24 cm3, for CH3Br and CH3Cl, respectively. The somewhat larger value obtained for methyl chloride is unexpected and we cannot explain it at the moment. From the quality of the fit of the theoretical electrostatic model to the experiment all the way from zero to 85% of full monolayer coverage (see insert of Figure 1), we conclude that the basic assumptions of the model are probably fulfilled, namely, parallel dipoles which are homogeneously distributed on the surface, at least up to 0.85 monolayer. The work function increase upon further adsorption beyond a monolayer is due to the opposite contribution to the work function by the second layer molecules. The majority of the second layer molecules are believed to adsorb with the methyl group facing the surface and the halides facing the vacuum side. Full account of this effect (24) Topping, J. Proc. R. Soc. London 1927, A114, 67.
is given elsewhere.21 It is interesting to note that in the bulk molecular crystal, methyl bromide structures in an antiparallel arrangement25 but methyl chloride does not.26 The fact that both molecules behave in a similar way while adsorbing on Ru(001) may be attributed to the strong polarizing effect of the surface chemical bond which is formed upon adsorption. 3.2. ∆p-TPD. Temperature-programmed desorption (∆p-TPD) spectra of the two methyl halides show clear differences. This is demonstrated in Figure 2. Methyl chloride presents simple spectra influenced by lateral repulsion, with peak desorption shifting gradually to lower temperatures and becoming broader (Figure 2b). The low coverage desorption peak near 210 K represents a relatively strong binding. Near a monolayer the peak shifts all the way to 150 K. In addition to the straightforward desorption behavior, there is clear evidence that the channel for dissociation does not exist. This is seen by the absence of any hydrogen desorption at higher temperatures. Hydrogen desorption is a fingerprint for dissociation since the methyl fragment further dissociates on the ruthenium surface, generating surface hydrogen atoms which eventually recombine and desorb.6,21 This channel is observed in high probability in the case of methyl bromide, as is discussed below. The absence of any competing kinetic process to desorption enables us to further examine the electrostatic model described above. We have found that the activation energy for desorption (Edes) of methyl chloride could be calculated using the model suggested by Maschhoff and Cowin.13,14 It fits the experimental data remarkably well between the coverage range of 0-0.9 monolayer, under the assumption of a coverage independent preexponential factor. Edes decreases from 55 ( 1 kJ/mol at close to zero coverage to 41 ( 1 kJ/mol near 0.9 monolayer. In contrast to methyl chloride, heating the surface from 82 K in the case of CH3Br results in dissociation. As seen in Figure 2a, at coverages Θ < 0.6 monolayer (here Θ represents the fractional coverage relative to full monolayer), only minor desorption of the molecular species is observed. Most of the molecules dissociate to CH3(ad) and Br(ad). At higher coverages the desorption channel becomes dominant. Eventually, at full monolayer coverage 50% of the adsorbed molecules have dissociated upon heating the surface above 150 K. The ∆p-TPD spectra in Figure 2a show an interesting behavior in which the minor fraction of molecules desorbing at coverages less than 0.6 monolayer, appears as a very wide peak with a maximum near 160 K, which is significantly lower than the equivalent desorption of methyl chloride molecules. At coverages above 0.6 monolayer, however, a new desorption peak emerges near 190 K. This peak is shifted to lower desorption temperature as coverage increases, a typical behavior for molecules which are characterized by repulsive interactions among neighbor adsorbates. This major difference in the ∆p-TPD spectra at coverages below and above 0.6 monolayer, is attributed to the dissociation which is the dominant reactivity channel below 0.6 monolayer. It is suggested that a small fraction of the parent molecules is displaced by the dissociation event of nearest neighbor molecules. Therefore this peak appears to follow the temperature range in which dissociation takes place; see details of work function measurements below. Above a coverage of 0.6 monolayer, however, where more of the parent molecules desorb, they appear in the “normal” (25) Kawaguchi, T.; Hijikigawa, M. ;Hayafuji, Y.; Ikeda, M.; Fukushima, R.; Tomie, Y. Bull. Chem. Soc. Jpn. 1973, 46, 53. (26) Burbank, R. D. J. Am. Chem. Soc. 1953, 75, 1211.
CH3Cl and CH3Br on Ru(001)
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Figure 2. ∆p-TPD spectra of the first layer of (a) CH3Br and (b) CH3Cl from Ru(001) at the indicated initial coverages in monolayers, (1 monolayers ) (3.6 ( 0.3) × 1014 molecules/cm2). The adsorption temperature was 97 K. In the inserts ∆p-TPD spectra include coverages above 1 monolayer.
desorption temperature range observed also in the case of methyl chloride, namely, 170-200 K. Differences in ∆p-TPD spectra of the two methyl halides are observed also at coverages above 1 monolayer. Methyl bromide presents a complex shift in desorption peak temperatures between the second, third, and coverages of the fourth monolayer and above. Methyl chloride, on the other hand, shows again a normal packing of higher layers, where the desorption always shifts to lower temperatures as coverage increases. The unique behavior of methyl bromide is understood again by the antiparallel structure of the molecular crystal. A higher degree of disorder at the third layer, induced by a very small interaction with the metal, is believed to cause a slightly lower desorption temperature of the third layer compared with that of the multilayer (here fourth layer and above). At these layers the bulk molecular antiparallel structure forms. The reason for not having this behavior in the case of CH3Cl (see Figure 2b) arises most probably from the fact that methyl chloride molecular structure is completely different and does not arrange in the antiparallel structure.26 Therefore the typical multilayer structure is formed, in which there is a gradually weaker influence of the metal surface. As a result, a gradual downward shift of the desorption temperature is observed as coverage increases above a monolayer. This is demonstrated in the insert of Figure 2b. ∆p-TPD spectra very similar to those of high coverage CH3Br mentioned above were found also in the desorption of CD3I from TiO219 and CH3I from Au(100).20 On the TiO2 sample there is no dissociation while on the Au(100)
surface 4% of the methyl iodide molecules decompose. Using X-ray photoelectron spectroscopy (XPS),19 these authors explained the change in TPD peak desorption between the second and the third layers as a result of 2D to 3D transition within the second adsorbed layer. The low temperature second layer desorption from Au(100) was attributed to a metastable bilayer structure, claimed to be less stable than the multilayer peak. The similarity in the ∆p-TPD of methyl bromide and methyl iodide on very different substrates supports the explanation that the formation of the bulk antiparallel structure is the origin for the shift to higher desorption temperature of the fourth layer and above (on ruthenium). This is because methyl iodide was shown to form the same antiparallel bulk structure as methyl bromide does,25 unlike methyl chloride.26 3.3. ∆φ-TPD. More details on the difference between the interaction of these two molecules on Ru(001) are revealed by recording and comparing work function change spectra taken while heating the substrate. The results of the ∆φ-TPD experiments are demonstrated in Figure 3. These spectra are simple to analyze in the case of CH3Cl, where as a function of initial coverage all the graphs line up on top of each other with a shift to lower temperatures of the initial work function increase. This shift is simply explained by the desorption of the parent molecules, which is shifted to lower temperatures (see Figure 2) as coverage increases due to repulsion among neighbors. Moreover, all the spectra combine at exactly zero work function change above 250 K, where all the
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Figure 3. Work function change during temperature-programmed desorption (∆φ-TPD) of the first layer of (a) CH3Br and (b) CH3Cl from Ru(001) at the indicated initial coverages. The adsorption temperature was 97 K and the heating rate 2 K/s. In the inserts ∆φ-TPD spectra are shown for covereges above 1 monolayer.
molecules have desorbed. This further supports the statement that there is no dissociation of CH3Cl on Ru(001). In the CH3Br case, however, the ∆φ-TPD spectra are far more complicated. This complication arises from the fact that work function measurements are sensitive to changes in density and dipole momentssboth may result from dissociation of the parent molecules in addition to desorption. It is clearly seen that there are significant changes in the work function at very low coverages where desorption practically does not occur. These changes are primarily due to the dissociation of the parent molecules to CH3(ad) and Br(ad) and appear already near 145 K, more than 50 K lower than the initial work function increase at the equivalent coverage of methyl chloride. Since both fragments decrease the work function,21 but far less than the parent molecule, the net result of the dissociation is an increase in work function. In addition to the difference observed in the initial increase in work function following sample heating between 82 and 200 K, at temperatures above 200 K there is clear additional structure in the ∆φ-TPD spectra of methyl bromide which is totally absent in the equivalent spectra of methyl chloride. Furthermore, at higher temperatures (above 700 K) the plateau in the work function change does not return to the same initial zero level. The unique pattern in the methyl bromide case is attributed to further dissociation of the methyl fragments. Details on the chemistry of methyl species on Ru(001) in the presence of iodide6 or bromide21 have been discussed elsewhere.
With work function change measurements, the features in the ∆φ-TPD spectra can most effectively be emphasized by differentiating the work function change (DWFC) with respect to temperature. Examples of such curves are shown in Figure 4, where thicker lines are ∆p-TPD spectra for comparison. In the case of CH3Cl, DWFC curves are practically identical to the normal ∆p-TPD lines, with typical small differences in width and normalization of amplitude. Both initial increase and the fall-off at high temperatures overlap in both spectra, another indication of the intact molecular desorption with no competing dissociation. The DWFC spectra of CH3Br, on the other hand, look quite different from the corresponding ∆p-TPD spectra, as seen in Figure 4. At a coverage of 0.12 monolayer, the DWFC (narrow dashed line) has a significantly different shape which starts at 140 K and peaks around 160 K, while molecular desorption is hardly seen and the small ∆p-TPD signal appears near 170 K. The DWFC spectrum thus directly records the dissociation process of the parent molecule, as stated above. At a coverage of 0.55 monolayer the difference between the two types of spectra becomes far more pronounced. Here the DWFC peak starts already near 100 K, due to increase which occurs before any dissociation or desorption, while the molecular desorption spectrum is only slightly more intense than that at 0.12 monolayer. The massive work function increase which starts near 100 K has been attributed to restructure within the submonolayer molecular adsorbates due primarily to dipole-dipole interactions. The restructure which has been discussed in detail elsewhere21 includes partial
CH3Cl and CH3Br on Ru(001)
Figure 4. Differential ∆φ-TPD spectra (d(∆φ)/dT) (narrow lines) of (a) CH3Br and (b) CH3Cl on Ru(001), together with the corresponding ∆p-TPD spectra (thick lines) for the indicated initial coverages in monolayers.
flipping of the molecular adsorbates so that antiparallel structure is formed, as in the molecular crystal.25 At a coverage of 0.85 monolayer, the restructure at low temperatures, the dissociation at the intermediate temperature range and the response to molecular desorption at temperatures of 160-220 K all appear as a double peak differential work function spectrum. Finally, as the crystal temperature rises above 250 K, more DWFC peaks are seen in the case of methyl bromide due to the dissociation of the methyl fragment, as mentioned above. Examples of such spectra at 0.1 and 1 monolayer are shown in Figure 5 together with the ∆pTPD of hydrogen, which is used as a marker for the dissociation of hydrocarbons at higher temperatures . At low coverage the major peaks in the DWFC spectrum at 220, 290, and 380 K are due to sequential dehydrogenation of CH3(ad). The methyl decomposes to CH2(ad) + H(ad) at 220 K, then at 290 K CH2 decomposes to CH(ad) + H(ad), and finally CH dissociates to carbon and hydrogen near 380 K, where all the accumulated hydrogen desorb; see the dashed-dotted line in Figure 5. At higher coverages approaching a monolayer, in addition to the reactive routes mentioned above, other intermediates are formed from surface reaction between methylene groups: CH2 + CH2 f CCH3 (methylidyne) + H(ad). The assignments of the various CH3 and CH2 decomposition steps are based on previous HREELS study of the dissociation of CH3I on Ru(001).6 These reactions and the decomposition of the new intermediates contribute also to the work function change at the temperature range of 300-450 K, therefore generating more complex spectra.21 Above 450 K the
Langmuir, Vol. 14, No. 6, 1998 1353
Figure 5. Differential ∆φ-TPD spectra (d(∆φ)/dT) after the decomposition of CH3 Br to CH3(ad) + Br(ad) has been completed (solid line), together with the corresponding hydrogen ∆p-TPD spectra (dashed-dotted line) for the indicated initial coverages in monolayers. The hydrogen ∆p-TPD spectra for temperatures exceeding 450 K are 10-fold magnified.
decomposition of CH groups contribute mostly to the DWFC spectrum. 4. Conclusions The chemistry of two methyl halides, CH3Cl and CH3Br, on Ru(001) have been compared by using work function change measurements in ∆φ-TPD and ∆p-TPD modes. At the adsorption temperature of 82 K both methyl halides adsorb molecularly. This enabled us to extract the dipole moment and polarizability of the two molecules while adsorbed on the ruthenium surface, in both cases with the halide in direct contact with the metal and the methyl group toward vacuum. The values obtained are 2.12 and 2.57 D for the dipole moments of the isolated molecules and 9.2 × 10-24 and 8.6 × 10-24 cm3 for the polarizability of CH3Cl and CH3Br, respectively. While heating the surface to record both ∆p-TPD and ∆φ-TPD spectra, a clear difference between the two molecules is observed. The methyl chloride desorbs molecularly without any trace of a competing dissociation channel, as evidenced by the return of the work function change spectra to the same initial zero value following sample heating. In addition, the nice overlap of the differential work function changeDWFC spectra with the normal TPD curves supports the same conclusion. In the case of methyl bromide, upon sample heating significant dissociation takes place at coverages up to 0.6 monolayer. This is the more important process at low coverages and only above it molecular desorption becomes the dominant one. At one monolayer coverage 50% of the methyl bromide molecules dissociate.
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Comparison of the DWFC and the normal TPD spectra enables direct monitoring of the dissociation process at low coverages. At coverages above 0.3 monolayer, work function increase precedes dissociation or desorption. This has been attributed to molecular rearrangement which leads to partial flipping to form a structure similar to that in the molecular crystal, namely, antiparallel packing of the methyl bromide molecules. While dissociation may cause a similar increase of the work function, we predict that the onset for dissociation should actually shift to higher temperatures with increasing coverage due to repulsion among neighbors which should slow down the
Livneh and Asscher
dissociation channel. In the experiment the opposite is observed. Acknowledgment. This work has been partially supported by the German-Israel Foundation and by a combined grant from the Israeli Ministry of Science and the Arts and the Commission of the European Communities. The Farkas Center for Light Induced Processes is supported by the Bundesministerium fu¨r Forschung and Technologie and the Minerva Gesellschaft fu¨r die Forschung mbH. LA970712B
