1-Chloro-2-fluoroethane Adsorption on Cu(111): Structure and

N. Bovet , D.I. Sayago , F. Allegretti , E.A. Kröger , M.J. Knight , J. Barrett , D.P. Woodruff , Robert G. Jones. Surface Science 2006 600 (2), 241-...
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J. Phys. Chem. B 2001, 105, 10600-10609

1-Chloro-2-fluoroethane Adsorption on Cu(111): Structure and Bonding Robert G. Jones,*,† A. S. Y. Chan,†,§ S. Turton,† G. J. Jackson,‡ N. K. Singh,†,⊥ D. P. Woodruff,‡ and B. C. C. Cowie| Department of Physical Chemistry, School of Chemistry, UniVersity of Nottingham, Nottingham NG7 2RD, U.K., Department of Physics, UniVersity of Warwick, CoVentry CV4 7AL, U.K., CLRC Daresbury Laboratory, Warrington WA4 4AD, U.K., and Department of Physical Chemistry, School of Chemistry, UniVersity of New South Wales, Sydney 2052, Australia ReceiVed: January 4, 2001

The adsorption of 1-chloro-2-fluoroethane on Cu(111) has been studied using low-energy electron diffraction, Auger electron spectroscopy, UV photoelectron spectroscopy, line of sight temperature-programmed desorption, and normal incidence X-ray standing wave (NIXSW) analysis. The molecule adsorbs and desorbs molecularly. The activation energy for desorption of the first layer increases linearly from 30.8 to 32.8 kJ mol-1 as the coverage increases from zero to full monolayer coverage with a preexponential of 1012 s-1. The activation energy for the multilayer is found to be 27.1 kJ mol-1 for the same preexponential value. The NIXSW structural study shows that the molecule bonds via its chlorine end, in an atop site, with a Cu‚‚‚Cl distance of 3.06 ( 0.06 Å. The fluorine end, at a distance of 3.02 ( 0.1 Å from the surface copper plane, is also located approximately in an atop site. This orientates the molecule with its Cl‚‚‚F axis parallel to the surface. These results are compared with similar ones for 1,2-dichloroethane and 1-bromo-2-chloroethane on Cu(111). All three molecules have similar adsorption geometries, bonding via a single halogen (the heaviest) in an atop site, while the bond strength is determined, predominantly, by this halogen.

1. Introduction This paper is the third in a series dealing with the adsorption structures of van der Waals-bonded dihaloethanes on Cu(111). The previous two were 1,2-dichloroethane (ClCH2CH2Cl, DCE)1 and 1-bromo-2-chloroethane (BrCH2CH2Cl, BCE)2 on Cu(111). Here, we report a combined study of the bonding and structure of 1-chloro-2-fluoroethane (ClCH2CH2F, CFE) on Cu(111). The structural technique used is normal incidence X-ray standing wave (NIXSW) analysis3 which allows us, using dihalocarbons, to locate experimentally the positions of the halogens at each end of the adsorbed dihaloethane and then infer the positions of the carbons and hydrogens between them. (In the future, it will be possible to carry out NIXSW studies directly on the carbon atoms also). To study the bonding, we have used line of sight temperature-programmed desorption (LOSTPD), a technique in which only temperature-programmed desorption (TPD) originating from a small patch on the sample surface is monitored.4,5 Our interest in these molecules lies in examining the interplay between surface structure and surface bonding for molecules bonded with what may be described as either strong physisorption bonds or weak chemisorption bonds. In the discussion, we compare the structure and bonding of CFE on Cu(111) with that of BCE on Cu(111), in which the bonding is predominantly through the bromine atom and with that of DCE on Cu(111), in which both chlorines could contribute to the surface bonding but only one appears to be active. In this work, * To whom correspondence should be addressed. E-mail: robert.g.jones@ nottingham.ac.uk. † University of Nottingham. § Present address: Department of Chemistry, Harvard University, 12 Oxford St., Cambridge MA 02138, USA. ‡ University of Warwick. | CLRC Daresbury Laboratory. ⊥ University of New South Wales.

we find that for CFE on Cu(111) the bonding is predominantly through the chlorine atom, with the fluorine end contributing little to the surface bonding. In the NIXSW technique, an X-ray beam incident along the normal to a set of planes in the crystalline substrate is scanned in energy through the Bragg diffraction condition for those planes. As the X-ray energy traverses the Bragg condition, the incident and diffracted waves combine to form an X-ray standing wave, which moves in a predictable way between the substrate scatterer lattice planes by half of a lattice spacing. Atoms residing at different positions relative to the extended set of scatterer planes, either within the bulk or above the surface, experience different electric field intensities as the X-ray energy is scanned and the standing wave passes through their position. When the intensity of some atomic property which depends on X-ray intensity (photoelectron intensity, Auger electron intensity or fluorescence emission intensity) is monitored, the intensity of the X-ray field at an adatom or substrate atom position can be monitored, and this can then be analyzed to yield the numerical value of the adatom position relative to the extended set of scatterer planes. The two fitting parameters of interest in NIXSW are the coherent position, d (Å), and the coherent fraction, f. The coherent position is the effective (see below) perpendicular distance of a particular type of atom from the set of planes defined by the substrate scattering atoms which generate the X-ray standing wave. Coherent positions are measured as 0 < d/dhkl < 1, where dhkl is the layer spacing of the (hkl) diffracting planes (2.08 Å for the copper (111) and (-111) planes used in this study). The coherent fraction, which lies between 0 and 1, depends on the distribution of coherent positions, a value of 1 implying that all contributing atoms are located at exactly the same coherent position, with no disorder or mixing of positions

10.1021/jp010016r CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001

1-Chloro-2-fluoroethane Adsorption on Cu(111) relative to the (hkl) scatterer planes. f values of less than 1 may be due to a continuous distribution of atomic positions (either static or dynamic) or may be caused by a small number of welldefined, but different, atomic positions (which can even have the effect of reducing f to zero in particular circumstances).3,6 When the atoms contributing to an experimental d and f pair lie in a range of sites, j, we assign each site a population, Pj (∑Pj ) 1), a layer spacing, zj (0 < zj/d111 < 1), and a site order parameter, sj, where zj and sj are the individual site equivalents of d and f. The structural determination then consists of working backward from one, or more, d and f pairs from NIXSW data taken from one or more different substrate crystal planes to extract the zj, and possibly sj, values for each atom of interest.2 2. Experimental Section The experiments were carried out at the University of Nottingham and the CLRC Daresbury Laboratory in the U.K. The equipment used in Nottingham has already been described.4,5,7 It consisted of a stainless steel UHV chamber equipped with a three grid front view low-energy electron diffraction (LEED) optic, a 50 mm mean radius concentric hemisphere analyzer for HeI ultraviolet photoelectron spectroscopy (UPS), and a liquid nitrogen-shrouded SX300 quadrupole mass spectrometer (Vacuum Generators) for LOSTPD studies. The mass spectrometer had channeltron amplification which was operated in analogue mode for these experiments. For UPS, the sample was held at a negative potential of 9V to enable accurate observation of the secondary electron cutoff and hence determination of the work function of the surface. The sample temperature range was 100-1000 K with a heating rate of 5 K s-1 for LOSTPD. The NIXSW experiments were carried out on beam line 6.3 of the synchrotron radiation source (SRS) in the CLRC Daresbury laboratory. This beam line has already been described.8 Because CFE has a chlorine atom at one end and a fluorine atom at the other, we can determine the position and general orientation of the molecule by determining the positions of these two elements. The copper (111) and (-111) Bragg reflections at a nominal energy of 2972 eV were used to triangulate the positions of the two halogens. The (111) NIXSW data provide the halogen distances above the (111) substrate scatterer planes parallel to the sample surface, while the (-111) NIXSW data provide the distances from the (-111) set of scatterer planes which lie at 70.5° to the (111) set. The relative X-ray absorption vs photon energy plots for copper and chlorine (XSW curves) were monitored using the copper 920 eV Auger peak intensity and the chlorine KL2,3L2,3 Auger peak intensity at a kinetic energy of ∼2382 eV,9 (calculated value is 2376 eV10). The peak intensities were determined by measuring the intensities at the top of the peak (the “on” signal) and the intensities of the background several electronvolts to higher kinetic energy of the peak (the “off” signal) and subtracting one from the other. The raw peak intensity data was manipulated in the usual way.2,3 The fluorine XSW curves were monitored using the fluorine 1s photoelectron peak intensity (binding energy of 686 eV). Unfortunately, at the Bragg energy for the {111} planes, the fluorine 1s photoelectron has a kinetic energy of 2286 eV, very close to the kinetic energy of the KL1L2,3 Auger transition of chlorine (calculated energy of ∼2304 eV10). This constitutes a difficult problem in data analysis because the fluorine photoelectron peak, which fluctuates in intensity according to the fluorine XSW absorption profile, sits on a secondary electron background, which fluctuates in intensity according to the substrate copper

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10601 XSW absorption profile, while moving across the chlorine KL1L3 Auger peak, which fluctuates in intensity according to the chlorine XSW absorption profile. The methodology used to extract the fluorine XSW shape is explained in detail in the appendix. For each X-ray scan across the standing wave region, the copper Auger data, chlorine (KL2,3L2,3) Auger data, and fluorine 1s photoelectron data were accumulated simultaneously. Once the XSW curves for the chlorine and fluorine atoms had been determined, the analysis proceeded as has already been described in previous publications.2,3 The copper Auger data were used to determine the Bragg energy, E0, and the experimental broadening, σ, by fixing the position of the copper atoms at the known (bulk) value of 2.08 Å and obtaining the best fit for E0, σ, and f. E0 and σ were then transferred and fixed for the analysis of the chlorine XSW data, which had been taken simultaneously. This yielded best fit values of d and f for the chlorine end of the atom. Analysis of the simultaneously obtained fluorine 1s data was the same as for the chlorine data, except that a correction term (Q) for the electric quadrupolar term in the photoemission experiment had to be included.11,12 The program for fitting E0, σ, d, f, and Q to an XSW shape is available from R.G.J.13 3. Results 3.1. Room-Temperature Adsorption. Adsorption of CFE on clean Cu(111) was attempted at 300 K using LEED, Auger electron spectroscopy (AES), UPS, and work function (φ) measurements. It was found that the Cu(111) surface remained clean, with no alteration in any of the properties measurable by these techniques, for CFE exposures up to 25 × 10-6 mbar s (the highest used). We conclude from this that CFE does not adsorb at room temperature and does not contain any contaminants that adsorb at room temperature. 3.2. 100 K Adsorption. Adsorption studies of CFE on Cu(111) at 100 K were attempted using LEED and AES to monitor the surface, but extensive electron-stimulated reactions were observed, which deposited both halogens and carbon on the surface. To avoid these electron-stimulated reactions, we used UPS to monitor the surface, for which the secondary electron flux was about 1000 times lower than that in LEED and AES. However; even UPS required care (described below) because the HeI radiation also affected the adsorbed CFE, though in a different way than the electron beams used for LEED and AES. Fortunately, for both monolayer and multilayer coverages of CFE, a significant effect from the HeI lamp was only observable for times longer than an hour and was barely perceptible on the time scales used for the results presented in this paper. In the NIXSW studies, no loss of either chlorine or fluorine was detected during the course of a NIXSW run. At the end of each experiment, the sample was warmed to room temperature to desorb the CFE, and after this process, no halogen residues were detected on the surface. These results indicate that for the ∼3000 eV X radiation used for NIXSW no discernible photoor electron-induced reactions were occurring during the course of the experiment. Figure 1 shows nested UP spectra for CFE adsorption on Cu(111) at 100 K. The clean surface spectrum shows the surface state (SS) at 0.4 eV binding energy (BE) and two intense sharp peaks at 2.8 and 3.8 eV.14 The behavior of these two peaks during adsorption in this study, and in previous studies, indicates that they are acutely sensitive to surface conditions, and yet band structure calculations and angle resolved UPS measurement both concur that they are bulk states.15 Because it is not clear

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Figure 1. UP spectra for increasing exposures of clean Cu(111) to CFE at 100 K. The clean surface state (SS), surface responsive (SR) peaks, and peaks due to molecularly adsorbed CFE (Mol1-Mol5) are indicated on the plots.

Figure 3. UP difference spectra showing the spectrum from each individual adlayer of CFE. The first-, second-, third-, and fourth-layer spectra were produced by subtracting the clean, 5.5, 10.8, 14.7, and 19.9 × 10-6 mbar s spectra from the next highest exposure spectrum in the list (∼5 × 10-6 mbar s is the exposure for one layer). Shown at the top is the gas-phase spectrum of CFE20 shifted by 5.94 eV in ionization energy to match the peaks in the first-layer spectrum.

Figure 2. Background-subtracted intensities measured at the peak positions of Mol1 (at ∼5.6 eV) and Mol2 (at ∼7.3 eV) and the raw intensity of the clean SR peak at 2.8 eV for increasing exposure of the clean surface to CFE. Also shown are the work function change (∆φ) and the binding energy (BE) of Mol1 for increasing exposure.

at present how these peaks can be surface sensitive bulk states, we shall refer to them here as “surface responsive” (SR) peaks, until a better description has been found. For increasing coverages of CFE, the surface state and the SR peaks collapse and five molecular peaks, Mol1-Mol5, grow at 5.6, 7.3, 9.0, 10.8, and 11.9 eV. It will be shown below that these peaks are characteristic of molecular CFE, see Figure 3 for the gas-phase UP spectrum of CFE. The intensities of the SR peak at 2.8 eV, the molecular peaks, Mol1 and Mol2, and the work function change are shown in Figure 2 as a function of coverage. It can be seen from Figures 1 and 2 that the SR peaks were quenched at ∼5 × 10-6 mbar s. Similar behavior has been observed for a range of halocarbons on Cu(111) and is thought to indicate the exposure for saturation of the first molecular layer on the surface.16 Below, we show, using TPD, that 5 × 10-6 mbar s does indeed correspond to saturation of the first monolayer. Figure 2 shows that the work function decreased, the change being approximately linear with coverage up to the first layer, after which it remained constant to completion of the second layer at about 11 × 10-6 mbar s. It then began to increase back toward the clean surface value for continued increase in CFE coverage. The Mol1 and Mol2

peaks increased continuously up to the highest exposure studied, 45 × 10-6 mbar s. The intensity of Mol1 (Figure 2) shows a dip between 5 × 10-6 and 11 × 10-6 mbar s, as the second layer adsorbs, which may be related to a difference in the adsorption geometry of the molecules in the first and second layers. Also shown in Figure 2 is the binding energy of Mol1 as a function of exposure, which remains constant as the first layer forms but, for exposures greater than this, shifts to higher binding energies. At ∼15 × 10-6 mbar s, the binding energy shift maximizes, and then on further exposure, it decreases again. It then minimizes at ∼24 × 10-6 mbar s, after which it again rises with increasing exposure. Examination of Figure 1 shows that the other molecular peaks, Mol2-Mol5, behave in a similar way, and similar behavior has been observed for 1,2-dichloroethane on Cu(111).16 Figure 3, in which the spectrum for each individual layer has been calculated by subtracting the UP spectrum for an exposure up to layer n from the UP spectrum for an exposure up to layer n + 1, shows this movement more clearly. These movements are probably caused initially, for low coverages, by a change in the relaxation shift as the adlayers build out from the metal surface and eventually, for very thick layers, by electrostatic charging due to the insulating nature of the CFE. For intermediate coverages, both effects occur, leading to the complex movements observed. 3.3. Desorption. UPS was not used to monitor desorption because the time required for heating, cooling, and data taking was sufficiently long for effects from the HeI radiation to become appreciable. However, experiments were carried out

1-Chloro-2-fluoroethane Adsorption on Cu(111)

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10603

Figure 5. Areas under peaks A, C, and A and C together (total) from Figure 4C. The two smooth dotted lines are to guide the eye. The solid straight line is a least-squares linear fit to the total area data.

Figure 4. LOSTPD plots using m/z ) 46 (A) and 27 (C) for increasing initial exposure to CFE at 100 K. Part B is the simulated TPD plots for 0 < θinitial e 1 (peak A) and 1 < θinitial e 2.7 ( peak C); (θinitial ) 1) ≈ (4.1 × 10-6 mbar s).

in which multilayers of CFE were deposited on the clean surface at 100 K and then desorbed by heating to room temperature without any exposure to HeI radiation. Subsequent analysis by UPS showed the surface to be clean, demonstrating that no decomposition occurred for adsorption and desorption at 100 K in the absence of HeI radiation. LOSTPD was carried out using two cracking fragments of CFE, m/z ) 27 (C2H3+) and m/z ) 46 (C2H3F+) (m is the mass of the ion in daltons and z is the charge on the ion). Both fragments gave the same family of TPD curves, Figure 4, for increasing initial coverage, confirming that CFE adsorbs and desorbs molecularly in the absence of HeI radiation. (If the CFE had cracked to ethene, as some other dihalocarbons do on Cu(111), the behavior of the m/z ) 27 fragment would have shown the presence of ethene as well as CFE.) Figure 4A shows TPD scans obtained using m/z ) 46, while Figure 4C shows TPD scans obtained using m/z ) 27. Each scan was a separate experiment with a heating rate of 5 K s-1. The high-temperature peak, peak A, at ∼140 K saturated after an exposure of ∼4 × 10-5 mbar s. We identify this peak as the first layer of CFE in contact with the copper surface. For higher coverages, a second peak C grew at ∼120 K, which we identify as being due to multilayer desorption (note that there is no B peak, as this is used to describe halocarbons desorbing from a surface covered in chemisorbed halogens,17,18 which is inappropriate here). Rather interestingly, for both the m/z ) 27 and the m/z ) 46 derived TPD curves, the height of peak A diminishes for the highest initial coverage studied. It is also noticeable that concurrent with this reduction in peak A a low temperature shoulder appears on peak C. This behavior is probably related to a redistribution of molecules between the first and subsequent layers as multilayer growth occurred.

The multilayer peak can be fitted using zero-order kinetics, a preexponential of 1 × 1012 s-1, and a fixed activation energy of 27.1 kJ mol-1. The monolayer peak A could be fitted using first-order kinetics, a preexponential of 1 × 1012 s-1, and an activation energy that increased linearly from 30.8 to 32.8 kJ mol-1 as the coverage increased from zero to full monolayer coverage. TPD curves calculated using these values are shown in Figure 4B. A preexponential of 1012 s-1 was chosen as this gave the best fit to the width of the submonolayer peaks. However, good fits could also be obtained for preexponential values between 1011 and 1013 s-1 in which for each increase of a factor of 10 in the preexponential, the above activation energies increased by approximately 2.6 kJ mol-1 for peak A or 2.2 kJ mol-1 for peak C. Figure 5 shows the areas under peaks A, C, and A and C together vs CFE exposure for m/z ) 27. It is clear that saturation of peak A occurs after ∼5 × 10-6 mbar s and that growth of the multilayer peak C, begins at this point. This proves that the monolayer point occurs at about 5 × 10-6 mbar s. The total area under both peaks vs exposure can be fitted by a straight line, showing that the sticking probability remains constant as the first and the second layers form. This strongly indicates that the sticking probability is in fact 1.0. The decrease in intensity of peak A as the second layer forms is also clearly visible in Figure 5. 3.4. NIXSW Structural Study. The clean surface was dosed with 10 × 10-6 mbar s of CFE at 100 K (i.e., more than one layer). It was then warmed to 125 K to desorb the multilayer of CFE, leaving just the monolayer, and then recooled to 100 K for NIXSW data taking. Energy distribution curves were taken across the chlorine KL2,3L2,3 Auger peak and the fluorine 1s photoelectron peak, using hν ) 3000 eV (i.e., well above the Bragg condition) before and after each NIXSW run. In Figure 6, we show two typical NIXSW curves obtained for the Cu(111) substrate in the (-111) Bragg reflection using the copper Auger peak and the secondary electron background (the “off” signal) measured on the high kinetic energy side of the Cl KL2,3L2,3 Auger peak. The fits were obtained by fixing the value of d/d111 at the known value of 1.00 for bulk copper and allowing E0, f, and σ to vary. It can be seen that the fits are good and are typical of such scans taken for Cu(111) in previous NIXSW experiments (see, for example, ref 2). As expected, the two curves agree closely. Similar curves, obtained concurrently

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TABLE 1: XSW Results for CFE on Cu(111) species

reflection

d/d111

d (Å)

f

Q

Cu Auger and Cl Auger background Cl Cl F F

(111) and (-111) (111) (-111) (111) (-111)

1.0a 0.47 ( 0.03 0.45 ( 0.03 0.45 ( 0.05 0.40 ( 0.07

2.08 0.98 ( 0.06 or 3.06 ( 0.06 0.94 ( 0.06 0.94 ( 0.1 or 3.02 ( 0.1 0.83 ( 0.15

0.85 ( 0.03 0.48 ( 0.10 0.24 ( 0.04 0.37 ( 0.07 0.29 ( 0.10

0.255a 0.255a

a

Fixed values.

Figure 6. Two substrate (-111) NIXSW curves taken using the copper Auger signal and the chlorine Auger “off” signal. Fitting parameters for the Cu Auger data were d/d111 ) 1.00, f ) 0.83. Fitting parameters for the Cl Auger “off” data were d/d111 ) 1.00, f ) 0.84.

with the chlorine and fluorine data presented below, were used to determine the values of E0 and σ for a particular data set, which were then used in fitting the halogen data. The results for all the data sets for both (111) and (-111) reflections are shown in Table 1; note that the substrate fits for (111) and (-111) reflections would be expected to be the same, as indeed they are. Figure 7 shows a typical set of chlorine NIXSW curves for the (111) and (-111) Bragg reflections. Although the signalto-noise ratio is poorer than that for the substrate, good fits can still be obtained. The averaged results from three (111) and three (-111) reflection data sets are shown in Table 1. The (111) reflection data show that the chlorine atom in CFE lies either 0.98 ( 0.06 or (2.08 + 0.98 )) 3.06 ( 0.06 Å above the surface (111) substrate scatterer plane. We will assume that this surface copper plane is coincident with the centers of the surface copper atoms, as the clean Cu(111) surface has only a very small contraction of -1.0% (0.02 Å) at 300 K19 and we would not expect this to be substantially altered by the physisorption interaction with CFE. The coherent fraction for the (111) reflection is f ) 0.48 ( 0.1, which is rather smaller than the value of ∼0.8, which one would expect for chemisorbed species. However, for a physisorbed species such as we have here with its weaker and possibly less-specific bonding and the possibility of large amplitude motions such as frustrated rotations, f is sufficiently large to suggest that the chlorine end of the CFE molecule is at least fairly well-ordered in a direction perpendicular to the surface. The (-111) data, which are sensitive to the lateral position of the chlorine, show a coherent position of

Figure 7. Chlorine NIXSW curves for one layer of CFE adsorbed on Cu(111) at 100 K using the (111) and (-111) Bragg reflections. The data were obtained by monitoring the KL2,3L2,3 Auger peak. Fitting parameters for the (111) data were d/d111 ) 0.48, f ) 0.57. Fitting parameters for the (-111) data were d/d111 ) 0.46, f ) 0.21.

0.94 ( 0.06 Å with a significantly smaller coherent fraction of f ) 0.24 ( 0.04. This smaller coherent fraction for the (-111) planes could be due to either a mixture of adsorption sites or a large vibrational amplitude parallel to the surface. A set of NIXSW curves for fluorine using the (111) and (-111) reflections are shown in Figure 8. The noise is noticeably higher than that for the chlorine data, and the spread of values across the two (111) and two (-111) reflection data sets is shown by the increased errors in Table 1. The value of Q used in the analysis11,12 was 0.255. The (111) data position the fluorine either 0.94 ( 0.10 or (2.08 + 0.94 )) 3.02 ( 0.10 Å above the surface plane of copper atoms. The coherent fraction for the (111) reflection of f ) 0.37 ( 0.07 is lower than that found for the chlorine end of the molecule (0.48 ( 0.01) but not substantially so. For a distance measured perpendicular to the surface, this value of f indicates a wider range of positions for the fluorine atoms than for the chlorine atoms. The coherent position for the (-111) reflection is 0.83 ( 0.15 Å with a coherent fraction of f ) 0.29 ( 0.1. This value of f is substantially above zero which indicates (as for the chlorine end) that the fluorine end may be in a mixture of adsorption sites or have a large vibrational amplitude parallel to the surface. The finite value of f does, however, show that the molecule cannot be randomly distributed across all horizontal positions in the surface. 4. Discussion 4.1. UPS Peak Assignment. Figure 3 shows the gas-phase UP spectrum for CFE,20 which has been shifted by 5.94 eV to match the difference spectrum for the first layer of the adsorbed species, which is shown in the same figure. Table 2 shows the

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J. Phys. Chem. B, Vol. 105, No. 43, 2001 10605 TABLE 3: Experimental and Calculated d-111 Values for Cl and Fa exptl. value (Å)

atop site (Å)

2-fold 3-fold (fcc) 3-fold (hcp) bridge site (both) (Å) site (Å) site (Å)

Chlorine Using d111 ) 3.06 ( 0.06 Å d/Å 0.94 ( 0.06 1.02 ( 0.02 0.33 ( 0.02 1.72 ( 0.02 2.06 ( 0.02 fb 0.24 ( 0.04 1 1 1 0.33 Fluorine Using d111 ) 3.02 ( 0.1 Å d/Å 0.83 ( 0.15 1.01 ( 0.05 0.32 ( 0.05 1.70 ( 0.05 2.05 ( 0.05 fb 0.29 ( 0.1 1 1 1 0.33 Fluorine Using d111 ) 4.61 ( 0.1 Å d/Å 0.83 ( 0.15 1.54 ( 0.05 0.85 ( 0.05 0.15 fb 0.29 ( 0.1 1 1 1

0.5 0.33

a Using d 111 ) 3.06 ( 0.06 Å for Cl and 3.02 ( 0.1 and 4.61 ( 0.1 Å for F. b Calculated f values are maximum possible.

Figure 8. Fluorine NIXSW curves for one layer of CFE adsorbed on Cu(111) at 100 K using the (111) and (-111) Bragg reflections. The data were obtained by monitoring the F 1s photoelectron peak and data reduction as described in the text. Fitting parameters for the (111) data were d/d111 ) 0.44, f ) 0.41. Fitting parameters for the (-111) data were d/d111 ) 0.44, f ) 0.20.

TABLE 2: Ionization Energies and Binding Energies of CFE Molecular Orbitals in the Gas and Adsorbed Phasea gas IP (eV)

molecular orbital

surface BE (eV)

peak

11.5 12.9 13.4 14.6 15.4 16.6 17.5

nCl

5.6 7.3 7.3 9.0 9.0 10.8 11.9

mol1 mol2 mol2 mol3 mol3 mol4 mol5

nF

a

Ionization energies (IP) and binding energies (BE) of CFE in the gas phase in the first adsorbed layer on Cu(111). For the gas phase, the nonbonding fluorine and chlorine molecular orbitals are identified. For the adsorbed phase, the labels Mol1-Mol5 are shown.

ionization potentials of gas-phase CFE and the binding energies of the adsorbed molecule in the first layer. The nonbonding chlorine, nCl, and fluorine, nF, orbitals can be unambiguously identified in the gas-phase spectrum by comparison with published data for chloroethane and fluoroethane21 and correspond to Mol1 and Mol5 in the adsorbed molecule. The remaining gas-phase peaks are due to the σC-C, σC-Cl, σC-F, and πCH2 molecular orbitals, but with no accurate molecular orbital calculation available for CFE at present, we have not attempted to assign these. However, for the adsorbed molecule, Mol2 may be identified as being due to the gas-phase 12.9 and 13.4 eV peaks, Mol3 as being due to the 14.6 and 15.4 eV peaks, and Mol4 as being due to the 16.6 eV peak. So as expected for a physisorbed species, the UP spectrum of the adsorbed molecule is very similar to that of the gas-phase molecule, apart from relaxation and work function shifts between the two. It is also noticeable that the relative peak intensities are different for the gas-phase and adsorbed molecule. This is to be expected because the adsorbed spectrum is from orientated molecules, while the gas-phase spectrum is an average of all orientations. 4.2. Surface Structure. From Table 1, the chlorine atom is at a distance of either 0.98 or 3.06 Å above the surface copper atomic plane. The distance of 0.98 Å is too short to be chemically meaningful, leaving 3.06 Å as the next possibility.

If we assume that the chlorine lies in an atop site, then the Cu‚‚‚Cl bond distance, given by the sum of the copper atomic radius (1.278 Å) and the chlorine van der Waals radius (1.80 Å), should be directly comparable with the experimentally determined distance. This sum is 3.078 Å, in remarkably close agreement with the experimentally determined value of 3.06 Å. The coherent fraction of 0.48 ( 0.1 is lower than would be expected from a chemisorbed species in a single site but is probably reasonable for a physisorbed species in which the surface vibration amplitude is greater. Apart from the atop site, the other possible high-symmetry sites for the chlorine are the hcp and fcc 3-fold hollow sites and the bridge sites. Given that the chlorine lies in particular sites at a distance of 3.06 Å from the (111) scatterer plane, it is a simple matter to calculate the distance expected in a (-111) NIXSW measurement for each type of site.2,3 This is done in Table 3. Single occupancy of either type of the 3-fold hollow sites, the fcc or the hcp, can be eliminated as possibilities, as can any combination of these two sites (the hcp 3-fold hollow has a copper atom directly below the adatom in the second copper layer down, while the fcc 3-fold hollow has a copper atom directly below the adatom in the third copper layer down). If we consider the bridge sites, there are two types on the (111) surface with distances of 1.02 Å (population 1/3) and 2.06 Å (population 2/3) relative to the (-111) scatterer planes (these sites are chemically identical but differ in their distances from the (-111) scatterer plane). When these populations and distances are combined,2,3 they give calculated values of d-111 ) 2.06 Å (and a theoretical maximum value of f ) 0.33). So bridge sites can also be eliminated as possible locations for the chlorine. When the d(111) value is used, excellent agreement is found between the experimental and calculated value for d(-111) for the atop position, confirming that the chlorine in CFE is adsorbed in an atop position. However, the experimental coherent fraction for the (-111) data (f ) 0.24) is very much lower than the theoretical maximum (1.0 for a single position) or the highest possible experimental value of ∼0.85 (i.e., the same as the copper substrate). This low value of f is compatible with adsorption in a single atop site provided that there is a large amplitude vibration parallel to the surface. This may be regarded as a reasonable model for CFE physisorbed on Cu(111) for which the corrugation at the bottom of the adsorption well is likely to be sufficiently shallow to allow such movement even at 100 K. For fluorine, d111 ) 0.94 ( 0.1 or 3.02 ( 0.1 Å. As the smaller value is physically unrealistic, we consider the next distance of 3.02 Å above the copper surface. The sum of the

10606 J. Phys. Chem. B, Vol. 105, No. 43, 2001

Figure 9. Plan (A), side (B), and end (C) views of chlorofluoroethane adsorbed on Cu(111) with the chlorine in an atop position and the fluorine close to an atop position. The molecule is shown in the anti conformation with the Cl-C-C-F plane aligned perpendicular to the surface. The atoms are shown with hard-sphere radii equal to their van der Waals radii. The experimentally determined heights of the two halogens are also shown.

copper atomic radius (1.278 Å) and the fluorine van der Waals radius (1.40 Å) is 2.678 Å, 0.34 Å shorter than the experimentally determined distance. This shows that the fluorine atom is not in contact with the copper surface but is some distance further out. The coherent fraction of 0.37 ( 0.07 is smaller than the equivalent (111) value for the chlorine end, implying that the fluorine is rather more free to move in a vertical plane than the chlorine. Table 3 shows the experimental value of d-111 (0.83 ( 0.15 Å) and the values calculated (as for chlorine) by assuming that the fluorine resides in the different high-symmetry sites. The closest match is, again, the atop site with a calculated value of 1.01 ( 0.05 Å. Although these value are close and clearly rule out any of the other adsorption sites, the two values are still significantly different. This suggests that there is a sufficient degree of heterogeneity of fluorine sites to pull the measured value of d-111 away from that expected for pure atop adsorption. A simple explanation for this would be as follows. If the molecule is bound primarily via the chlorine end in an atop site, then we must look for possible fluorine atop adsorption sites around this position. From the gas-phase geometry of CFE, the Cl‚‚‚F distance is calculated to be 3.922 Å.22 On Cu(111), the nearest neighbor distance between copper atoms within the surface plane is 2.556 Å, while the next nearest neighbor distance is 4.427 Å (x3 × 2.556, see Figure 9). So for a CFE molecule lying with the chlorine in an atop position, lying with the Cl‚‚‚F axis horizontal (in agreement with the NIXSW analysis above), and orientated with the Cl‚‚‚F axis pointing in the direction of the next nearest neighbor atop site, the F atom would be only ∼0.5 Å short of lying directly above this atop site. Figure 9 shows plan, side, and end views of a hard-sphere model of CFE on Cu(111) that places the halogens in these positions. The molecule is in the anti conformation with gasphase bond lengths and angles22 and hard-sphere van der Waals radii of H (0.9 Å), C (1.5 Å), F (1.4 Å), and Cl (1.8 Å). It should be pointed out that the (111) NIXSW data for the fluorine atom are also compatible with the fluorine lying at a distance of (0.45 + 2 × 2.08) ) 4.61 Å from the surface. If this were the case, the Cl‚‚‚F axis would lie at an angle of ∼23° to the surface and the fluorine would be closer to lying above a 3-fold hollow than lying above the atop site. In Table 3, we list the calculated (-111) distances for fluorine at a (111)

Jones et al.

Figure 10. Chlorofluoroethane adsorbed on Cu(111) as for Figure 9 but with the molecule rotated by 180°, parts A and B, and 90°, parts C and D, about the horizontal Cl‚‚‚F axis. Notice that in parts C and D the Cl-C-C-F plane is now parallel to the copper surface.

distance of 4.61 Å. The calculated d value for the fcc 3-fold hollow (0.85 Å) is very close to the experimental value (0.83 Å), and hence our NIXSW data is also consistent with the CFE molecule lying at angle of ∼23° to the surface with the fluorine atom located at a distance of 4.61 Å above an fcc 3-fold hollow. However, one might reasonably ask why the fluorine, such a long way from the surface, is able to locate so precisely above an fcc 3-fold hollow? One would expect equal population of both types of 3-fold hollow, giving a calculated (-111) distance of 0.5 Å, in poor agreement with the experimental value. The (-111) coherent fraction (0.29) is also very low, which is incompatible with a single specific adsorption site. We therefore reject the possibility of the molecule being tilted at 23°. Although the positioning of the atoms within the C2H4 group is at present conjectural, it is instructive to consider the possibilities. In Figure 9, the molecule is orientated such that the hydrogens in the FCH2 group point toward the surface, while those in the ClCH2 group point away. The side and end views in Figure 9 show that this places the two hydrogens in the FCH2 group in contact with two surface copper atoms. In Figure 10A,B, the molecule has been rotated by 180° about the horizontal Cl‚‚‚F axis such that the hydrogens in the ClCH2 group are now pointing toward the surface. The two hydrogens are again in van der Waals contact with the two surface copper atoms, and the situation is very similar, though not identical, to Figure 9. If the molecule in Figure 9 is rotated by 90° about the Cl‚‚‚F axis, we get the situation in Figure 10C,D. Here again, there are two H‚‚‚Cu atom contacts, one from the ClCH2 group and one from the FCH2 group. Future studies using chemical shift XSW to separately determine the positions of the carbons in the ClCH2 and FCH2 groups should be able to differentiate among these three models and the possibility that the molecule can undergo free rotation about the Cl‚‚‚F axis. For all four models, we predict that the carbons should be in approximately bridge sites. For Figure 10C,D, both carbons will be located 3.1 Å above the copper surface atomic plane. For Figure 9, the carbon in the ClCH2 group will be 3.7 Å, and the carbon in the FCH2 group 2.5 Å, above the copper surface atomic plane. For Figure 10A,B, these carbon positions will be reversed. If the molecule were spinning about the Cl‚‚‚F axis, then both carbons would have a position of 3.1 Å but with a coherent fraction in the (111) standing wave data considerably lower than that of the chlorine or fluorine atoms. 4.3. Comparison of Bonding and Structure of BCE, DCE, and CFE on Cu(111). 1-Bromo-2-chloroethane, BrCH2CH2Cl, bonds to Cu(111) via its bromine end, which lies in an atop

1-Chloro-2-fluoroethane Adsorption on Cu(111)

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10607

TABLE 4: Adsorption Geometries for XCH2CH2Ya Molecules on Cu(111) adsorbate XCH2CH2Y

X site

rXb (Å)

rCuc + rXb (Å)

rCu‚‚‚Xd (Å)

θX‚‚‚Ye

rCu‚‚‚Yd (Å)

Y site

ref

BrCH2CH2Cl ClCH2CH2Cl ClCH2CH2F

atop ∼atop atop

2.00 1.80 1.80

3.278 3.078 3.076

2.63 ( 0.2 3.08 ( 0.1 3.06 ( 0.06

11° 9° 0°

3.78 ( 0.2f 3.68 ( 0.1 3.02 ( 0.1

∼hcp 3-fold hollow ∼atop ∼atop

2 1 this work

a X is the heavier halogen; Y is the lighter halogen. b r is the van der Waals radius of X. c r d X Cu is the atomic radius of Cu. rCu...X and rCu...Y are the experimentally determined bond lengths. e θX...Y is the angle the X‚‚‚Y axis makes with the copper surface. f Calculated from the chlorine layer spacing of 3.48 ( 0.2 Å and assuming a 3-fold hollow adsorption site.

site with a Br‚‚‚Cu distance of 2.63 ( 0.2 Å. The Br‚‚‚Cl axis is tilted away from the surface at an angle of ∼11°, placing the chlorine above an hcp 3-fold hollow site at a distance of 3.48 ( 0.2 Å from the surface copper plane.2 1,2-Dichloroethane, ClCH2CH2Cl, bonds to Cu(111) with the two chlorines ∼3.1 and ∼3.7 Å above the surface copper plane, with the one closer to the surface probably in an atop site. The Cl‚‚‚Cl axis is tilted at ∼9° to the surface plane.1 For 1-chloro-2-fluoroethane, we have shown above that the molecule bonds via its chlorine end in an atop position with a Cl‚‚‚Cu distance of 3.06 ( 0.06 Å. The Cl‚‚‚F axis is parallel to the surface, the fluorine atom lying close to an atop position. This information is collected together in Table 4. All three molecules bond via the heaviest halogen, located in an atop position. For DCE and CFE, which both bond via a chlorine atom, the bond lengths are the same and match the sum of the copper atomic radius, rCu (1.278 Å), and the halogen van der Waals radius, rX (1.80 Å for chlorine). For BCE, the Cu‚‚‚Br bond length is markedly shorter than rCu + rX but longer than the sum of the copper and bromine covalent radii (1.17 + 1.14 ) 2.31 Å). This indicates that the Cu‚‚‚Br bond for physisorbed BCE is stronger than the type of van der Waals bonds formed by DCE and CFE but not as strong as a covalent Cu-Br bond. This fits with the adsorption behavior of BCE, in which the molecule dissociates rather than desorbing intact, as DCE and CFE do. The outer chlorine atom in BCE was found to be located approximately in an hcp 3-fold hollow, while for DCE a model was proposed in which the outer of the two halogens was located approximately above an atop site. For CFE, the outer fluorine is located approximately above an atop site. So, DCE and CFE are in good agreement, but BCE is different. This may be because in the BCE study the molecules were coadsorbed with chemisorbed chlorine and bromine, and this may have influenced the positioning of the chlorine end of the molecule. For CFE, the X‚‚‚Y angle with the surface was approximately zero, and this has been rationalized, above, as being due to bonding via the chlorine plus van der Waals contact between two surface copper atoms and two hydrogens on the molecule. For DCE and BCE, this angle increases to 9° and 11°, respectively. Because both CFE and DCE have the same CHCl2 group closest to the surface, this behavior cannot be explained by any simple hard-sphere-type model. We now consider the bonding strength of these molecules as demonstrated by the activation energy, Ea, for desorption of the intact molecule, Table 5. As noted above, BCE dissociates before it can desorb and so is not considered further here. DCE has a significantly higher adsorption energy (43 kJ mol-1) compared to that of CFE (34 kJ mol-1) using a preexponential of 1013 Hz to aid comparisons. These numbers should be compared with the adsorption energy of chloroethane (ClCH2CH3) on Cu(111) (38 kJ mol-1) derived from the work of Lin and Bent.24 Starting from chloroethane, it would seem that the replacement of an H in the CH3 group by Cl only increases the adsorption energy by about 5 kJ mol-1, while replacement by an F decreases it by about 4 kJ mol-1. In other words, the second

TABLE 5: Desorption Parameters for XCH2CH2Y Molecules from Cu(111)a adsorbate XCH2CH2Y Ea (kJ mol-1) BrCH2CH2Cl ClCH2CH2Cl ClCH2CH2F ClCH2CH3

dissociates 43 31 (34)b 38c

ν (Hz)

ref

18 1 × 1013 17 1 × 1012 (1 × 1013)b this work 1 × 1013 24

a Activation energy for desorption, Ea, and preexponential factor, ν, for molecular desorption of XCH2CH2Y from Cu(111). b Values in parentheses are for an assumed preexponential of 1 × 1013 Hz. c Extrapolated to chloroethane from an alkyl chloride TPD data set ranging from chloropropane to chloroheptane.

chlorine in DCE does not contribute much more to the bonding, in agreement with its greater distance from the surface, while the F in CFE actually decreases the adsorption energy. Bent has shown24 that each CH2 group in the series CH3(CH2)nCl increases the adsorption energy by 5 ( 1 kJ mol-1, so it would appear that the electron-withdrawing F in CFE effectively eliminates the bonding contribution of the CH2 to which it is attached. 5. Conclusion 1-Chloro-2-fluoroethane adsorbs molecularly on Cu(111) at 100 K. It bonds via the chlorine end in an atop site with a Cu‚‚‚Cl bond length of 3.06 ( 0.06 Å. The Cl‚‚‚F axis of the molecule is approximately parallel with the surface with the fluorine atom in an approximately atop site at a distance of 3.02 ( 0.1 Å from the surface copper atom plane. Desorption occurs molecularly with first-order kinetics, a preexponential of 1 × 1012 s-1, and an activation energy that increased linearly from 30.8 to 32.8 kJ mol-1 as the coverage increased from zero to full monolayer coverage. The adsorption geometry and bonding of 1-chloro-2-fluoroethane, 1,2-dichloroethane, and 1-bromo2-chloroethane to Cu(111) show systematic differences. Acknowledgment. We would like to thank the EPSRC for supporting this work and providing a studentship (S.T.). We would also thank the University of Nottingham for a studentship (A.S.Y.C.) and the CLRC for beam time on the Synchrotron Radiation Source at Daresbury laboratory. Appendix In this appendix, we describe how the intensity of the fluorine 1s photoelectron peak is measured even though it moves across a weak chlorine Auger peak (KL1L2,3) during the photon energy scan. First, we describe an idealized situation in which there is no X-ray standing wave. Then, we show how the equations can be modified to accommodate the effect of the standing wave in the X-ray scan. Finally, we show how the photoelectron intensity can be extracted for the general case in which substrate and adsorbate Auger electron peaks are encountered in the scan. Figure 11A shows an idealized electron energy distribution curve (edc) of the fluorine 1s photoelectron peak and the chlorine Auger electron peak sitting on a flat secondary electron

10608 J. Phys. Chem. B, Vol. 105, No. 43, 2001

Jones et al. allowing Cledc(EX) to be determined from the experimental measurements. Because the position of Fb precedes that of Fu by δ eV (Figure 11B), Cledc(EX) can be shifted by δ eV, Cledc(EX+δ), allowing us to write

Fu(EX) ) Fb,0(1 + Cledc(EX+δ))

(A3)

and hence we can deduce the fluorine 1s photoelectron peak intensity, Fp - Fu, using Fp, Fb, δ, and Fb,0. To the above case, we now add the extra complication that the X-rays pass through the Bragg condition during the X-ray energy scan. Our object is still to measure the variation of the fluorine photoelectron peak intensity (Fp - Fu) as a function of photon energy, which we can then use to obtain the X-ray absorption profile for the fluorine atoms. If Cuxsw(EX) is the relative X-ray absorption of the copper substrate as a function of photon energy (see Figure 6, for example), then the intensity of the copper background, as a function of photon energy, will be Fb,0Cuxsw(EX). Similarly, if Clxsw(EX) is the relative X-ray absorption of the chlorine atoms as a function of photon energy (see Figure 7, for example), then the intensity of the KL1L2,3 chlorine Auger electron peak, as a function of photon energy, will be Fb,0Clxsw(EX)Cledc(EX). Hence, we can write

Fb(EX) ) Fb,0(Cuxsw(EX) + Clxsw(EX)Cledc(EX)) (A4) which can be rearranged as Figure 11. Idealized situation for measuring the F 1s photoelectron intensity in the presence of a Cl Auger peak on a flat secondary electron background due to Cu in the absence of any standing wave: (A) An energy distribution curve showing how the F 1s photoelectron peak moves across the Cl Auger electron peak for increasing X-ray energy. Fp and Fb are the experimentally measured “on” and “off” positions and Fu is the true background under the 1s peak. (B) The intensities of Fp and Fb plotted as a function of X-ray energy (Ex). (C) The Cl Auger electron peak shape as a function of X-ray energy normalized to the intensity of the Cu background electrons for Fb and shifted by δ to allow Fu to be calculated. See appendix for more details.

background due to the copper substrate. If we take the simplest case first and assume that the X-ray energy is far removed from the Bragg condition, then as the photon energy increases, the fluorine 1s photoelectron peak will move to higher kinetic energies along the flat secondary electron background, over the chlorine Auger electron peak, and back onto the background again. During this movement, we experimentally monitor the intensity, Fp, at the top of the photoelectron peak and on the background, Fb, δ eV ahead of the peak (usually 10-20 eV), generating two data sets, Fp(EX) and Fb(EX), where EX is the X-ray energy in eV, Figure 11B. To extract the peak height of the fluorine 1s peak at each photon energy, we need the intensity of the background directly under the peak, Fu. If we normalize the constant secondary electron background due to copper to a value of 1.0 and write the chlorine Auger peak as a function of the photon energy (instead of kinetic energy), see lower set of arrows in Figure 11A and the plot in Figure 11C, then

Fb(EX) ) Fb,0(1 + Cledc(EX))

Fb,0 is the background intensity, in counts, at the beginning of the scan before either the chlorine Auger peak or the X-ray standing wave are encountered. Fb(EX) is measured experimentally, and Cuxsw(EX) and Clxsw(EX) are determined simultaneously, but separately, during the run by measuring the peak and background intensities for the chlorine KL2,3L2,3 Auger peak and the copper Auger peak. Hence, Cledc(EX) can be determined from equation (A5). Fu can now be written as

Fu(EX) ) Fb,0(Cuxsw(EX) + Clxsw(EX)Cledc(EX+δ)) (A6) where Cledc(EX+δ) is Cledc(EX) that has been shifted by the separation, δ, between Fb and Fu. All the terms on the righthand side of eq A6 are known, allowing Fu(EX) to be determined. It is then a simple matter to evaluate Fp(EX) Fu(EX), which is the fluorine 1s photoelectron peak height as a function of photon energy. It is possible to envisage one more level of complexity in which the photoelectron peak moves across a structured background containing copper Auger peaks, given by the normalized function Cuedc(EX), with chlorine Auger electron peaks on top of these. We can then write

Fb(EX) ) Fb,0(Cuxsw(EX)Cuedc(EX) + Clxsw(EX)Cledc(EX)) (A7) and

(A1)

Fb,0 is the intensity, in counts, of the constant copper secondary electron background, while Cledc(EX) is the intensity of the chlorine Auger peak as a fraction of the secondary electron background. Equation A1 can be rearranged as

Cledc(EX) ) Fb(EX)/Fb,0

Cledc(EX) ) {Fb(EX)/Fb,0 - Cuxsw(EX)}/Clxsw(EX) (A5)

(A2)

Fu(EX) ) Fb,0(Cuxsw(EX)Cuedc(EX+δ) + Clxsw(EX)Cledc(EX+δ)) (A8) In this case, two functions, Cuedc(EX) and Cledc(EX), have to be determined, and there is insufficient information in equation (A7) to allow this. However, by experimental measurement of these edcs as functions of kinetic energy, Cuedc(KE) and

1-Chloro-2-fluoroethane Adsorption on Cu(111) Cledc(KE), at a fixed photon energy well removed from the Bragg condition, followed by normalization to a particular background level, it is a simple matter to shift the abscissa appropriately to give Cuedc(EX+δ) and Cledc(EX+δ). These functions may then be used in equation (A8) to calculate Fu(EX), which allows the photoelectron intensity, Fp - Fu, to be determined. The solution to the case in which the photoelectron peak passes over just a structured substrate Auger peak but not an adsorbate Auger peak is given by equation (A8) with Cledc(EX+δ) ) 0 and has already been described for another system.23 For the situation in which an Auger electron peak is being monitored and a substrate or adsorbate photoelectron peak sweeps through the Auger peak during the photon energy scan, similar reasoning to the above can be employed to extract the intensity of the Auger electron peak as a function of photon energy. References and Notes (1) Kerkar, M.; Walter, W. K.; Woodruff, D. P.; Jones, R. G.; Ashwin, M. J.; Morgan, C. Surf. Sci. 1992, 268, 36. (2) Kadodwala, M. F.; Davis, A. A.; Scragg, G.; Cowie, B. C. C.; Kerkar, M.; Woodruff, D. P.; Jones, R. G. Surf. Sci. 1997, 392, 199. (3) Woodruff, D. P. Prog. Surf. Sci. 1998, 57, 1. (4) Jones, R. G.; Fisher, C. J. Surf. Sci. 1999, 424, 127. (5) Jones, R. G.; Clifford, C. A. Phys. Chem. Chem. Phys. 1999, 1, 5223. (6) Woodruff, D. P.; Cowie, B. C. C.; Ettema, A. R. H. F. J. Phys.: Condens. Matter 1994, 6, 10633.

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10609 (7) Singh, N. K.; Jones, R. G. Surf. Sci. 1990, 232, 229. (8) McDowell, A.; Norman, D.; West, J.; Campuzano, J.; Jones, R. G. Nucl. Instrum. Methods Phys. Res., Sect. A 1986, 246, 131. (9) McGuire, G. E. AES reference manual; Plenum: New York, 1979. (10) Coglan, W. A.; Clausing, R. E. At. Data 1973, 5, 317. (11) Fisher, C. J.; Ithnin, R.; Jones, R. G.; Jackson, G. J.; Woodruff, D. P.; Cowie, B. C. C.; Kadodwala, M. F. J. Phys. Condens. Matter 1998, 10, L623. (12) Jackson, G. J.; Cowie, B. C. C.; Woodruff, D. P.; Jones, R. G.; Kariapper, M. S.; Fisher, C. J.; Chan, A. S. Y.; Butterfield, M. Phys. ReV. Lett. 2000, 84, 2346. (13) The fitting program runs under Igor Pro 3 on either a Macintosh or a PC. It is available from [email protected]. (14) Westphal, D.; Goldmann, A. Surf. Sci. 1983, 131, 92. (15) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science, 2nd ed; Cambridge University Press: Cambridge, U.K., 1994; p 226. (16) Walter, W. K.; Jones, R. G. Surf. Sci. 1992, 264, 391. (17) Chan, A. S. Y.; Turton, S.; Jones, R. G. Surf. Sci. 1999, 433-435, 234. (18) Turton, S.; Kadodwala, M.; Jones, R. G. Surf. Sci. 1999, 442, 517. (19) Chae, K. H.; Lu, H. C.; Gustafsson, T. Phys. ReV. B 1996, 54, 14082. (20) Gas-phase spectrum of CFE measured by J. Dyke, University of Southampton. (21) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeI photoelectron spectra of fundamental organic molecules; Japan Scientific Societies Press: Tokyo, Japan, 1981. (22) Huang, J.; Hedberg, K. J. Am. Chem. Soc. 1990, 112, 2070. (23) Jones, R. G.; Abrams, N. E.; Jackson, G. J.; Booth, N. A.; Butterfield, M. T.; Cowie, B. C. C.; Woodruff, D. P.; Crapper, M. D. Surf. Sci.1998, 414, 396. (24) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1992, 96, 8529.