2886
J. Phys. Chem. 1985,89, 2886-2891
the gauche to cis barrier is greater by 15%. The dihedral angle for the gauche conformer is increased by 2O which is not significant. Therefore, the observed asymmetric torsional frequencies can be satisfactorily fit with the smaller A H value, but the experimentally determined value is well determined statistically from the 920/905-cm-l d0ub1et.I~ It is possible that the 920-cm-l band of the gauche conformer could have a difference band of ~ ' 1 7dZ1z 920 in coincidence so that the increased intensity of the 920-cm-I line with temperature could be enhanced, which would lead to a larger AH value. The corresponding sum band could have sufficient anharmonicity so that it would be obscured by vl0 at 11 18 cm-'. Unfortunately, there are no other doublets in the Raman spectrum which can be used to check the value obtained earlier.14 Since the barriers between the conformers as determined experimentally differ by 30-90 cm-I from those obtained by the 6-31G* ab initio calculations, it is possible that the AHobtained theoretically may be in error by at least these values. Therefore, we will continue to prefer the larger AH value obtained experimentally until it is shown that the a b initio values agree with experimental values for a sufficient number of molecules so they can be confidently accepted. At this time, there appear to be discrepancies between experimental and theoretically determined values with the 6-31G* or 4-31G basis sets for several similar molecules so that further evaluations need to be carried out.3847
Conclusions In the present study it has been shown that two conformers of 3-fluoropropene exist at ambient temperature and the conformer which has the fluorine atom cis to the double bond is the more stable form in the gaseous, liquid, and solid phases. From a variable-temperature study of the relative intensities of the Raman lines at 918 cm-I (gauche) and 903 cm-' (cis) in the liquid, the enthalpy difference was found to be 58 cm-' (166 cal/mol), with the cis rotamer as the thermodynamically preferred conformation. This value is about one-sixth the value for the gas.I4 A complete vibrational assignment has been presented for the cis conformer, and, where the gauche conformer has different frequencies from the cis, suggested assignments for these bands have also been presented. The assignments are supported by a normal-coordinate calculation. Complete equilibrium geometries have been determined for both rotamers by ab initio Hartree-Fock gradient calculations, and the calculated parameters are compared for both the r, and r, structural parameters. The a b initio calculations predict that the cis rotamer lies 148 cm-' (423 cal/mol) lower in energy than the gauche conformer; however, this determined energy difference is about one-half of the experimental value. The potential barriers have been calculated and compared to the experimental values and found to be in agreement within the expected uncertainties of the calculated barriers.
(38) Currie, G. W.; Ramsay, D. A. Can. J . Phys. 1971, 49, 317. (39) Durig, J. R.; Bucy, W. E.; Cole, A. R. H. Can. J. Phys. 1975, 53, 1832. (40) George, P.; Bock, C. W.; Trachtman, M. J . Mol. Struct. 1980, 69, 183. (41) Osamura, Y.; Schaefer, 111, H. F. J . Chem. Phys. 1981, 74, 4576. (42) Carreira, L. A. J . Phys. Chem. 1976, 80, 1149. (43) Alves, A. C. P.; Christoffenen, J.; Hollas, J. M. Mol. Phys. 1971,20, 625. (44) Durin. J. R.: ComDton. D. A. C. J . Phvs. Chem. 1979. 83. 265. (45) Campion, D.' A. C:; Montero, S.; Murphy, W. F. J . Phys. Chem. 1980,84, 3587.
Acknowledgment. We gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-83-11279. Registry No. 3-Fluoropropene,818-92-8; allyl bromide, 106-95-6; KF, 7789-23-3.
(46) (47)
Raghavachari, K. J . Chem. Phys. 1984,81, 1383. Compton, D. A. C. In "Vibrational Spectra and Structure"; Durig,
J. R., Ed.; Elsevier: Amsterdam, 1980; Vol. 9.
Surface Chemistry of Dimethyl Methylphosphonate on Rh( 100) R. I. Hegde, C. M. Greenlief, and J. M. White* Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 (Received: January 22, 1985)
The adsorption of dimethyl methylphosphonate (DMMP) has been studied on clean and carbon-covered Rh( 100) surfaces. Temperatureprogrammed desorption (TPD), X ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), and work function change (A@) measurements were used to characterize the adsorbed layer as a function of temperature. At 100 K the DMMP adsorbed into two states, a monolayer phase and a multilayer phase which are distinguishable by TPD and spectroscopy measurements. For one monolayer of DMMP on carbon-free Rh(100), between 60 and 70% decomposes upon heating, leaving carbon, phosphorus, and oxygen on the surface. On carbon-covered Rh, the decomposition of DMMP is strongly inhibited. On carbon-free Rh, there are two distinct molecular DMMP desorption peaks at 210-225 and 200 K (monolayer and multilayer) with first-order desorption energies of 13.8 and 8.6 kcal/mol, respectively. On the C-covered surface, the higher temperature DMMP desorption peak shifts to slightly lower temperature. X-ray photoelectron spectra of multilayer and monolayer DMMP indicate some dissociative adsorption at 100 K. Ultraviolet photoelectron spectra of multilayer and gas-phase DMMP are compared. A bonding configuration of DMMP to the Rh( 100) surface is proposed.
Introduction The surface chemistry of adsorbed organophosphorus molecules is largely unexplored. It is our goal to understand the surface chemistry of such molecules, including phosphine (PH,), which we have reported in an earlier paper,' and dimethyl methyl. (1) R. I. Hegde, J. Tobin, and J. M. White, J . Vac. Sci. Technol., A, 3, 339 (1985).
0022-3654/85/2089-2886$01.50/0
phosphonate (DMMP), which is discussed here. DMMP can coordinate to transition-metal atoms via either a lone pair or a orbital. Other molecules with similar coordinating ability include alcohols, ethers, esters, ketones, amides, and water. Recently we have examined the behavior of HzO adsorbed at low temperatures on Rh(100).* We have also studied the interaction of PH3 on (2) R. I.
Hegde and J. M. White, Surf. Sci., in press.
0 1985 American Chemical Society
Surface Chemistry of DMMP on Rh( 100)
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2887
I
Rh( 100)' and established that it binds to the surface via the P atom. DMMP, an organophosphorus compound, is a liquid at room temperature with a vapor pressure given by the equation p(torr) = 2.844 X IO8 exp[-l1500/RT]? At 298 K, this gives 1.06 torr in agreement with the 1.0 0.1 torr we measured for our sample. It has the following structura! configuration
200K
I
DMMPIRh(100)
+
CH3O ,3>,=0
CH3O
in which the phosphorus is in a tetrahedral en~ironment.~The possibility of both phosphorus and oxygen coordination to transition-metal surfaces must be considered. The object of the present work is to report the results of DMMP adsorption on Rh( 100) as studied by TPD, AES, XPS, and A+ measurements. There is no known literature on the adsorption of DMMP on well-characterized transition-metal surfaces. Rh was chosen as reasonably representative of group 8-10 transition metals and because of our previous experience with it.5,6 Carbon-covered Rh was studied as it is reasonably representative of much less active surfaces.
Experimental Section The experiments were camed out in two separate UHV systems with different Rh( 100) crystals, which have been described elsewhere.',' The base pressure in both systems was in the low torr range. One system was equipped with a single-pass cylindrical mirror analyzer and an electron gun for AES, a standard four-grid LEED optics, and a line-of-sight mass spectrometer which was computer interfaced to multiplex up to nine peaks in TPD. TPD was done with a heating rate of -3 K/s. In the other UHV system, XPS data were taken with A1 Ka radiation (1486.6 eV) at 480-W power, and binding energies were calibrated against the Rh(3d,,2) peak at 307.3 eV. The doublepass CMA was operated at 25-eV pass energy. The data were accumulated on a multichannel analyzer with multiple scans. UPS data were taken with both He I (21.2 eV) and He I1 (40.8 eV) radiation. Work function measurements of clean and adsorbate-covered Rh(100) surfaces were made from He I spectra by observing the low kinetic energy cutoffs of the secondary electron energy distribution curves. He I1 spectra were used to probe the valence levels of adsorbed DMMP. Each Rh( 100) crystal was mounted on a liquid-nitrogen-cooled sample holder. Controlled temperatures between 100 and 1400 K were reached by resistive heating. A chromel-alumel thermocouple was spot-welded to the back of the crystal for temperature measurements. Cleaning before each run was accomplished by heating at 1300 K in 5 X lo-' torr of O2 for 15 min followed by annealing for 5 min at 1400 K. This was sufficient to produce a sharp (1 X 1) LEED pattern and, by AES, no detectable surface impurities. Commercial spectroscopic grade DMMP was further purified by using four to five freeze-pumpthaw cycles. It was then dosed into the UHV chamber by a multichannel array doser. Results and Discussion Temperature-Programmed Desorption from Rh( 100). TPD spectra for varying coverages of DMMP (parent ion at 124 amu) adsorbed on Rh(100) at 100 K are shown in Figure 1. At low coverages, DMMP exhibits a single desorption state at 225 K. With increasing exposure, the peak grows in intensity and shifts to lower temperatures (210 K). When the peak has shifted to 210 K, this state is saturated. Further exposures yield another (3) J. W. Barlow, private communication. (4) D. A. Bafus, E.J. Gallegos, and R. W. Kisher, J . Phys. Chem., 70, 2614 _ ...(1966). , - - - - I
Y.Kim, H. C. Peebles, and J. M. White, Sur$ Sci., 114, 313 (1982). (6) P. Ho and J. M.White, Surf.Sci., 137, 117 (1984). (7) D. E. Peebles, H. C. Pcebles, and J. M. White, Surf.Sci., 136, 463 (5)
(1984).
100 150
200
225
250
Temperature (K) TPD spectra of molecular DMMP from Rh(100) with varying DMMP exposure times. The exposure times are 10, 25, 50, and 75 s. Figure 1.
10
20
3J
41
50
60
70
OMMP Exposure Tme (Seconds)
Figure 2. Area under the molecular DMMP desorption peaks as a function of DMMP exposure time.
peak near 200 K which is due to multilayer formation and which could not be saturated. Figure 2 shows the total area under the molecular DMMP desorption spectra. The change in slope at the low coverage end indicates that partial decomposition of DMMP occurs on a clean Rh(100) surface at 100 K. The high exposure time portion of the curve corresponds to desorption of DMMP from the adsorbed multilayers and represents a measure of the rate of deposition of DMMP (constant sticking probability). A line drawn parallel to the high coverage portion of the curve and passing through the origin indicates that at low doses 60 f 5% of the adsorbed DMMP decomposes. The state between 210 and 225 K is attributed to desorption from the first layer of DMMP since it appears first and saturates with increasing exposure. The coverage-dependent peak shift to lower temperatures for this state is indicative of repulsive inter-
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Hegde et al.
Rh
P
w
-T? Y
Ii 100
300
550
Klnetic Energy ( e V )
Figure 3. AES for DMMP decomposition on Rh(100): (A) clean Rh(100) and (B) after DMMP exposure and heating to 500 K. actions in the DMMP adlayer. Using a simple first-order Redhead analysis,8 we estimated the heat of desorption of DMMP from the monolayer state to be 13.8 f 1.0 kcal/mol at low coverages. It is possible to separate empirically various contributions to the heats of adsorption (AHads)by making measurements on a series of molecules, for example, alcohols, ethers, and alkane^.^ For alcohols and ethers the contribution to the heat of adsorption from an oxygen lone pair orbital is approximately 10.0 kcal/mol, with the remaining contribution coming from van der Waals bonding of the hydrocarbon chains to the metal surface. If a similar contribution is assumed from the oxygen lone pair, interaction in DMMP leaves -3.8 kcal/mol for bonding of other parts of the molecule to the surface. That the heat of adsorption is dominated by a single lone pair (from the -P==O group) is consistent not only with this literature but with work function change measurements as well (see below). The heat of sublimation (AHsub)of the multilayer can be estimated from the leading edge of the DMMP multilayer desorption. A plot of the desorption rate vs. 1/T for the leading edge of the TPD curve was linear and gave a zero-order desorption energy of 8.6 f 1.0 kcal/mol. Data for sublimation of DMMP are not available. A value of 8.8 f 1.0 kcal/mol for the latent heat of vaporization of (CH,O),P, a geometrical isomer of DMMP, has been reported from the liquid state.I0 The heat of sublimation from the molecular solid is 5.2 kcal/mol smaller than the heat of desorption for the DMMP monolayer. This indicates that there is a specific chemical interaction (oxygen lone pair bond) of DMMP with the Rh surface. Decomposition products are observed in thermal desorption for every exposure. However, these signals saturate after an exposure of 25-30 s. This is consistent with the break seen in Figure 2. In desorption, no phosphorus-containing molecules are observed. Only low molecular weight species are detected, including hydrogen, water, methane, methanol, carbon monoxide, and carbon dioxide. Only CO and C 0 2 are observed in desorption above room temperature. The other molecules all desorb at lower temperatures. Hydrogen is found at 205 and 230 K, water between 184 and 200 K, methane at 220 K, methanol at 200 K, carbon monoxide at 210 and 475 K, and carbon dioxide at 155 and 380 K. In addition, adsorbed P, C, and 0 were detected by AES after the thermal desorption as shown in Figure 3. Work is in progress to characterize the relative amounts of these products and the kinetics of these reaction processes. (8) P. Redhead, Vacuum, 12, 203 (1962). (9) B. A. Sexton and A. E. Hughes, Surf: Sci., 140, 227 (1984). (10) J. D. Cox and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds", Academic Press, New York, 1970, p 480.
100
225
200
250
Temperature (K)
Figure 4. TPD spectra of molecular DMMP from carbon-covered Rh(100) with varying DMMP exposures. The exposure times were 10,25, 50, 75, 100, and 150 s.
50
100
150
200
DMMP Exposure Time (Seconds)
Figure 5. Area under the TPD peaks of DMMP from carbon-covered Rh(100) as a function of DMMP exposure time.
-
TPD from Carbon-Covered Rh( 100). The carbon-covered Rh( 100) surface was generated by exposure to C2H4(pCZHa 5 X lo-' torr for 30 min) at 550 K and then flashing the surface to 800 K. This procedure gives approximately a monolayer of carbon. The molecular DMMP desorption spectra from the carbon-covered Rh surface show two peaks just as the carbon-free surface. Figure 4 shows the desorption spectra observed with increasing DMMP exposure. For low exposures, a single desorption peak is observed a t 21 5 K while a second peak appears at low temperature (-200 K) with increasing coverage. The low-temperature peak shifts to higher values with increasing coverage, but the leading edges of the desorption peaks coincide. These are characteristic of zero-order desorption kinetics normally observed during the desorption of condensed multilayers. The high-temperature peak (21 5 K), which corresponds to molecular DMMP in the first monolayer, saturates first, while the condensed multilayer continues to grow and does not saturate. The monolayer peak is 10 K lower on the carbon-covered Rh surface than on the carbon-free Rh( 100) surface. The monolayer desorption energy is 12.9 kcal/mol based on the location of peak maximum (215 K) and assuming that the rate of desorption is
-
Surface Chemistry of DMMP on Rh( 100)
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2889 A
B
DMMP Monolayer
~
140
535
540
~~
135
530
130
Binding Energy (eV)
Figure 6. P(2p) peaks for multilayer (A) and monolayer (B) DMMP adsorbed on Rh(100).
first order with a preexponential factor of lOI3 s-I. Thus, the desorption energy of DMMP is reduced, but only slightly, by the presence of carbon on Rh(100). The area under the TPD spectra grows linearly (Figure 5) with DMMP exposure from zero coverage to multilayer formation, indicating a constant sticking probability over the entire range at 100 K. On the basis of this result and on the lack of any observable decomposition products by TPD and AES, which will be discussed below, we conclude that the adsorbed DMMP does not decompose measurably on a carbon-covered Rh( 100) surface. TPD experiments were performed with detection at various &asses (2, 16, 18, 28, 32, and 44 amu) in order to determine whether any decomposition of the adsorbed DMMP occurs in the carbon-covered Rh(100). All the TPD peaks tracked molecular DMMP desorption, and the relative intensities were identical with those obtained when gasphase DMMP was measured directly. Furthermore, no significant P or 0 residues were left on the C/Rh( 100) surface as monitored by AES. Thus, in contrast to clean Rh( loo), the carbon-covered Rh surface blocks the formation of strong dissociative chemisorption bends with the surface atoms. X-ray Photoelectron Spectroscopy. XPS spectra of the P(2p), O( Is), and C( 1s) peaks of multilayer DMMP are shown in Figures 6A-8A. Each of these spectra was smoothed by using a 15-point quadratic fit. Two types of oxygen and carbon atoms appear. The XPS spectra of a monolayer, prepared by annealing a multilayer to 190 K and cooling to 100 K, are shown in Figures 6B-8B. In each case, the overlapping peaks for O( 1s) and C( 1s) were separated and their relative peak areas measured (Table I). The multilayer shows a single P(2p) peak at 135.5 f 0.1 eV, a doublet O(1s) at 533.0 f 0.1 and 534.6 f 0.1 eV, and a doublet
Binding Energy ( e V )
Figure 7. O(1s) peaks for multilayer (A) and monolayer (B) DMMP adsorbed on Rh(100). TABLE I: XPS Binding Energies (BE) for DMMP/Rh(100) BE, eV peak
DMMP WP) O(W OUS)
C(W (31s)
multilayer
monolayer
areaa
135.5 f 0.1 533.0 f 0.1 534.6 f 0.1 286.3 f 0.1 288.0 f 0.1
134.5 f 0.1 532.0 f 0.1 533.6 f 0.1 286.3 f 0.1 287.9 f 0.1
1 .oo 1.85 1 .oo 1.81
ONormalized to the weaker C(1s) and O(1s) peaks. C(1s) at 286.3 f 0.1 and 288.0 f 0.1 eV (see Table I). XPS spectra of the monolayer show the P(2p) peak at 134.5 f 0.1 eV and the doublet O(1s) peaks at 532.0 f 0.1 and 533.6 & 0.1 eV. The C(1s) spectrum is not well resolved and cannot be readily decomposed into two peaks. Thus, these spectra show a uniform shift of approximately 1.O eV for the P(2p) and O(1s) core levels between the two phases. This shift is attributed to decreased extra-atomic relaxation in the condensed DMMP. It is not due to electrostatic charging, since the Rh(3d) peaks did not shift. Although the C(1s) cannot be resolved, it is clear that there is not a 1-eV shift of C(ls) intensity on the low binding energy side (Figure 8B) as there is for O(1s) and P(2p). The P(2p) spectrum (Figure 6B) shows a shoulder on the low binding energy side which we take as evidence that some dissociation occurs in the first monolayer. The binding energies of P(2p) and O(1s) are in excellent agreement with the binding energy values reported for (C6H5O ) 3 P 0 which is analogous to DMMP." The higher binding (1 1) W. E. Morgan, W. J. Stec, R. G. Albridge, and J. R. V. Wazer, Inorg. Chem., 10, 926 (1971).
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Hegde et al.
’he Journal of Physical Chemistry, Vol. 89, No. 13, I985
~-
100
200
300
400
T e m p e r a t u r e (K)
B
Figure 9. Work function change (A@) measurements of DMMP/Rh(100) as a function of annealing temperature.
intermediates from DMMP on Rh( 100) while annealing to 300 DMMP Monolayer
ca
K.
energy, more intense peak (cb) of the C( 1s) doublet is assigned to carbon atoms in the methoxy (-OCH3) groups, while the less intense, lower binding energy peak (C,) is assigned to the carbon in the methyl (-CH,) group. These assignments are made on the basis of simple considerations of electronegativity where the carbon atoms bound to oxygen are expected to yield electrons at higher binding energy. From the stoichiometry of DMMP, we would expect that Cb would have twice the area of C,. The O(1s) doublet can be assigned in a similar fashion. For the monolayer (Figure 8B), there is an apparent reduction in the relative intensity of cb, while the ob/o, intensity ratio is about the same as for the multilayer. Moreover, there is no significant change in the P(2p) and O(1s) peak positions between 200 and 300 K (not shown). The C(1s) peak did show a binding energy shift (-0.8-1 .O eV) between 200 and 300 K. Between 200 and 300 K, CH4 and CH,OH desorb; this is probably accompanied by cleavage of phosphorus-oxygen and carbon-oxygen bonds of the -P-O-CH, group. Although no detailed binding energy standards are available, the following comments can be made. The observed O( 1s) binding energy (532.0 eV) indicates that atomic oxygen is not formed, since dissociatively adsorbed oxygen on Rh(100) has an O( 1s) binding energy of 529.8 f 0.2 eV.I2 Our recent XPS meas u r e m e n t ~on ’ ~ interaction of PH3 on Rh(100) show that atomically adsorbed phosphorus has a P(2p) binding energy of 129.0 f 0.1 eV. The P(2p) peak appears at 134.3 eV after heating DMMP/Rh(100) to 300 K. These observations support the formation of small amounts of CH,PO-Rh and/or CH3P03-Rh
From the decrease of C( Is), P(2p), and O( 1s) peak areas, the DMMP dissociation probability is estimated to be 70 f 5% after annealing to 300 K. This is in good agreement with the TPD extrapolation shown in Figure 2. DMMP is a tetrahedral molecule. Its dimensions, computed by using gas-phase bond lengths, are approximately 6.7 and 3.4 8, for the maximum and minimum diameters perpendicular and parallel, respectively, to the symmetry axis (-P=O axis). From density and molecular weight data, the van der Waals diameter of the DMMP molecule is approximately 3.25 8,. The nearestneighbor distance in Rh is 2.69 A. If one uses the maximum and minimum diameters (6.7 and 3.25 A), the first layer coverage would liebetween 0.16 and 0.68 monolayer (ML) (1 ML is defined as one adsorbed species per surface Rh atom). These estimates are useful as guides for qualitatively analyzing the desorption spectra and work function change measurements. Work Function (A%) Measurement. A% measurements were made for DMMP adsorbed on Rh( 100) as a function of annealing temperature. The work function change measurements were made by following the secondary electron energy cutoff in the He I spectra after annealing the adsorbed layer to specific temperatures (Figure 9). With an adsorbed multilayer at 100 K, a A% value of -2.5 eV was observed. Removal of the multilayer raised A@ by 0.25 to -2.25 eV. Such a large A@value at monolayer coverage of DMMP/Rh(100) is not surprising since the free DMMP molecule has a rather high dipole moment (between 3.62 and 2.48 D depending on the structure used).’, The negative sign of A@ suggests that the adsorbed species is oriented with the positive end (-CH3 group) of the molecule away from the surface and the end toward the surface. The extended structure negative ,(-p=O) of DMMP has a resultant dipole moment of 2.48 D directed along the -P=O axis.” If one assumes this form and a coverage of 0.16 ML, a A@ of 2.10 eV is predicted on the basis of the simple expression A% = p n / e o where A% is the work function change in electronvolts, 1.1 is the dipole per adsorbed species, eo = 2.66 X 1014D/(eV cm2),14and n is the saturation coverage of DMMP. This is in surprisingly good agreement with the measured value of -2.5 eV. However, the large uncertainty in the dipole moment makes this estimate semiquantitative.
(12) G. B. Fisher and S. J. Schmieg, J. VUC.Sci. Techno/.,A, 1, 1064 (1983).
(1 3) G. M. Kosolapoff, J. Chem. Soc., 3222 (1 954). (14) G. Ertl and J. Kuppers, “Low Energy Electrons and Surface Chemistry”, Verlag Chemie, Weinheim, 1974, p 123.
290
285
280
Binding E n e r g y (eV)
Figure 8. C(1s) peaks for multilayer (A) and monolayer (B) DMMP adsorbed on Rh(100).
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2891
Surface Chemistry of DMMP on Rh( 100)
S h ii f i i i i A
16
12
8
4
0
Binding Energy (eV)
Figure 10. UPS He I1 difference spectra for multilayer DMMP on Rh(100) at 100 K. The gas-phase spectrum taken at 21.2 eV is shown as a bar graph above the difference spectrum, and the highest lying bands have been aligned as described in the text.
As the monolayer desorbs, A@ rises to about -0.5 eV. Because of the presence of electronegative dissociation products such as P and 0 (see Figure 3), A@ remains negative after annealing to 300 K. The following diagrams show four different ways in which molecular DMMP might be bonded to the Rh surface.
Rh
I
Rh
II
Rh
111
Rh
IV
We consider I and I1 to be unlikely because the phosphorus is fully coordinated in DMMP and has no lone pair electrons. We also consider I11 unlikely because of steric considerations which make it difficult to coordinate three oxygens simultaneously to the Rh surface and because PO and the heat of adsorption do not support such a model. We favor bonding geometry IV because it is consistent with all the TPD, XPS, and A@ results and is chemically sensible. UPS Measurements. The He I1 difference spectra for multilayer DMMP adsorbed on Rh(100) are shown in Figure 10. The difference spectra were obtained by subtracting a normalized spectrum of the clean Rh(100) surface from that of the DMMP multilayer and were subsequently smoothed. Four groups of adsorbate-induced bands labeled A, B, C, and D are distinguished.
The observed gas-phase spectrum of DMMP15 shows several bands in the range 10-20 eV (see the bar graph in Figure 10). The first five gas-phase bands can be assigned with fair confidence to oxygen lone pair orbitals. The band at 10.05 eV is due to ionization of an oxygen lone pair orbital on the P=O oxygen. Those bands at 10.55, 11.20, 11.85, and 13.20 eV are connected with oxygen lone pair orbitals on the methoxy groups that combine in a complicated manner.I6 There are at least four more bands at higher ionization potential, namely at 13.7, 14.9, 17.0, and 18.8 eV. The band at 13.7 eV is p s i b l y due to a(P-C) and/or u(P-0) bonding 0rbita1s.l~ The band centered at 14.91 is broad and can be assigned to a(C-H) 0rbita1s.l~ The remaining bands are probably due to core like C(2s) and P(3s) orbitals.'* Alignment of the least tightly bound valence band of the DMMP gas-phase spectrum (Figure 10) with the corresponding band in the adsorbed DMMP results gives quite satisfactory alignment of the other bands, as expected. Further analysis of the multilayer spectrum is not warranted. We were unable to analyze the spectra for the monolayer; the spectra were complex and suggest that some dissociation occurs in the first monolayer just as in the P(2p) XPS spectra (Figure 6B). However, our bonding model would suggest that the P-0 lone pair would be shifted relative t? the deeper lying peaks.
Summary Both multilayer and monolayer states of DMMP were identified from TPD, A@, and XPS measurements. The monolayer gave a distinct thermal desorption peak, a large work function change of -2.25 eV, O(1s) binding energies of 532.0 and 533.6 eV, and a P(2p) binding energy of 134.5 eV. The C(1s) spectrum was broad and difficult to analyze. The multilayer has a distinct thermal desorption peak at a lower temperature than the monolayer. In addition, the A@ value was slightly more negative than the monolayer (-2.50 eV) and the XPS O(1s) and P(2p) binding energies were higher by 1.O eV due to less extra-atomic relaxation. Adsorption of DMMP on Rh( 100) in the monolayer state is characterized by the heat of adsorption of 13.8 kcal/mol. This value is consistent with bonding of DMMP through the -P=O (lone pair oxygen orbital). The A@ results also support this model. On clean Rh( 100) a fraction of the adsorbed DMMP desorbs molecularly, while the remainder decomposes,. The decomposition products include H2, H 2 0 , CO, COz, CHI, and C H 3 0 H . Adsorbed P, C, and 0 were detected by AES after the desorption experiments. We estimate, using TPD and XPS, that 60-70% of the first layer decomposes. The molecular DMMP desorption spectra from the carboncovered Rh(100) surface show two peaks (monolayer and multilayer) just as for the carbon-free surface. The area under the TPD spectra grows linearly with DMMP exposure from zero coverage to multilayer formation, indicating a constant sticking probability over the entire range at 100 K for the carbon covered Rh(100). On the basis of this result and on the lack of any observable decomposition products by TPD and AES, we concluded that the adsorbed DMMP does not decompose measurably on the C/Rh( 100) surface.
-
Acknowledgment. We are grateful to S . D. Worley for supplying the gas-phase UPS of DMMP. This work was supported in part by the U S . Army Research Office. Registry No. Rh, 7440-16-6;DMMP, 756-79-6. (15) S. D.Worley, private communication. (16)S. D. Worley, J. H. Hargis, L. Chang, and G. A. Mattson, J . Electron Spectrosc. Relar. Phenom., 25, 135 (1982). (17) H. Bock, Pure Appl. Chem., 44,343 (1975). (18) J. W. Rogers, Jr., and J. M. White, J . Vac. Sci. Technol., 16,485 (1979). (19) C. M.Greenlicf, R. I. Hegde, and J. M. White, in preparation.
