Electronic Structures and Excitation Mechanism - American Chemical

programmed desorption (TPD) and two-photon photoemission spectroscopy ... local maxima located at 195, 135, 121, and 115 K. From the 2PPE spectra, thr...
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J. Phys. Chem. B 2006, 110, 5547-5552

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Two-Photon Photoemission Spectroscopy Study of 1,3-Butadiene on Cu(111): Electronic Structures and Excitation Mechanism Weixin Huang,† Wei Wei,‡ Wei Zhao,‡ and J. M. White*,‡ Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, 230026, China, and Department of Chemistry and Biochemistry, Center for Materials Chemistry, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: July 28, 2005; In Final Form: January 3, 2006

The characteristics of 1,3-butadiene (C4H6) adsorbed on Cu(111) were investigated with temperatureprogrammed desorption (TPD) and two-photon photoemission spectroscopy (2PPE). Dosed at 90 K, the work function drops by 0.4 eV and TPD provides no evidence for dissociation, but there are four coverage-dependent local maxima located at 195, 135, 121, and 115 K. From the 2PPE spectra, three unoccupied electronic states of the C4H6-Cu(111) system were identified: the LUMO (π1*, 2au), LUMO + 1 (π2*, 2bg), and LUMO + 2 (σ*, 7bu), lying 1.3, 3.4, and 4.8 eV above the Fermi level, respectively. Although the excitation mechanisms for the LUMO and LUMO + 1 are substrate mediated, the excitation of the LUMO + 2 is attributed to intramolecular excitation.

1. Introduction Mapping the electronic structures of an adsorbed molecule, both its occupied and unoccupied molecular orbitals, is fundamentally important. The molecular orbital approach provides a powerful means for describing molecular structures, electronic transitions, and chemical reactivities. Frontier orbital theories of chemical reactivity, for example, employ information about the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of the reactants to predict reaction pathways. Recently, the interfacial electronic structures of species adsorbed on metal and semiconductor substrates has received more attention, motivated, in part, by applications involving excited state interfacial charge transfer, e.g., organic light-emitting diodes (OLED), field effect transistors (FET), and substrate-mediated surface photochemistry. Understanding photosynthesis and vision also depend, in part, on identifying the relevant interfacial charge transfer processes. Interfacial electronic structure is also important for heterogeneous catalysis and, in part, motivates the present work studying a relatively weakly adsorbed molecule with π-bonds on a metal substrate. Significant insight into the electronic structures of molecules comes from comparing measured and calculated densities of states. While experimental densities of occupied molecular states can be conveniently obtained using ultraviolet photoemission spectroscopy (PES), analogous assessments of unoccupied states are relatively complicated. Optical spectroscopy, multiphoton ionization spectra (MPI), electron transmission spectroscopy (ETS), electron energy loss spectroscopy (EELS), and inverse photoemission spectroscopy (IPES) have been employed to characterize the unoccupied states of gas and condensed phase molecules.1 Because it can access both the occupied and unoccupied states of an adsorbate, the recently developed * Corresponding author. Fax: + 1-512-4719495. [email protected]. † University of Science and Technology of China. ‡ The University of Texas at Austin.

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technique of two-photon photoemission spectroscopy (2PPE) is especially powerful.2 Here, it was used to probe the occupied and unoccupied orbitals of a model system, namely, 1,3butadiene (C4H6) adsorbed on Cu(111). The adsorbate, 1,3-butadiene, is important in heterogeneous catalysis and mimics some properties of long-chain polyenes, which are of special interest due to their importance in the chemistry of vision. For example, butadiene adsorption on Pd and Pt is a crucial step for understanding the selective hydrogenation of diolefins.3,4 Mapping its electronic structures in adsorbed form will help explain its reactivity on solid surfaces. The electronic structures of isolated C4H8 have been investigated using both experimental and theoretical methods. Although there is good agreement between experiment and ab initio theory for the two lowest triplet electronic states and a number of the low-lying Rydberg states, the assignment of the lowest excited singlet state is still a matter of debate.5-13 The states of the cations14,15 and anions16-18 of butadiene have also been studied, assuming the approximate validity of Koopman’s theorem (KT),19 which associates the experimentally determined ionization potentials (IP) and electron affinities (EA) with the negative energies of the filled and unfilled orbitals, respectively. The energies (with respect to the vacuum level) of the occupied states of gas-phase butadienes9.03 eV (π2, 1 bg), 11.46 eV (π1, 1 au), and 12.23 eV (σ7, 7 ag)shave been measured using PES.13 Adding an electron forms the radical anion, and its absorption spectrum in a glassy medium at low temperature showed two intense peaks at 2.2 and 3.2 eV, which were assigned to the 12Au f 12Bg and 12Au f 22Bg transitions, respectively.16 The 2.2 eV transition was attributed to a 2au(π*) f 2bg(π*) orbital excitation and the 3.2 eV transition to a 1bg(π) f 2au(π*) excitation. ETS measurements of gas-phase butadiene located the 12Au and 12Bg anion states at 0.62 and 2.8 eV, respectively; an unassigned broad, weak feature also appeared at 5 eV.17 A detailed EELS study of gas-phase butadiene identified π* anion states near 0.9 and 2.8 eV and π-1(π*)2 anion states near 5.0, 6.7, 8.1, 11.0, and 12.5 eV. Recently, theoretical studies20,21 of adsorbed butadiene have appeared. For example, a DFT

10.1021/jp0541922 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006

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calculation20 has shown that for weakly adsorbed 1,3-butadiene on Pd(111), the lowest unoccupied state is located 1.8 eV above the Fermi level. In the present paper, we report TPD and 2PPE studies of the adsorption behavior and electronic structure of butadiene on Cu(111). From the 2PPE results, we successfully identified, with respect to the Fermi level (EF) of Cu(111), the three lowest lying LUMOs of adsorbed butadiene. 2. Experimental Section Experiments were performed in a two-level stainless steel ultrahigh vacuum chamber with a base pressure of 3 × 10-10 torr.22 The upper level comprised Auger electron spectroscopy (AES) for surface analysis, time-of-flight mass spectrometry (TOF-MS) for TPD and residual gas analysis, and an ion sputtering gun for surface cleaning. The lower level housed 2PPE instrumentation, including a homemade electron timeof-flight spectrometer (e-TOF) and optics for directing laser radiation (see below) onto the sample. A tunable Spectra-Physics MOPO/FDO-900 laser system pumped by a Quanta-Ray 170-10 Nd:YAG laser was used for 2PPE. The laser pulse (3 mJ/cm2 and 7 ns full width at halfmaximum) was tunable between 274 and 344 nm. A polarizer and a half-wave plate were inserted into the light path to eliminate s-polarized light. The laser beam was incident at 45° with respect to the surface normal and irradiated a 0.5 mm diameter spot near the center of the surface. The 2PPE spectrum was normalized to the relative laser intensity, the latter determined by reflection off the chamber window, measured with a photodiode. The time-dependent output signal (I(t) vs t) was recorded by a Tektronix TDS 724D digital phosphor oscilloscope and transferred to a computer for further analysis. Photoelectron kinetic energies were determined using the e-TOF. The electron flight distance (d) was calibrated on the basis of the clean Cu(111) sample, and the flight time (t) was measured with respect to the arrival time (2.2 × 10-7 s) of reflected laser light. When gathering 2PPE spectra, a +0.5 V bias was applied between the TOF spectrometer entrance aperture and the sample to enhance the intensity of the signals. Once calibrated, electron kinetic energies (EK (eV)) were calculated using EK + 0.5 ) 1/ m (d/t)2. The 2PPE spectra in this paper were plotted as 2PPE 2 e intensity versus final state energy above the Fermi level (EK + 4.9). The Cu(111) sample was mounted on a loop of tungsten wire that was connected to a standard sample manipulator. The sample could be cooled to 86 K by contact with a liquid nitrogen reservoir and resistively heated to 850 K. A commercial temperature controller was used to control the sample temperature, measured with a type K thermocouple inserted into a radial hole in the edge of the crystal. Surface contamination was removed by sputtering and annealing cycles until no impurities were evident in AES. Butadiene was dosed through a preset leak valve ending in a stainless steel tube positioned within 10 mm of the sample surface. To dose, a fixed pressure of butadiene was added to a vessel behind a butterfly valve (closed) that was connected by an evacuated tube leading to the preset leak valve. The butterfly valve was opened to initiate the dose. The dose was terminated, not by closing the leak valve, but by evacuating the gas behind the leak valve with a turbomolecular pump. Since the C4H8 TPD spectra are complex, and the saturation first layer coverage is not well-defined, coverages are reported in standard dose (SD) units. The latter is arbitrarily defined in terms of the integrated TPD spectrum following a saturation dose with the substrate

Figure 1. TPD spectra of butadiene from Cu(111) following various butadiene exposures at 90 K. The ramping rate is 2 K/s.

held at 170 K, i.e., significantly above the bulk multilayer desorption temperature (115 K). 3. Results 3.1. TPD. For doses with the substrate held at selected temperatures between 90 and 170 K, TPD indicated no dissociative adsorption; only fragments belonging to butadiene were detected, and AES found no carbon accumulation after repeated cycles of dosing and TPD to 500 K. Figure 1 presents the TPD spectra following butadiene dosing at 90 K. At the lowest dose, 0.31 SD, there is a single desorption peak at 219 K that is attributed to C4H6 in contact with Cu(111). With higher doses, this peak shifts downward to 195 K, and three additional features evolve consecutively at 138, 121, and 109 K. The feature at 109 K does not saturate and shifts upward to 115 K as coverage increases, implying that it comes from the desorption of multilayer. Keeping in mind that the standard dose (SD) scale is defined in terms of the dose required to saturate the surface at 170 K, Figure 1 indicates that the adsorbate organization of 0.96 SD at 90 K differs from that at 170 K. The TPD spectrum shows significant population of the 138 K peak and less population at 195 K. Clearly different structures (organizations) are formed from the same number of C4H6 molecules dosed at 90 and 170 K. These structures may respond distinctively to two-photon probes. The TPD data suggest that during dosing at 90 K, 3-D islands of C4H6 form before the first layer is completely saturated. The TPD results indicate weak interactions between butadiene and Cu(111). Assuming a pseudo first-order preexponential factor of 1013 s-1, Redhead analysis23 yields a desorption activation energy for monolayer (195 K peak) butadiene of 57 kJ mol-1. The monolayer peak shifts downward with increasing coverage, suggesting repulsive interactions between adsorbed butadiene molecules. During the course of 2PPE experiments, we prepared Cu(111) surfaces with various butadiene coverages by saturated dosing at 170, 125, and 112 K, and a dose of 1500 s at 90 K (Figure 2). These doses give C4H6 coverages of 1, 2, 3, and 8.6 SD, respectively.

Spectroscopy of 1,3 Butadiene on Cu(111)

Figure 2. TPD spectra of butadiene from Cu(111) dosed at various substrate temperatures. The ramping rate is 2 K/s.

3.2. Work Function Changes. The change in surface work function as a function of butadiene coverage on Cu(111) at 90 K was determined using the 2PPE spectra. The left panel of Figure 3 displays a series of 2PPE spectra of Cu(111) covered by different amounts of butadiene. A sharp secondary electron edge, the 3d orbital peak (2 eV below EF), and the surface state peak (0.4 eV below EF) appear in the spectrum of the clean surface, consistent with previous reports.2 Adding 1 SD butadiene results in the apparent attenuation and downward shift of the secondary electron peak, indicating a lower surface work function. The intensity attenuation and downward shift of the secondary electron peak are clearly visible in an expanded figure showing the low kinetic energy part of the left panel of Figure 3 (included in the Supporting Information). Assuming 4.9 eV for the work function of clean Cu(111), the work functions of butadiene-covered Cu(111) were obtained from the onset of the secondary electron peak in the 2PPE spectra. Three sets of data excited by photons with different energies have been analyzed, and the agreement is satisfactory (right panel of Figure 3). Adsorption of 1 SD butadiene reduces the work function from

J. Phys. Chem. B, Vol. 110, No. 11, 2006 5549 4.9 eV to 4.5 ( 0.02 eV. Larger doses reduce the work function further, but by no more than 0.03 eV. The work function reduction and its magnitude are consistent with data reported for C4H6 on Pd(110),24 implying a weak interaction between butadiene and Cu(111), i.e., the formation of π-bonded C4H6. The lower surface work function is expected and implies that the electron density of adsorbed C4H6 is polarized toward the Cu(111) surface. On Pt(111), the work function reduction is much larger, 1.7 eV, and the bonding is described as di-σ, i.e., there are σ-bonds between the C atoms of butadiene and the surface Pt atoms.24 3.3. 2PPE and Assignments. As shown in the 2PPE spectra (left panel of Figure 3), after the addition of butadiene to Cu(111), the surface state peak (SS) is significantly attenuated. Besides the 3d peak, there are several new features. As shown in the Supporting Information (expanding of Figure 3), the secondary electron peak greatly attenuates and shifts downward upon butadiene adsorption; thus, it is unlikely that this peak contributes to the observed new features, and we attributes these features to adsorbed butadiene. Whether photoelectrons originate from occupied or unoccupied states in the 2PPE spectra can be determined from the dependence of the electron kinetic energy on the photon energy.2 Photoelectrons originating from an occupied initial state have kinetic energies that vary as twice the change in photon energy (i.e., 2 ∆Ehν), whereas those arising from a transiently populated but initially unoccupied orbital have kinetic energies that vary as 1 ∆Ehν. If the detected photoelectrons are formed by excitation to an unoccupied final state above the vacuum level, the kinetic energy will not vary with photon energy. Figure 4 presents the 2PPE spectra of 1SD C4H6 on Cu(111), probed with various photon energies, and the photon energy dependence of the observed peaks. 2PPE spectra with increasing butadiene coverages appear in Figure 5. In general, the Cu(111) 3d band and three new features (peaks A, B, and C) are well resolved in these spectra. In addition, another peak, D, with the highest final state energy is clearly observed for 1SD C4H6/ Cu(111) when the photon energy exceeds 4.07 eV. The observed peaks show different photon energy dependences (right panel of Figure 4). As expected, the 3d peak varies with 2 ∆Ehν. Three other peaks (B, C, and D) exhibit a 1 ∆Ehν dependence, identifying involvement of unoccupied orbitals of the initial state. The remaining peak A position does not vary with photon energy, which indicates that it involves an unoc-

Figure 3. Left panel: 2PPE spectra of adsorbed butadiene on Cu(111) with various coverages, excited by photons with 3.94 eV energy. The spectra were recorded at 86 K. Right panel: work function change of Cu(111) upon butadiene dosing, determined from the secondary electron peak shift in the 2PPE spectrum. Three sets of data excited by photons with different energies were analyzed.

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Figure 4. Left panel: 2PPE spectra of 1 SD butadiene on Cu(111) excited by photons with different energies. The spectra were recorded at 86 K. Right panel: the wavelength dependence of the observed 2PPE peak positions.

Figure 5. 2PPE spectra of 2 SD (left panel), 3 SD (middle panel), and 8.6 SD (right panel) butadiene on Cu(111) excited by photons with different energies. The spectra were recorded at 86 K.

cupied orbital of the system that is populated in the final state and lies above the vacuum level. The following equations were used to calculate orbital energies (E) with respect to the Fermi level of Cu(111): Occupied states: E ) EK - 2hν. Unoccupied intermediate states: E ) EK - hν. Unoccupied final states above the vacuum level: E ) EK. EK is the final state energy above EF. Within the error of 0.1 eV, the 3d band of Cu(111) lies 2 eV below EF; the three unoccupied intermediate states lie 1.3 (peak B), 3.4 (peak C), and 3.8 eV (peak D) above EF; and the unoccupied final state lies 4.8 eV above EF. The unoccupied intermediate state D, lying 3.8 eV above the Fermi level, becomes unresolvable as C4H6 coverage increases. Thus, this state probably does not involve transitions in butadiene. We assign it to an image state (n ) 1) of Cu(111). The analogous n ) 1 image state was observed in 2PPE at 4.1, 3.1, and between 3.3 and 3.45 eV for bare Cu(111), naphthalene/ Cu(111), and C6H6/Cu(111), respectively.25-26 Three other peaks were assigned, on the basis of their energies, to the three lowest unoccupied molecular orbitalss (LUMO, 1.3 eV) (π1*, 2au), (LUMO + 1, 3.4 eV) (π2*, 2bg), and (LUMO + 2, 4.78 eV) (σ*, 7bu)sof adsorbed butadiene, respectively. Because it comprises 99% of the dosed species derived from a 300 K source,27 trans-1,3-butadiene is assumed, and the corresponding symmetry designations are adopted. No term values have been reported for the unoccupied states of adsorbed butadiene. Near-edge X-ray absorption fine structure (NEXAFS) revealed π1*-π2* splittings of 2.7, 2.0, 2.4, and 2.4 eV for butadiene in the following four situations: condensed

phase and adsorbed at 300 K on Pd(111), Pd(110), and Pd50Cu50(111), respectively.24 The assignments made above agree, giving a π1*-π2* splitting of 2.1 eV. There is no literature concerning the 7bu unoccupied state (LUMO + 2). 4. Discussion Figure 6 summarizes the energies of all assignments made in the present study. The Fermi level of Cu(111) serves as the reference. The vacuum level is identified using the work function. The energies of the observed states of adsorbed butadiene vary only slightly with coverage, consistent with the results of TPD and work function change, implying weak interactions of butadiene with Cu(111). It is important to examine the differences in the initial, intermediate, and final states involved when discussing orbital energies and transition energies. 2PPE probes both occupied and unoccupied states. Occupied states are probed in a 2PPE process by coherent two-photon excitation, leaving behind an N - 1 electron final state. This is similar to the excitation mechanism occurring in one-photon photoemission, yielding the ionization potential (IP). On the other hand, when unoccupied states are involved, the absorption of the first photon in the 2PPE process may result in photoinduced metal-to-molecule electron transfer or intramolecular excitation, and the excited electron transiently populates an unoccupied state of the adsorbate molecule. The second photon excites the transient electron above the vacuum level. Photoinduced metal-to-molecule electron transfer results in a transient molecular anion possessing N + 1 electrons, and the final state of the 2PPE process is a neutral molecule. Thus, the energy associated with such unoccupied

Spectroscopy of 1,3 Butadiene on Cu(111)

Figure 6. Schematic energy level of butadiene on Cu(111) with various coverages. The Fermi level of Cu(111) serves as the reference.

states may approach those determined from IPES. Intramolecular excitation from occupied states to an unoccupied state differs in that a transient excited but neutral state is formed. This transiently populated unoccupied level may be stabilized by an exciton-like electron-hole attraction, and relaxation may result in light emission. Furthermore, removing the excited electron produces a nascent cation. Because initial and final states differ, as do associated relaxation and correlation effects, the energies of unoccupied states must account for the method used and are expected to differ. The LUMO + 2 peak of butadiene, detected as an unoccupied final state above the vacuum level in 2PPE, is excited by twophoton absorption. The 2PPE peak of LUMO + 2 grows dramatically with both the butadiene coverage and the photon energy. This indicates that the excitation of the LUMO + 2 of butadiene is dominated by intramolecular excitation. If metalto-molecule electron transfer were dominant, the features would at least not intensify with increasing butadiene coverage. The dramatic growth of LUMO + 2 with photon energy indicates that excitation of the LUMO + 2 approaches resonance absorption of two photons. The gap between the 3d orbital of Cu(111) and the LUMO + 2 of butadiene is only 6.73 eV, far less than the energy required for resonance absorption. There is no literature reporting the HOMO (1bg) position of butadiene on Cu(111). Previous PES results on Pd(111) and Pd50Cu50(111) placed the HOMO for butadiene ∼3.4 eV below the Fermi level.24 Assuming that this value holds for butadiene on Cu(111), the gap between the HOMO and the LUMO + 2 will be 8.2 eV. Thus, it is reasonable that the excitation mechanism of the LUMO + 2 in 2PPE involves the resonant absorption of two photons by an electron in the HOMO of butadiene on Cu(111). The excitation mechanism for the LUMO and LUMO + 1 peaks is different. Detected as the unoccupied intermediate states, excitation of the LUMO and LUMO + 1 involves only one-photon absorption. However, the energy gap between the HOMO and LUMO (4.7 eV) and the LUMO + 1 (7.8 eV) is beyond the photon energy we employed. Thus, photoinduced metal-to-molecule electron transfer contributes to the excitation of the electron into the LUMO and LUMO + 1. We have assigned the peak located 1.3 eV above the Fermi level to the LUMO of adsorbed butadiene. If we assume the

J. Phys. Chem. B, Vol. 110, No. 11, 2006 5551 electron affinity of isolated butadiene is -0.62 eV,17 the energy shift of the affinity level will be 3.8 eV. While this shift is unexpectedly large, a recent DFT calculation20 showed that for a similar weakly bonded system, 1,3-butadiene on Pd(111) at low temperature, the LUMO was found at 1.8 eV above the Fermi level. In general, upon adsorption, adsorbate molecular orbitals undergo a relaxation shift (Coulomb relaxation) toward lower binding energies with respect to the Fermi level. This is attributed mainly to interactions of an adsorbate with the substrate and neighboring molecules. For example, the gas-phase molecular orbital positions (HOMOs)24 of butadiene uniformly shifted by 6 eV when weakly adsorbed on Pd(110) at 95 K. Our TPD and work function change results show that the butadiene is also weakly adsorbed on Cu(111). Therefore, we propose that the interaction of the anion state with neighboring molecules likely contributes to the large energy shift of the LUMO of adsorbed butadiene on Cu(111). Since butadiene molecules adsorb with the π-bond parallel to the surface, the trans geometry we propose makes it possible that the double bonds in one molecule can align parallel with those of any nearest neighbor. In this configuration it is plausible that the wave functions of the π* orbitals of neighboring adsorbate molecules mix together to form the LUMO of adsorbed state. The parallel configuration of butadiene molecules on Pd(110) was proved by a STM study,28 and the DFT calculation noted above20 shows that a c(4 × 2) superstructure is significantly more stable than a p(2 × 2) of butadiene on Pd(111). Since the coverages for these two structures are the same (0.25 ML), we infer that the adsorbed butadienes in the c(4 × 2) likely have a parallel geometry that leads to a significant reduction of the repulsive lateral interactions. The LUMO + 2 σ* orbital lying 4.8 eV above the Fermi level of Cu(111) is reported for the first time. For gas-phase C4H6, both ETS and EELS revealed a feature at 5 eV (4.4 eV above the anion ground state 12Au),17,18 later assigned to the 22Bg anion state of butadiene.18 However, the absorption spectrum of the radical anion in a glassy medium at low temperature placed the 22Bg anion state 3.2 eV above the 12Au anion ground state.4 It has been argued that the energy of the 22Bg anion state differs by as much as 1.2 eV in the condensed and gas phases.18 On the basis of the 2PPE results, we reassign the aforementioned 5 eV feature to promotion of an electron into the 7bu unoccupied state (σ*) of butadiene (12Bu anion state) in the gas phase. 5. Conclusion In summary, TDS and 2PPE were employed to investigate interactions between C4H6 and Cu(111). Butadiene adsorbs weakly with a three-dimensional island growth mode at 90 K. Four coverage-dependent molecular desorption peaks appear in the TPD spectrum. In the 2PPE spectra, three peaks involving orbitals of adsorbed butadiene are identified and assigned, respectively, as π1* 2au (LUMO), π2* 2bg (LUMO + 1), and σ* 7bu (LUMO + 2). These lie 1.3, 3.4, and 4.8 eV above the Fermi level of Cu(111), respectively. The 2PPE processes of the LUMO and LUMO + 1 of adsorbed butadiene are initiated by substrate-mediated excitation, but that of LUMO + 2 is attributed to intramolecular excitation. Acknowledgment. This work was supported in part by the National Science Foundation (CHE0412609), by the Robert A. Welch Foundation (F-0032), and by the Center for Materials Chemistry of the University of Texas. W.H. acknowledges the financial support of the “talent program” of the Chinese Academy of Sciences.

5552 J. Phys. Chem. B, Vol. 110, No. 11, 2006 Supporting Information Available: An expanded figure showing the low kinetic energy part of the left panel of Figure 3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Robin, M. B. Higher Excited States of Polyatomic Molecules Vol. 1; Academic Press: New York, 1974; Higher Excited States of Polyatomic Molecules Vol. 2; Academic Press: New York, 1975; Higher Excited States of Polyatomic Molecules Vol. 3; Academic Press: New York, 1985. (2) Zhu, X. Y. Annu. ReV. Phys. Chem. 2002, 53, 221. (3) Pradier, C.-M.; Margot, E.; Berthier, Y.; Oudar, J. C. R. Acad. Sci., Ser. II 1988, 306, 561. (4) Ouchaib, T.; Massardier, J.; Renouprez, A. J. Catal. 1989, 119, 517. (5) Buenker, R. J.; Whitten, J. L. J. Chem. Phys. 1968, 49, 5381. (6) Shih, S.; Buenker, R. J.; Peyerimhoff, S. D. Chem. Phys. Lett. 1972, 16, 244. (7) Hosteny, R. P.; Dunning, T. H., Jr.; Gilman, R. R.; Pipano, A.; Shavitt, I. J. Chem. Phys. 1975, 62, 4764. (8) Buenker, R. J.; Shih, S.; Peyerimhoff, S. D. Chem. Phys. Lett. 1976, 44, 385. (9) Nascimento, M. A. C.; Goddard, W. A., III. Chem. Phys. 1979, 36, 147. (10) Nascimento, M. A. C.; Goddard, W. A., III. Chem. Phys. 1980, 53, 257.

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