J. Phys. Chem. 1991,95, 3687-3691 i, with pH showed that the band energies of FeS2 suspensions were functions of pH and allowed estimation of the potential of photogenerated electrons ( - 4 . 9 V vs SCE at pH 14). Modification of the surface with Pt can change the position of the energy levels (from -0.81 to -1.01 V for CdS3'). A similar result was
3687
also found for Pt-modified FeS2 particles.
Acknowledgment. The support of this research by the National Science Foundation (Grant No. CHE8805865) is gratefully acknowledged.
NEXAFS and EELS Study of the Orientation of Sulfur Dioxide on Ag(110) J. L. Solomon,+R. J. Madix,* Departments of Chemical Engineering and Chemistry, Stanford University, Stanford, California 94305-5025
W . Wurth, and J. Stohr IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 (Received: August 27, 1990)
The angular dependence of the bl* resonance in the near-edge X-ray absorption fine-structure(NEXAFS) spectra indicates that the plane of the SO2 molecule is perpendicular to the plane of the surface and perpendicular to the close-packed direction of the Ag( 110) surface ([ If01 azimuth). SO2forms a *-acceptor bond between the 3bl molecular orbital of SO2 and the Ag( 110) surface, and the bl* resonance corresponds to transitions into the antibonding combination of this *-acceptor bond while still retaining its strong angular dependence on the orientation of the ?i vector. This antbonding combination lies 531.9 eV above the O(1s) level. Off-specular electron energy loss spectroscopy (EELS) measurements confirm the dipole activity of the observed vibrational modes and show that the C, axis of the SOzmolecule must be oriented along the surface normal.
Introduction
The adsorption of SO2on Ag( 1 10) has been studied previously with temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), ultraviolet and X-ray photoelectron s p e c t m p y (Upsand XPS),' and high-resolution electron energy loss spectroscopy (EELS).2 These studies showed that SO2 adsorbs and desorbs molecularly without decomposition or dissociation on the clean Ag( 110) surface. At a surface temperature of 100 K multilayers of SO2are formed, which desorb in a sharp peak at 120 K. Three desorption states labeled a l ,a2,and a3are also observed at 170, 225, and 275 K, respectively. These states are associated with desorption from the second layer (aI) and first layer (a2,a3),respectively. The binding energies of these states, 41, 53, and 64 kJ mol-', respectively, indicate significant bonding of the SO2 molecule to the Ag( 110) surface. On the basis of previous EELS measurements, it was suggested that the first layer of SO2is bound to the surface via a Ag-S bond, with the plane of the SO2molecule tilted toward the surface with the oxygen atoms parallel to the surface, assuming the vibrational modes to be dipole active. This deduction was based on the presence of a mode assigned to the S02-Ag out-of-plane wag and the strict application of the surface 'selection rule". Indeed the appearance of weak modes, even if they are dipole active, is not necessarily indicative of the orientation of the adsorbed species. The outof-plane wag of surface-bound SO2in fact must exhibit a weak dynamic dipole moment perpendicular to the surface even if metal-molecule charge transfer does not accompany the wagging motion. If such charge transfer does occur, as would be expected due to the extensive donation of metal electrons into the r* orbital of SO2,the wagging motion of an upright SO2would become easily observable. The ambiguity of deducing the orientation of the adsorbed SO2 is thus apparent. Similar conclusions have been reached for the binding of H2S on Ni(001).3 To further investigate the orientation of SO2on Ag( 1 IO), we have measured the near-edge X-ray absorption fine-structure structure (NEXAFS) of SO2 on Ag(l10). The orientation of adsorbed molecules was Current Address: Process Technologies Laboratory, Building 208-1-01, 3M Center, St. Paul, MN 55144-1000. To whom correspondence should be addressed.
probed by determining the dependence of transition intensities from the O( 1s) level into unoccupied molecular orbitals of the adsorbed species on the angle of incidence of the linearly polarized light? For SO2the three lowest unoccupied molecular orbitals in order of increasing energy are the bl*, a'*, and b2*.5 These transitions are referred to as resonances hereafter. Xa calculations provide a means of determining the angular dependence of transitions into the unoccupied molecular orbitals? We have carried out such selfconsistentcalculationsfor SO2using the transition state method, and the results are shown in Figure 1. The potential was constructed by choosing the sphere radii such that the calculated O( 1s) ionization potential matched the experimental value. The X a calculations show that the polarization dependence of the al* and b2* resonances of SO2is complicated (compare curves b and c of Figure 1). If the orbital symmetry of the SO2molecule were not reduced by-the core hole, transitions to al* and bl* would be produced by E either in the 2 or 9 directions. From Xa calculations whick take into account the core hole and therefore the syymetry reduction the al* resonance is observed only when the E is in th_e9 direction (Figure 1). The b2* resonance is observed when E is along both the 2 and j directions (Figure 1). The angular dependence of the transitions are the result of the local geometry of the molecular orbital on the excited atom, and both the a'* and the b2* reson a n m correspond to transitions into molecular orbitals that have significant intensity in the plane of the molecule (u symmetry). Due to the complicated angular dependenceof these two resonance, they provide little information in determining the orientation of SO2. However, the X a calculations also show that the bl* resonance does not mix with the other resonances, and it remains a valid orbital to determine the z' direction or, equivalently, the (1) Outka, D. A.; Madix, R. J. Surf. Sci 1984, 137, 242. (2) Outka, D. A.; Madix, R. J. Longmuir 1986, 2, 406. (3) McGrath, R.; MacDowell, A. A.; Hashimme, T.; Sette, F.; Citrin, P. H. Phys. Rev. B 1989, 40, 9457. (4) StBhr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891. (5) Sze. K. H.; Brion, C. E.; Tong, X. M.; Li, J. M. Chem. Phys. 1987. 115, 433. (6) Horsley, J. A. In Chemistry and Physics of Solid Surfaces, Vanselow, V. R., Howe, R., EMS.;Springer Series in Surface Science; Springer: Berlin, 1988: Vol. 10.
0022-365419112095-3681302.50/0 0 1991 American Chemical Society
3408 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
Solomon et al. SO2 / Ag( 110) NEXAFS - Multilayer
SO2 X a Calculations
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Figure 1. Xu calculations of the oxygen K edge of SO2shown for the l? along the 3 (b), 3 (c), and Z (d) directions. Curve a is the sum over all three directions. The first three resonances are the bl*, al*, and the b2* resonances, respectively.
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rygen K-edge NEXAFS spectra of a multilayer of SO2on
TABLE I: Summrry of the CuncFlt Parameterg for the NEXAFS Spectra of MultiJayem of SOz on Ag( 110) and Spectra Recorded after Ameatiig Briefly to 185 K annealing X-ray energy, width: eV asym area assgnt temp, K inc (6') azimuth eV multilayer 530.7 2.9 5.00 bI* 535.8 2.6 1.60 al*,b2* 185
90
[IT01
90
[OOl]
20
[IT01
20
[001]
Experimental Section
(7) Rodriguez, J. A. Surf. Sei. 1990, 226, 101. (8) Stbhr. J. In X-ray Absorption: Principles, Applications. Techniques of EXAFS, SEXAFS. and XANES; Koningsberger, - D.C., Prins. R., Eds.; Wiley: New York. 1988. ( 9 ) Stahr, .I. In Chemistry and Physics of Solid Surfaces; Vanselow, R., Howe, R.,Us.; Surinner - Series in Chemical Physics 35: Surinaer: . - Berlin, 1984; Vol. 5, p 23.1. (IO) Outka, D.A.; Stbhr. J. J. Chem. Phys. 1988.88, 3539. (1 1) Solomon,J. L.; Madix, R. J.; Stbhr, J., submitted to J. Chem. Phys. (12) Brundle, C. R.; Carley, A. F. SO2 multilayer O(ls) XPS binding energy measured on Au. Faraday Discuss. Chem. Soc. 1975,60, 51. (13) Dwcydari, A. W.;Mee, C. H.E. Phys. Status Solidi A 1975,27,223.
/
540
Photon Energy (eV)
Figure 2. Ag( 110).
orientation of the plane of the SO2molecule (Figure 1). The bl* resonance corresponds to transitions into the 3bl molecular orbital, which is predominantly centered on the sulfur atom (60% S(3pZ))' and has only T character. To determine the rotation of the SO2 molecule within the plane determined from NEXAFS, we have performed off-specular EELS measurements to determine the dipole activity of the observed vibrational modes. NEXAFS experiments were performed at Stanford Synchrotron Radiation Laboratory on beam line 1-1 with a grasshopper monochromator (1200 lines/" holographic grating). Spectra were recorded at the oxygen K edge with partial-electron-yield detection using a retarding voltage of -400 V.*3 The crystal was mounted so that it could be rotated about a vertical axis to change the polar angle of the X-rays relative to the surface normal from glancing X-ray incidence (e = 20°) to normal X-ray incidence (e = goo). It could also be rotated in situ about a horizontal axis to change the azimuthal angle of the electric field vector relative to the rows and troughs of the Ag(ll0) surface. The final normalized NEXAFS spectra were prepared by subtracting the spectra recorded for the S02-coveredsurface by spectra recorded for the clean surface. The final spectra were curvefit to determine peak positions and areas.I0 Since SO2 forms a bond to the Ag( 110) surface,lV6the edge-jump corresponds to transitions to the continuum of states above the Fermi level, and it was, therefore, set a t the O(1s) XPS binding energy of 530.6 eV for the monolayer spectra.'JI For the multilayer spectrum the edge-jump corresponds to transitions to the continuum above the vacuum level;1° it was set a t 537.0 eV, which is the sum of the O(ls)binding energy of 532.5 eVI2 and the work function of the clean Ag(ll0) surface (4.5 eVI3). The resonances were fit with
,
542.7 554.1 531.9 535.2 541.3 557.5 531.9 535.2 541.3 556.1 531.9 534.3 542.4 557.5 531.9 534.3 542.5 556.8
5.3 13 3.7 5.2 5.1 10 3.7 5.2 5.1 10 3.7 4.5 6.0 10 3.7 4.4 6.4 11
0.21 0.25 0.20 0.20
1.66 2.00 3.56 bl* 0.69 al*,b2* 2.36
1.50 0.20 0.63 bl* 0.20 3.93 al*,b2* 1.20 2.40 0.20 0.42 bl* 4.14 al*,b2* 0.64 2.02 1.77 0.20 0.63 bl* 3.97 al*,b2* 0.74 2.58 2.06
"Seeref 14 for a complete mathematical explanation of the curve-fit parameters. bThe widths listed are the full width at half-maximum values.
either symmetric or asymmetric Gaussian peaks?*" The Ag( 1 10) surface was cleaned by argon bombardment and oxygen treatments according to standard procedures.15 Sharp (1 X 1) LEED patterns were observed prior to adsorption of sulfur dioxide. The monolayer-covered surface was prepared by adsorbing a multilayer of SO2 at 100 K and annealing briefly to 185 K to desorb the multilayer and second-layer state. NEXAFS spectra were also recorded after annealing the multilayer-covered surface to 155 and 215 K. The 155 and 21 5 K anneal spectra are not included here since they showed the same polar and azimuthal dependence as the 185 K anneal. The 155 K anneal penetrates the leading edge of the second-layer desorption feature, and although TPD measurements have not been performed to confirm this, this anneal leaving likely desorbs a significant amount of the second layer (al), primarily the first-layer states (a2and aj).The anneal to 215 K is expected to significantly depopulate the a2state, if not desorb it completely. There thus appears to be no significant difference in the orientation of the SO2at coverages that yield the u2or a3 state. All NEXAFS spectra were recorded a t a surface temperature of 100 K. The EELS spectra were recorded in a separate ultrahighvacuum system equipped with Auger electronics, four-grid LEED optics, an argon gun, a quadrupole mass spectrometer, and an (14) Liu, A. C.; Friend, C. M. J . Chem. Phys. 1988, 89, 4396. (15) Bowker, M.; Barteau, M. A.; Madix, R. J. Surf. Sci. 1980, 92, 528.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3689
Orientation of Sulfur Dioxide on Ag(ll0)
SO2 I Ag( I 10) NEXAFS - 185 K
TABLE 11: Resonance Energies near the OxyEen K Edge of resonance multilayer monolayer Xd gas phaseb bl *
ab*,b2*
530.7 535.8 542.7 554.1
531.9 535.2, 534.3' 542 557
532.0 536.2, 537.6 545 556
530.56 535.05, 535.81 542d
e
.The Xa peak energies listed are those for the angle-averaged spectrum. bunless noted otherwise, gas-phase data were recorded by using ISEELS and are from ref 5 . 'For the monolayer spectra the first number is for normal X-ra incidence and the second number is for glancing X-ray incidence. dlAlthough this resonance was not observed with ISEELS, it was observed in the gas-phase soft X-ray absorption spectrum, from ref 16. uThis resonance was not observed with ISEELS, and the X-ray absorption spectrum is reported only to 548 eV. EELS spectrometer with a double-pass monochromator and a rotatable analyzer. In addition to the specular angle, spectra were recorded for analyzer angles up to 20° from the specular direction, which is 60° from the surface normal. A beam energy of 1.9 eV and a resolution of 8 meV were used for all the spectra reported here. The crystal was cleaned and prepared in the same way as described above for NEXAFS experiments. The EELS spectra were recorded at a surface temperature of 100 K.
Results Near-Edge X-ray Absorption Fine Structure. The oxygen K-edge NEXAFS spectrum of a multilayer of SO2on Ag(ll0) is shown in Figure 2. The resonance energies and widths are listed in Table I. The gas-phase inner shell electron energy loss spectra (ISEELS) of SO2have been recorded previously and were used to assign the first two features in the multilayer spectrum. Resonances were observed at 530.56, 535.05, and 535.81 eV for gas-phase SO2,which have been assigned to transitions into the bl*, al*, and b2* molecular orbitals, respectively, by employing multichannel quantum defect theory calculations! Since the al* and b2* resonances are separated by only 0.8 eV and since the resolution of the X-ray monochromator used to collect the NEXAFS spectra is approximately 3.5 eV at the oxygen K edge, the al* and b2* resonances for SO2 are not resolved in the NEXAFS spectra. Due to the complicated angular dependence of these two resonances on the orientation of the electric field vector and the impossibility of resolving them, a single peak has been used to fit the contribution of both of these resonances to the NEXAFS spectra. However, the energies of the first two resonances observed for SO2multilayers and gas phase SO2agree reasonably well (Table 11). In addition to the first two resonances, two higher energy resonances were observed at 542.7 and 554.1 eV, respectively, for multilayers of SO2on Ag( 110). Although neither of these resonances was observed in the ISEELS spectrum, an unassigned resonance was observed at 542 eV in the soft X-ray absorption spectrum of gas-phase SO2.I6 Further, X a calculations of the oxygen K-edge region predict resonances at 545 and 556 eV, respectively (see Figure 1 and Table II), which are due to transitions from the O( 1s) level into higher lying molecular orbitals. Hartree-Fock-Roothaan calculations also indicate the presence of higher lying molecular orbitals." Therefore, the resonances at 542.7 and 554.1 eV are likely due to transitions into higher lying unoccupied molecular orbitals. More specific assignments of these resonances cannot be made at this time. Even though the al* and resonances are not resolved, a clear azimuthal and polar dependence of the spectra is observed for monolayer coverage of SO2on Ag( 110) (Figures 3 and 4), strongly suggesting a preferred orientation. The oxygen K-edge NEXAFS spectrum recorded at normal incidence with the?!, along the [ 1SO] azimuth (Figure 3, curve a) shows a prominent peak at 531.9 eV (16) Akimov, V. N.; Vinogradov, A. S.;Zimkina, T. M.Opr. Specrrosc. (USSR) 1982,53, 548. (17) Kondratcnko, A. V.; Mazalov, L. N.; Neiman, K.M.Opr. Spectros. (USSR) 1980,49,266.
' 520
XPS B.E
530 540 550 Photon Energy (eV)
560
570
Oxygen K-edge NEXAFS2pectra of SO2on Ag( 1IO) rmrded at normal X-ray incidence with the E along ([ 1TO] azimuth) and across ([OOl] azimuth) the close-packed direction. Figure 3.
SO2 / Ag( 110) NEXAFS - 185 K
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Figure 4. Oxygen K-edge NEXAFS s . r a of SO2on Ag( 1IO) mrded at glancing X-ray incidence with the E along and across the close-packed
direction.
and is clearly distinct from the spectra recorded at the other orientations. The peak positions and peak widths of the curvefits are listed in Table I. From the Xa calculationsthe only resonance that shows a strong angular dependence is the bl*. The 3bl is also the lowest unoccupied molecular orbital for SO2,'*and clearly curve a of Figure 3 contains the lowest energy resonance of any of the spectra. We have, therefore, assigned t_he lowest energy peak in the normal incidence spectra with the E vector along the rows to be primarily t_hebl* resonance. Since the bl* resonance is prominent with the E vector in the plane of the surface and along the close-packed dirytion, and since transitions into the bl* are expected when the E vector is perpendicular to the plane of the SO2 molecule, we conclude that the plane of the SO2molecule is perpendicular to the close-packed direction and perpendicular to the plane of the surface. In addition, a small bl* resonance is expected in the spectra at other orientations due to the incomplete polarization of the (18) Roos, B.; Siegbahn, P. Theor. Chim. Acra 1971,2I, 368. Hillier, I. H.; Saunders, V. R. Mol. Phys. 1971, 22, 193.
3690 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
Solomon et al.
SO2 / Ag( I IO) EELS SPECULAR
i o '?IFF-SPECULAR XI00 I
t O;t, --S /O-
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400 800 1200 Energy Loss ( m i ' )
Figure 5. EELS spectra of SO2on Ag(ll0) recorded with the analyzer at the specular angle (60' from the surface normal) and 10' off-specular (50' from the surface normal). The spectra were recorded with a primary electron energy of 1.9 eV and at a surface temperature of 100 K.
X-rays and the vector component of the 2 vector in the plane of the surface due to the finite angle of incidence of the light. Given t j e intensity of the bl* resonance at normal incidence with the E along the [ 1TO] azimuth (curve a, Figure 3), the contribution of the bl* resonance to the spectra recorded a t the other three orientations was calculated' (Figures 3 and 4). The remaining contribution to the first feature of each spectrum was then fit with a single peak that is due to both the aI* and the b2* resonances. The NEXAFS spectra recorded at glancing X-ray incidence (Figure!) do not show a dependence on the azimuthal orientation of the E vector, but they do differ from the normal-incidence spectra. The position of the al*,b2*resonance observed in the glancing incidence spectra is 0.9 eV lower in energy than the position observed in normal X-ray incidence spectra. The X a calculations indicate that the peak combining the al* and_the b2* resonances should be shifted to lower energy with the E in the j direction rather than the 3 direction. Tje width of the combined al*,b2*peak should also be wider for E along j . However, the al*,b2*peak has a fwhm of 4.5 eV a t glancing X-ray incidence, whereas the fwhm is 5.2 eV a t normal X-ray incidence (Table I). Hence, the peak position of the al*,b2*resonance suggests that the SO2molecule is tilted so that the j direction is nearly perpendicular to the plane of the surface, but the peak widths suggests that the SO2molecule stands upright so that the 3 direction is perpendicular to the surface plane. Therefore, the NEXAFS measurements of SO2do not provide a means of accurately determining the tilt angle of the SO2molecule in the plane perpendicular to the close-packed direction. However, the angular dependence of the bl* raonance means that the plane of the SO2 molecule is at least nearly perpendicular to- the close-packed direction (e.g., the z direction is along the [ 1101 azimuth), and EELS measurements can be used to determine the orientation within this plane. Electron Energy Loss Vibrational Spectroscopy. EELS spectra recorded for a single layer of SO2on Ag( 110) with the analyzer at the specular angle and at IOo off-specular are shown in Figure 5. The assignment of the vibrational modes follows directly from the assignments made previously for SO2on Ag( 110) (see Figure 6).2 The intensity of the observed vibrational modes as a function of analyzer angle is shown in Figure 7. Since dipole-active modes are observed when the analyzer collects inelastic electrons emitted at an angle close to the direction of specular reflection, the intensity of the dipole-active modes follows the intensity of the elastically scattered e1ectr0ns.l~ In contrast, energy losses resulting from impact scattering typically have a broad angular distributionals The absolute intensities of vibrational modes that are predominantly impact scattered typically do not decrease in intensity
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105
104
103
102
101
100
11 " I 0
-10
.5
0
5
10
15
20
Deviation from Specular (degrees) Figure 7. Intensity variation of the EELS vibrational modes of SO2on Ag( 110) following annealling the surface to 185 K as a function of the analyzer angle. The analyzer angles are relative to the specular direction, and a positive deviation from specular indicates the analyzer was closer to the surface normal. The direction of the scattered electrons (specular
direction) was held constant at 60' from the surface normal. Since the intensity of the observed vibrational modes follows the intensity of the elastically scattered electron, the modes have primarily dipole character. off-specular,Is which is clearly not the case for the vibrational modes of SO2on Ag(ll0). All the observed vibrational modes of SO2 on Ag( 110) have significant dipole character.
Discussion The bonding of SO2on Ag( 110) has been examined recently with semiempirical MO-SCF calculations (INDO/S)! The large electron affinity of SO2in the gas phase (1.1 eVm) suggests that SO2should be an electron acceptor, and the MO-SCF calculations show that SO2does indeed act as a a-donor and a *-acceptor of electrons on a-top sites on Ag( 110): The S02-Ag( 110) bond is primarily the consequence of bonding between the occupied 8al molecular orbital of SO2,which is mostly a combination of sulfur atomic orbitals (u-donor) with the Ag(5s,5p) orbitals and the bonding of the unoccupied 3bl orbital of SO2 (*-acceptor) with the Ag(5s,5p) orbitals: The donation of charge into the 3bl orbital reduces the S-0 bond order from the gas-phase value of 2.27 to a value of 2.05 for SO2on Ag( 110): This weakening of the S-O bond due to the formation of a *-acceptor bond was observed with EELS as a reduction in the symmetric S - 0 stretching frequency of adsorbed SO2to 985 cm-I from the gas-phase frequency of 1151 cm-1.2 The bonding interaction of SO2with the Ag( 110) surface is also reflected in the NEXAFS spectra. The shift of the al*,bz* resonance of the monolayer to lower energy by 0.6 eV relative to the multilayer is also consistent with a lengthening of the S-0 bond since both of these orbitals have primarily u* character and since it has been shown that the u* resonances associated with
~~
(19) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surfoce Vibrclfions;Academic Press: New York, 1982; pp 103, 202.
(20) Chen, E. C. M.; Wentworth, W. E.J. Chem. Phys. 1!375,63,3183.
Orientation of Sulfur Dioxide on Ag( 110)
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3691 AFS measurements indicate that the SO2plane is not tilted but is nearly perpendicular to the plane of the surface. The apparent contradiction between the two results can, however, be explained in two ways, First, the uncertainty in the orientation determined from NEXAFS of 1 5 O could accommodate a small tilt that m a y be large enough to allow the wag mode in the EELS spectrum. Alternatively, even if the SOz plane were not tilted, the wag motion corresponds to the oxygen atoms moving in an arc, which in addition to having a large component parallel to the surface also has a small component perpendicular to the surface. The small perpendicular component of the wag motion would not be screened by the image charge and could give rise to the small loss observed in the EELS spectrum. Given this explanation of the small wag mode, the adsorbed SO2 molecule retains its C, symmetry.2 Regardless of the explanation, the plane of the SO2 molecule appears to be perpendicular to the surface within 15'. It was previously suggested that the oxygen atoms in the adsorbed SO2molecule are equivalently bound to the surface, since the asymmetric S a stretch is not observedU2This conclusion was based on the assumption that the asymmetric stretch (v,) is dipole active. Since off-specular EELS measurement have confirmed that the symmetric stretch (v,) is dipole active, it is a near certainty that the asymmetric stretch is also dipole active, particularly since v, is very intense for multilayers of SO2. To fully screen the v, vibrational mode, the oxygen atoms must be equidistant from the surface so that the asymmetric stretch is parallel to the surface. The combination of the equivalence of the oxygen atoms as determined from EELS and orientation of the plane of the SO2 molecule from NEXAFS completely defines the orientation of SO2on Ag( 110). The plane of the SO2molecule is perpendicular to the close-packed direction with the C, axis of the molecule (2 in Figure 1) perpendicular to the plane of the surface (Figure 8). Since the v, and 6 losses have been shown to be dipole active, the presence of these losses in the EELS spectrum is also quite consistent with this orientation.
-
Side Vlew
L [ITOI Top View
Figure 8. Orientation of SOzon Ag( 110) determined by NEXAFS and EELS measurements. The C , axis of the SOzmolecule is perpendicular to the plane of the surface (side view), and the plane of the SO2molecule is perpendicular to the close-packed direction (top view).
longer bonds occur at lower energies.21 In contrast to the al*,bz* resonance, the bl* resonance is shifted up in energy by 1.0 eV for the monolayer coverage of SO2relative to the multilayer. As shown by the MO-SCF calculations, the 3bI orbital of SO2forms a bond with the Ag(5s,Sp) orbitals, which results in the formation of a bonding and antibonding combinations of these orbitals.22 The hybrid bonding combination lies below the Fermi level, is occupied according to the MO-SCF calculations,6 and is observed in the ultraviolet photoelectron spectroscopy of SO2on Ag( 1lO).I Since NEXAFS probes unoccupied molecular orbitals, the bl* resonance corresponds to transitions from the O(1s) level into the antibonding combination of the 3bl and the Ag(Ss,5p). The antibonding combination lies above the 3bl energy level, which explains the upward shift of 1.O eV for chemically bound SO2. Even though bl* is clearly involved in the bonding of SO2to Ag( 1 lo), transitions into this hybrid orbital retain their strong angular dependence on the orientation of the l? vector. _Dominate transitions into this orbital are observed only when the E is in the plane of the surface and along the close-packed direction, strongly suggesting that the symmetry of the hybrid orbital is not reduced. If the bonding interaction with the surface created a perpendicular component of this hybrid orbital or caused the SOz molecule to tilt, transitions to the bl* hybrid orbital would be observed at glancing X-ray incidence. Since the intensity of @e bl* resonance at glancing incidence and normal incidence with E along the [Ool] azimuth can be completely accounted for by the incomplete polarization of the X-rays and the vector component of the E vector in the plane of the surface due to the finite angle of incidence of the light, the hybrid orbital appears to retain its symmetry and remains a valid measure of orientation of the SO2 molecule. Previously reported EELS measurements have suggested that the plane of the SO2molecule is tilted toward the surface because the wag mode of low intensity is observed. However, the NEX(21) Sette, F.; StBhr, J.; Hitchcock, A. P. J . Chem. Phys. 1984,81,4096. Stehr, J.; Sette, F.; Johnson, A. L. Phys. Rev. Leu. 1984, 53, 1684. ( 2 2 ) Rodriguez, J. A., private communication.
Conclusion
Due to an ambiguity of the polarization dependence of transitions to in-plane molecular orbitals the NEXAFS measurements alone are not enough to establish the orientation of SOz on Ag(1 IO). NEXAFS measurements have been combined with offspecular EELS measurements to completely determine the orientation of SO2on Ag( 110). The angular dependence of the bl* resonance in the NEXAFS spectra indicates that the plane of the SO2 molecule is perpendicular to the plane of the surface and perpendicular to the close-packed direction. Off-specular EELS measurements confirm the dipole activity of the observed vibrational modes and show that the C, axis of the SO2molecule must be oriented along the surface normal. Therefore, sulfur dioxide is oriented with its molecular plane perpendicular to the closepacked direction and with its C, axis along the surface normal.
Acknowledgment. We express our sincere appreciation to D. A. Outka, P. A. Stevens, and W. Jark for their assistance in collecting the NEXAFS data. The NEXAFS experiments were performed at Stanford Synchrotron Radiation Laboratory, which is funded by the Department of Energy under Contract DEAC03-82ER- 13000, Office of Basic Energy Sciences, Division of Chemical Sciences. We also gratefully acknowledge the support of J.L.S. by the Center for Materials Research at Stanford and the support of the National Science Foundation (NSF CHE 8615910). Registry No. SO2, 7446-09-5; Ag, 7440-22-4.
