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Symmetry reduction of benzene on Rh(111) by coadsorbed Na
CHEMICAL PHYSICS LETTERS
Volume 140, number 2
SYMMETRY
REDUCTION
OF BENZENE
ON Rh(ll1)
25 September 1987
BY COADSORBED Na
G. ROSINA =, G. RANGELOV a,‘, E. BERTEL b, H. SAALFELD ’ and F.P. NETZER a a Institut j% PhysikalischeChemie, Universitiit Innsbruck,A-6020 Innsbruck,Austria b Max-Planck-Institut ftir Plasmaphysik,D-8046 Garching,Federal Republicof Germany c Fachbereich Physik,UniversitiitOsnabriick,D-4500 Osnabriick,Federal Republicof Germany Received 2 May 1987; in final form 6 July 1987
Angle-resolved photoemission experiments using synchrotron radiation reveal a reduction from C& to C3v,avin the local symmetry of benzene on Rh( 11 I ) in the presence of coadsorbed Na. This is interpreted via a model of the coadsorbate layer in which the benzene molecules, adsorbed on-top in a pure benzene layer, occupy threefold hollow positions with a possible out-of-plane C-H bending distortion. The benzene energetics are influenced only weakly by coadsorbed Na.
During the interaction of small molecules with metal surfaces promoted by alkali preadsorption or coadsorption, the alkali additive often has drastic effects on the physics and chemistry of the adsorbed molecules. These effects, mostly studied on adsorbed CO, include increased metal-molecule bond strength [ l-51, intramolecular bond weakening as reflected by substantial decreases in intramolecular stretching frequencies [ 61, and bond rehybridization [ 71 or enhanced dissociation [ 8,9]. Different molecular bonding geometries have been inferred in the presence of alkali promoters [ 10,111, and enhanced sticking probabilities into a precursor state for dissociation have been reported [ 121. The motivation for the widespread interest in this area is due not only to the importance of promoters in practical catalysis, but also by their role as model systems in studies of the modification of electronic properties of metal surfaces, with the ultimate aim of tailoring the properties of chemically active surfaces. In this Letter we report the first detailed study of the interaction of a molecule of some complexity, namely benzene, with a Na-modified Rh( 111) surface. We find that the energetics of the adsorbed benzene molecules as revealed by thermal desorption (TDS) is only slightly affected by coadsorbed Na. ’ Permanent address: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria.
200
This observation is different from that usually found with smaller molecules. The local symmetry of the benzene adsorption complex, however, is significantly reduced by the presence of Na atoms as deduced from angle-resolved UV photoemission (ARUPS) experiments. This symmetry reduction is discussed in terms of a model resulting from a change in the adsorption site of benzene molecules in the benzene-Na coadsorbate layer. The experiments were performed in two separate angle-resolving electron spectrometers (VG ADES 400) equipped with the usual facilities for surface analysis and crystal characterization. The ARUPS spectra reported here were recorded at the TGM 2 beamline of the BESSY synchrotron radiation laboratory in Berlin. The total energy resolution of the system (toroidal grating monochromator and energy analyzer) was typically 200 meV during our experiments. The Rb( 111) surface was cleaned by standard procedures as described previously [ 131. Benzene dosing was carried out from the system ambience with the crystal at room temperature; sodium was deposited from a SAES getter source and the Na surface coverage was characterized by work LEED function, and thermal desorption measurements. The structure of benzene on Rh( 111) has been investigated previously in some detail [ 13- 15 1, and the essential aspects can be summarized as follows.
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
CHEMICAL PHYSICS LETTERS
Volume 140, number 2
MO
hnper*tur.
( n)
Fig. 1. Thermal desorption spectra of benzene (amu 78) and hydrogen (amu 2) from pure benzene and benzene coadsorbed with 0.1 ML of Na on Rh(ll1). Heating rate: 6 K s-‘. Insert: Work function changes A@after benzene saturation of Na-precovered surfaces. The bottom scale gives the Na A@values and the top scale the corresponding Na precoverage in monolayers (ML).
The benzene moieties are essentially undistorted and adsorbed with their ring planes parallel to the surface, bound via the aromatic n-electron system [ 131. This adsorption geometry derives from the Cbv symmetry as deduced from angle-resolved photoemission. The molecules are azimuthally oriented and aligned with the corners of their carbon hexagons along the FLU crystal mirror planes [ 151. In conjunction with an ordered LEED pattern [ 161 this indicates that the on-top positions are the favoured adsorption sites. Coadsorption of Na has little influence on the energetics of the adsorbed benzene molecules. This is deduced from the TDS results as shown in fig. 1, where desorption curves of benzene (amu 78) and
25 September 1987
H2 (amu 2) from a pure benzene and a benzene-Na (0.1 monolayer (ML)) coadsorbate layer are displayed. Note that generally only a fraction of adsorbed benzene molecules can be desorbed from transition metal surfaces in intact form, and that a large proportion dissociates during the heating cycle into surface carbon and hydrogen, the latter then desorbing from the surface. Na precoverage reduces the amount of molecular benzene desorption, presumably an effect of reduced surface coverage, and shifts the first H2 desorption peak by x 20 K to lower temperature. This may be indicative of an increased tendency for C-H bond breaking, but the effect is not pronounced. Whereas the desorption energy for molecular benzene desorption from Rh(ll1) is essentially unaffected by Na coadsorption, more pronounced destabilization of benzene molecules in TDS has been found on Pt (111) in the presence of K [ 171. The insert to fig. 1 shows a plot of the change of work function obtained after benzene saturation of the Na-precovered surface. Both coadsorbate species reduce the Rh work function and thus may be regarded, somewhat loosely, as electron donors. For Na precoverage beyond 0.25 ML the Rh surface is blocked for subsequent benzene adsorption. The benzene A@values for 19,~< 0.25 ML follow a straight line which intersects the ordinate ( oNa=O) at A@= - 1.15 eV; this corresponds to the saturation value for the pure benzene adlayer. This linear A@ relationship may be partly a result of reduced benzene surface coverage, and partly due to a reduction in the donor properties of the benzene molecules in the presence of Na. Angle-resolved UV photoemission reveals significant differences between benzene in a pure and in a Na coadsorbate layer. Fig. 2 compares ARUPS spectra stimulated by p-polarised light, hv= 45 eV for benzene on Rh( 111) and coadsorbed with 0.1 ML Na. The normal emission spectrum of pure benzene (bottom curve) shows emission from adsorbate bands A, D and the leading component of band C, but at off-normal emission geometry (8 = 50”, top curve of fig. 2) additional features appear from bands B, E and the higher-energy components of band C. The physical basis of this emission behaviour is well understood and may be rationalized in terms of polarisation-dependent symmetry-derived selection 201
Volume 140, number 2
CHEMICAL PHYSICS LETTERS
Fig. 2. Angle-resolved photoemission spectra of pure benzene and benzene coadsorbed with 0.1 ML of Na on Rh(ll1). Photon energy hv=45 eV, p polarisation, photon angle of incidence ar=45 “; 0 is the electron take-off angle.
rules [ 13,181. As illustrated in the correlation table (table 1)) where allowed and forbidden emission in the normal direction separated into the various components of the photon polarisation vector are indicated, adsorbate bands B, E and the higher-energy components of band C - corresponding to the 2e2g, le,,, lb*, and lb,, orbitals of free benzene, respectively - cannot be stimulated by any polarisation component for CbVsymmetry. This forms the experimental basis which establishes CbVsymmetry for benzene on Rh(ll1) [ 13,151. Coadsorption of Na introduces important changes in the ARUPS spectra (fig. 2, middle curves): in normal emission band B and the lb,, component of band C are clearly apparent; in addition, the relative intensity of band D is significantly altered both at normal and off-normal emission. The normal emission behaviour indicates symmetry reduction of the 202
25 September 1987
benzene adsorption complex in the presence of Na. In fig. 3 the polarisation dependence of the normal emission spectra of the benzene-Na coadsorbate layer is displayed. The polarisation composition of the photon beam was varied by changing the photon angle of incidence cy;at (Y= 25 ’ x polarisation is prevailing, but at a! = 65 ’ the z polarisation component contributes significantly [ 191. Pronounced polarisation effects are indicated in fig. 3, in particular two components of band C and D are obviously stimulated by the z polarisation component at (Y=65”. An analysis of the data in terms of the selection rules for emission in high-symmetry directions (here normal emission) [20] yields the best agreement between group theoretical predictions and experimental results for C3V,av symmetry as derived in table 1. The a, components of band C ( laZu and 1bl, in D6,J and band D (2a,,) show experimental z polarisation dependence as required by theory, and the forbidden a2 component of band C ( lb2”) coincides with a clear minimum in the normal emission spectra. C3V,oVsymmetry is therefore indicated, which signifies that the mirror planes perpendicular to the molecular plane cut through the comers of the benzene hexagons. The behaviour exhibited by band E (1 e2*) is interesting. Although allowed on symmetry grounds it is not visible in normal emission spectra (figs. 2 and 3). In fact, there is no point group symmetry lower than CeVin which this band would be forbidden. We conjecture that the absence of band E in normal emission is due to a directional cross-section effect. Emission from the 1e29orbitals of benzene adsorbed on Pd has been investigated in some detail [ 2 11, and negligible intensity was found in directions near the ring plane normal. Significant intensity was observed only at angles > 30” off the surface (i.e. the ring plane) normal. It has to be added that there is evidence that benzene adsorbed on Pd surfaces is not distorted [ 22-241. On the other hand, in cases where distortion of the carbon ring skeleton is indicated (e.g. for benzene on Os( 000 1) [ 251) emission from the le,, derived molecular orbitals is detected in the normal direction. This suggests that distortion of the carbon ring skeleton is not causing the observed symmetry reduction of benzene in the presence of Na. We propose instead a change in the adsorption site of benzene to account for the symmetry reduction
CHEMICAL PHYSICS LETTERS
Volume 140, number 2
25 September 1987
Table 1 Correlation table for the molecular orbitals of benzene. Allowed and forbidden emission in the normal direction for respective polarisation components of the photon beam are indicated Dlh
C JV.0”
Cl” JGY
z
X,Y
2
Experimental band (eV)
ler,(x)
e,
+
-
e
+
-
A
5
2e2s(o)
e2
-
-
e
+
-
B
5.6
laz.(x) 2elU(o) lb,,(o) lb,.(o)
aI e1 bz b,
+ -
ai e a2 aI
+ -
+ +
C
7-10
-
+ _
2a,,(o)
aI
-
+
aI
_
+
D
11
lezs(o)
e2
-
-
e
+
-
E
13
from CbVto C3_. In this model, Na adatoms force the benzene molecules from the on-top positions into the threefold hollow sites. An illustration of a possible model arrangement is shown in fig. 4. If the Na atoms occupy threefold hollow sites, the coadsorbed benzene molecules are most naturally accommo-
Rh (111) +
NB
h”. 15.”
dated in threefold hollow sites as well. The three hydrogen atoms located on top of the Rh atoms in fig. 4 may be slightly bent away from the surface, thereby substantiating the C3V,0V symmetry imposed by the adsorption site. We expect that the 2alg orbital of benzene (band D), which is purely C-H derived [ 261, should reflect this out-of-plane bending dis-
+ Benzene P-PI
Fig. 3. Normal emission spectra of benzene on Rh( 111) precovered with 0.1 ML of Na for two photon angles of incidence (Y.
Fig. 4. Schematic model of a local arrangement in the benzene-Na coadsorbate layer. The benzene molecule is represented by its van der Waals size, and the radius of Na adatoms has been taken as lying between ionic and metallic. ov designate vertical mirror planes.
203
Volume 140, number 2
CHEMICAL PHYSICS LETTERS
tortion. The ARUPS spectra show that the intensities of band D are indeed those that are influenced most. In conclusion we wish to comment on the differences in adsorption behaviour between small molecules, such as CO, and benzene in the presence of alkali additives. Alkali coadsorption increases the CO-metal bond strength on most metal surfaces (see e.g. refs. [ l-l 0 3). This is generally attributed to increased metal d-CO 27c*backdonation. For benzene no such stabilization of the adsorption energy is observed. Anderson et al. [ 271 have calculated the benzene adsorption energy on a Pt, cluster as a functin of shifts of the Pt valence electron ionization potentials, which are taken to model electron donation from coadsorbed K. They find a decrease of the adsorption energy for moderate upward shifts of the Pt valence bands, and this is ascribed to reduced donation from the benzene x orbitals into the Pt valence band. These calculations are in qualitative agreement with what is observed in this study on Rh(ll1) and by Garfunkel et al. [17] on Pt(lll), and they emphasize the dominance of 7c donation bonding over backbonding into empty IC*orbitals for adsorbed benzene. The results of the present study suggest that the energetics of adsorption sites are modified by the alkali additive, so that benzene is stabilized on a site different from that on the bare surface. The observed symmetry reduction is imposed by the adsorption site, but possibly substantiated by out-of-plane C-H bending. The influence of Na on coadsorbed benzene is presumably mediated by both geometric and electronic effects. This experimental programme has been supported by the Fonds zur Forderung der Wissenschaftlichen Forschung of Austria and by the Austrian National Bank. We wish to acknowledge the staff of the BESSY synchrotron in Berlin for their technical support, and D.R. Lloyd, Trinity College, Dublin and J.C. Riviere, AERE Harwell for discussions and critical comments on the manuscript. References [ 1] H.P. Bonzel, J. Vacuum Sci. Technol. A2 (1984) 866.
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25 September 1987
[ 21 J.E. Crowell, E.L. Garfunkel and G.A. Somorjai, Surface Sci. 121 (1982) 303. [3] J. Lee, C.P. Hanrahan, J. Arias, R.M. Martin and H. Metiu, Phys. Rev. Letters 5 1 ( 1983) 1803. [4] F. Sette, J. Stijhr, E.B. Kollin, D.J. Dwyer, J.L. Gland, J.L. Robbins and A.L. Johnson, Phys. Rev. Letters 54 (1985) 935. [ 51D. Heskett, E.W. Plummer, R.A. de Paola and W. Eberhardt, Phys. Rev. B33 ( 1986) 5 171. [6] F.M. Hoffmann and R.A. de Paola, Phys. Rev. Letters 52 (1984) 1697. [7] J.J. Weimer and E. Umbach, Phys. Rev. B30 (1984) 4863. [8] G. Broden, G. Gafner and H.P. Bonzel, Surface Sci. 84 (1979) 295. [ 91 E. Wimmer, C.L. Fu and A.J. Freeman, Phys. Rev. Letters 55 (1985) 2618. [lo] W. Eberhardt, F.M. Hoffmann, R. de Paola, D. Heskett, I. Strathy, E.W. Plummer and H.R. Moser, Phys. Rev. Letters 54 (1985) 1856. [ 111 F.P. Netzer, D.L. Doering and T.E. Madey, Surface Sci. 143 (1984) L363. [ 121 L.J. Whitman, C.E. Bartosch, W. Ho, G. Strasser and M. Grunze, Phys. Rev. Letters 56 (1986) 1984. [ 131 M. Neumann, J.U. Mack, E. Bertel and F.P. Netzer, Surface Sci. 155 (1985) 629. [ 141 E. Bertel, G. Rosina and F.P. Netzer, Surface Sci. 172 (1986) L515. [ 151 F.P. Netzer, G. Rosina, E. Bertel and H. Saalfeld, Surface Sci. 184 (1987) L397. [ 161 R.F. Lin, R.J. Koestner, M.A. van Hove and G.A. Somorjai,SurfaceSci. 134 (1983) 161. [ 171 E.L. Garfunkel, J.J. Maj, J.C. Frost, M.H. Farias and G.A. Somorjai, J. Phys. Chem. 87 (1983) 3629. [ 181 N.V. Richardson and A.M. Bradshaw, in: Electron spectroscopy, Vol. 4, eds. C.R. Brundle and A.D. Baker (Academic Press, New York, 198 1) ch 3; E.W. Plummer and W. Eberhardt, Advances in chemical physics, Vol. 49 (Wiley, New York, 1982) p. 533. [ 191 M. Schemer, K. Kambe and K. Forstmann, Solid State Commun. 25 (1978) 93. [ 201 J. Hermanson, Solid State Commun. 22 (1977) 9. [ 2 1 ] F.P. Netzer, G. Rosina, G. Rangelov and H. Saalfeld, to be published. [22] G.L. Nyberg and N.V. Richardson, Surface Sci. 85 (1979) 335. [23] F.P. Netzer and J.U. Mack, J. Chem. Phys. 79 (1983) 1017. [24] G.D. Waddill and L.L. Kesmodel, Phys. Rev. B31 (1985) 4940. [25] F.P. Netzer, H.H. Graen, H. Kuhlenbeck and M. Neumann, Chem. Phys. Letters 133 (1987) 49. [ 261 W.L. Jorgensen and L. Salem, The organic chemist’s book of orbitals (Academic Press, New York, 1973). [27] A.B. Anderson, M.R. McDevitt and EL. Urbach, Surface Sci. 146 (1984) 80.
Volume 140, number 2
SYMMETRY
REDUCTION
OF BENZENE
ON Rh(ll1)
25 September 1987
BY COADSORBED Na
G. ROSINA =, G. RANGELOV a,‘, E. BERTEL b, H. SAALFELD ’ and F.P. NETZER a a Institut j% PhysikalischeChemie, Universitiit Innsbruck,A-6020 Innsbruck,Austria b Max-Planck-Institut ftir Plasmaphysik,D-8046 Garching,Federal Republicof Germany c Fachbereich Physik,UniversitiitOsnabriick,D-4500 Osnabriick,Federal Republicof Germany Received 2 May 1987; in final form 6 July 1987
Angle-resolved photoemission experiments using synchrotron radiation reveal a reduction from C& to C3v,avin the local symmetry of benzene on Rh( 11 I ) in the presence of coadsorbed Na. This is interpreted via a model of the coadsorbate layer in which the benzene molecules, adsorbed on-top in a pure benzene layer, occupy threefold hollow positions with a possible out-of-plane C-H bending distortion. The benzene energetics are influenced only weakly by coadsorbed Na.
During the interaction of small molecules with metal surfaces promoted by alkali preadsorption or coadsorption, the alkali additive often has drastic effects on the physics and chemistry of the adsorbed molecules. These effects, mostly studied on adsorbed CO, include increased metal-molecule bond strength [ l-51, intramolecular bond weakening as reflected by substantial decreases in intramolecular stretching frequencies [ 61, and bond rehybridization [ 71 or enhanced dissociation [ 8,9]. Different molecular bonding geometries have been inferred in the presence of alkali promoters [ 10,111, and enhanced sticking probabilities into a precursor state for dissociation have been reported [ 121. The motivation for the widespread interest in this area is due not only to the importance of promoters in practical catalysis, but also by their role as model systems in studies of the modification of electronic properties of metal surfaces, with the ultimate aim of tailoring the properties of chemically active surfaces. In this Letter we report the first detailed study of the interaction of a molecule of some complexity, namely benzene, with a Na-modified Rh( 111) surface. We find that the energetics of the adsorbed benzene molecules as revealed by thermal desorption (TDS) is only slightly affected by coadsorbed Na. ’ Permanent address: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria.
200
This observation is different from that usually found with smaller molecules. The local symmetry of the benzene adsorption complex, however, is significantly reduced by the presence of Na atoms as deduced from angle-resolved UV photoemission (ARUPS) experiments. This symmetry reduction is discussed in terms of a model resulting from a change in the adsorption site of benzene molecules in the benzene-Na coadsorbate layer. The experiments were performed in two separate angle-resolving electron spectrometers (VG ADES 400) equipped with the usual facilities for surface analysis and crystal characterization. The ARUPS spectra reported here were recorded at the TGM 2 beamline of the BESSY synchrotron radiation laboratory in Berlin. The total energy resolution of the system (toroidal grating monochromator and energy analyzer) was typically 200 meV during our experiments. The Rb( 111) surface was cleaned by standard procedures as described previously [ 131. Benzene dosing was carried out from the system ambience with the crystal at room temperature; sodium was deposited from a SAES getter source and the Na surface coverage was characterized by work LEED function, and thermal desorption measurements. The structure of benzene on Rh( 111) has been investigated previously in some detail [ 13- 15 1, and the essential aspects can be summarized as follows.
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
CHEMICAL PHYSICS LETTERS
Volume 140, number 2
MO
hnper*tur.
( n)
Fig. 1. Thermal desorption spectra of benzene (amu 78) and hydrogen (amu 2) from pure benzene and benzene coadsorbed with 0.1 ML of Na on Rh(ll1). Heating rate: 6 K s-‘. Insert: Work function changes A@after benzene saturation of Na-precovered surfaces. The bottom scale gives the Na A@values and the top scale the corresponding Na precoverage in monolayers (ML).
The benzene moieties are essentially undistorted and adsorbed with their ring planes parallel to the surface, bound via the aromatic n-electron system [ 131. This adsorption geometry derives from the Cbv symmetry as deduced from angle-resolved photoemission. The molecules are azimuthally oriented and aligned with the corners of their carbon hexagons along the FLU crystal mirror planes [ 151. In conjunction with an ordered LEED pattern [ 161 this indicates that the on-top positions are the favoured adsorption sites. Coadsorption of Na has little influence on the energetics of the adsorbed benzene molecules. This is deduced from the TDS results as shown in fig. 1, where desorption curves of benzene (amu 78) and
25 September 1987
H2 (amu 2) from a pure benzene and a benzene-Na (0.1 monolayer (ML)) coadsorbate layer are displayed. Note that generally only a fraction of adsorbed benzene molecules can be desorbed from transition metal surfaces in intact form, and that a large proportion dissociates during the heating cycle into surface carbon and hydrogen, the latter then desorbing from the surface. Na precoverage reduces the amount of molecular benzene desorption, presumably an effect of reduced surface coverage, and shifts the first H2 desorption peak by x 20 K to lower temperature. This may be indicative of an increased tendency for C-H bond breaking, but the effect is not pronounced. Whereas the desorption energy for molecular benzene desorption from Rh(ll1) is essentially unaffected by Na coadsorption, more pronounced destabilization of benzene molecules in TDS has been found on Pt (111) in the presence of K [ 171. The insert to fig. 1 shows a plot of the change of work function obtained after benzene saturation of the Na-precovered surface. Both coadsorbate species reduce the Rh work function and thus may be regarded, somewhat loosely, as electron donors. For Na precoverage beyond 0.25 ML the Rh surface is blocked for subsequent benzene adsorption. The benzene A@values for 19,~< 0.25 ML follow a straight line which intersects the ordinate ( oNa=O) at A@= - 1.15 eV; this corresponds to the saturation value for the pure benzene adlayer. This linear A@ relationship may be partly a result of reduced benzene surface coverage, and partly due to a reduction in the donor properties of the benzene molecules in the presence of Na. Angle-resolved UV photoemission reveals significant differences between benzene in a pure and in a Na coadsorbate layer. Fig. 2 compares ARUPS spectra stimulated by p-polarised light, hv= 45 eV for benzene on Rh( 111) and coadsorbed with 0.1 ML Na. The normal emission spectrum of pure benzene (bottom curve) shows emission from adsorbate bands A, D and the leading component of band C, but at off-normal emission geometry (8 = 50”, top curve of fig. 2) additional features appear from bands B, E and the higher-energy components of band C. The physical basis of this emission behaviour is well understood and may be rationalized in terms of polarisation-dependent symmetry-derived selection 201
Volume 140, number 2
CHEMICAL PHYSICS LETTERS
Fig. 2. Angle-resolved photoemission spectra of pure benzene and benzene coadsorbed with 0.1 ML of Na on Rh(ll1). Photon energy hv=45 eV, p polarisation, photon angle of incidence ar=45 “; 0 is the electron take-off angle.
rules [ 13,181. As illustrated in the correlation table (table 1)) where allowed and forbidden emission in the normal direction separated into the various components of the photon polarisation vector are indicated, adsorbate bands B, E and the higher-energy components of band C - corresponding to the 2e2g, le,,, lb*, and lb,, orbitals of free benzene, respectively - cannot be stimulated by any polarisation component for CbVsymmetry. This forms the experimental basis which establishes CbVsymmetry for benzene on Rh(ll1) [ 13,151. Coadsorption of Na introduces important changes in the ARUPS spectra (fig. 2, middle curves): in normal emission band B and the lb,, component of band C are clearly apparent; in addition, the relative intensity of band D is significantly altered both at normal and off-normal emission. The normal emission behaviour indicates symmetry reduction of the 202
25 September 1987
benzene adsorption complex in the presence of Na. In fig. 3 the polarisation dependence of the normal emission spectra of the benzene-Na coadsorbate layer is displayed. The polarisation composition of the photon beam was varied by changing the photon angle of incidence cy;at (Y= 25 ’ x polarisation is prevailing, but at a! = 65 ’ the z polarisation component contributes significantly [ 191. Pronounced polarisation effects are indicated in fig. 3, in particular two components of band C and D are obviously stimulated by the z polarisation component at (Y=65”. An analysis of the data in terms of the selection rules for emission in high-symmetry directions (here normal emission) [20] yields the best agreement between group theoretical predictions and experimental results for C3V,av symmetry as derived in table 1. The a, components of band C ( laZu and 1bl, in D6,J and band D (2a,,) show experimental z polarisation dependence as required by theory, and the forbidden a2 component of band C ( lb2”) coincides with a clear minimum in the normal emission spectra. C3V,oVsymmetry is therefore indicated, which signifies that the mirror planes perpendicular to the molecular plane cut through the comers of the benzene hexagons. The behaviour exhibited by band E (1 e2*) is interesting. Although allowed on symmetry grounds it is not visible in normal emission spectra (figs. 2 and 3). In fact, there is no point group symmetry lower than CeVin which this band would be forbidden. We conjecture that the absence of band E in normal emission is due to a directional cross-section effect. Emission from the 1e29orbitals of benzene adsorbed on Pd has been investigated in some detail [ 2 11, and negligible intensity was found in directions near the ring plane normal. Significant intensity was observed only at angles > 30” off the surface (i.e. the ring plane) normal. It has to be added that there is evidence that benzene adsorbed on Pd surfaces is not distorted [ 22-241. On the other hand, in cases where distortion of the carbon ring skeleton is indicated (e.g. for benzene on Os( 000 1) [ 251) emission from the le,, derived molecular orbitals is detected in the normal direction. This suggests that distortion of the carbon ring skeleton is not causing the observed symmetry reduction of benzene in the presence of Na. We propose instead a change in the adsorption site of benzene to account for the symmetry reduction
CHEMICAL PHYSICS LETTERS
Volume 140, number 2
25 September 1987
Table 1 Correlation table for the molecular orbitals of benzene. Allowed and forbidden emission in the normal direction for respective polarisation components of the photon beam are indicated Dlh
C JV.0”
Cl” JGY
z
X,Y
2
Experimental band (eV)
ler,(x)
e,
+
-
e
+
-
A
5
2e2s(o)
e2
-
-
e
+
-
B
5.6
laz.(x) 2elU(o) lb,,(o) lb,.(o)
aI e1 bz b,
+ -
ai e a2 aI
+ -
+ +
C
7-10
-
+ _
2a,,(o)
aI
-
+
aI
_
+
D
11
lezs(o)
e2
-
-
e
+
-
E
13
from CbVto C3_. In this model, Na adatoms force the benzene molecules from the on-top positions into the threefold hollow sites. An illustration of a possible model arrangement is shown in fig. 4. If the Na atoms occupy threefold hollow sites, the coadsorbed benzene molecules are most naturally accommo-
Rh (111) +
NB
h”. 15.”
dated in threefold hollow sites as well. The three hydrogen atoms located on top of the Rh atoms in fig. 4 may be slightly bent away from the surface, thereby substantiating the C3V,0V symmetry imposed by the adsorption site. We expect that the 2alg orbital of benzene (band D), which is purely C-H derived [ 261, should reflect this out-of-plane bending dis-
+ Benzene P-PI
Fig. 3. Normal emission spectra of benzene on Rh( 111) precovered with 0.1 ML of Na for two photon angles of incidence (Y.
Fig. 4. Schematic model of a local arrangement in the benzene-Na coadsorbate layer. The benzene molecule is represented by its van der Waals size, and the radius of Na adatoms has been taken as lying between ionic and metallic. ov designate vertical mirror planes.
203
Volume 140, number 2
CHEMICAL PHYSICS LETTERS
tortion. The ARUPS spectra show that the intensities of band D are indeed those that are influenced most. In conclusion we wish to comment on the differences in adsorption behaviour between small molecules, such as CO, and benzene in the presence of alkali additives. Alkali coadsorption increases the CO-metal bond strength on most metal surfaces (see e.g. refs. [ l-l 0 3). This is generally attributed to increased metal d-CO 27c*backdonation. For benzene no such stabilization of the adsorption energy is observed. Anderson et al. [ 271 have calculated the benzene adsorption energy on a Pt, cluster as a functin of shifts of the Pt valence electron ionization potentials, which are taken to model electron donation from coadsorbed K. They find a decrease of the adsorption energy for moderate upward shifts of the Pt valence bands, and this is ascribed to reduced donation from the benzene x orbitals into the Pt valence band. These calculations are in qualitative agreement with what is observed in this study on Rh(ll1) and by Garfunkel et al. [17] on Pt(lll), and they emphasize the dominance of 7c donation bonding over backbonding into empty IC*orbitals for adsorbed benzene. The results of the present study suggest that the energetics of adsorption sites are modified by the alkali additive, so that benzene is stabilized on a site different from that on the bare surface. The observed symmetry reduction is imposed by the adsorption site, but possibly substantiated by out-of-plane C-H bending. The influence of Na on coadsorbed benzene is presumably mediated by both geometric and electronic effects. This experimental programme has been supported by the Fonds zur Forderung der Wissenschaftlichen Forschung of Austria and by the Austrian National Bank. We wish to acknowledge the staff of the BESSY synchrotron in Berlin for their technical support, and D.R. Lloyd, Trinity College, Dublin and J.C. Riviere, AERE Harwell for discussions and critical comments on the manuscript. References [ 1] H.P. Bonzel, J. Vacuum Sci. Technol. A2 (1984) 866.
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