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Benzene and ethylene chemisorbed on transition metals; The measurement of energy level shifts which accompany chemisorption
Surface Science 0 North-Holland
102 (1981) Publishing
45-55 Company
BENZENE AND ETHYLENE CHEMISORBED ON TRANSITION METALS; THE MEASUREMENT OF ENERGY LEVEL SHIFTS WHICH ACCOMPANY CHEMISORPTION S.R. KELEMEN and T.E. FISCHER Exxon Research and Engineering New Jersey 07036, USA
Company, Corporate Research Science Laboratories,
Received
for publication
8 April 1980; accepted
Linden,
8 July 1980
Ultraviolet photoemission spectra for ethylene chemisorbed on Ag(ll0) and benzene on Ag(ll0) and Ru(001) indicate adsorption with minimal molecular distortions. A survey of the photoemission data for ethylene and benzene similarly adsorbed on transition metals indicate that the relaxation shift in the binding energy of molecular orbitals not contributing to the chemisorption bond is essentially independent of the metal when obtained using the value of the work function when the surface is saturated with adsorbate. This observation is in agreement with a theoretical analysis where final state image charge screening is the dominant relaxation mechanism. These observations support the choice of the work function of the surface saturated with adsorbate as the approximation of the adsorption site potential.
1. Introduction When as gas adsorbs onto a metal surface, peaks are observed in the photo emitted electron energy distributions which can be identified with electrons ionized from orbitals of the adsorbed gas. The extent to which these energy levels are modifield with respect to the free molecule ionizations gives an indication of the state of the adsorbed molecule. The ionization energies or binding energies for these orbitals, E,,, are given by the Einstein relation: Eb = hv -&kin,
(1)
where hv is the photon energy and Ekin the kinetic energy of the emitted electrons. The binding energy is rigorously defined as the difference between the ionized and the neutral system and in the case of adsorbed gases on metals as Eb = (E - EF) + @ sample,
(2)
where (E - EF) is the energy difference between an observed peak and the Fermi level and @ the work function. In general, these ionization energies are smaller for 45
46
S.R. Kelernen, T.E. Fischer /Benzene and ethylene on transition metals
an adsorbed molecule than for a free molecule in the gas phase. The difference is called the relaxation shift, which reflects differences in the initial and final state energies of the gaseous and adsorbed molecule. In addition, one or several of the highest occupied orbitals of the adsorbed molecule can be displaced to a relative higher biding energy. This displacement is labelled a chemical shift and accompanies the combination of these orbitals with substrate orbitals and is partly responsible for the chemisorption bond [I]. Distortions in molecular geometry give rise to additional displacements [2,3]. The delineation of contributing factors which govern the observed peak positions is a topic of vital interest and has received considerable attention [4-l 91. It is usually observed that adsorbate peaks that are not directly involved in the chemisorption bond appear at fixed energy with respect to the Fermi level in the whole range of adsorbate density from zero to a monolayer even though the measured work function may vary. Use of the work function at the partial coverage corresponding to the measured spectrum results in an apparent ionization potential and relaxation shift that vary with coverage. This has caused controversy concerning a proper experimental prescription for approximation of the adsorption site potential used in reporting ionization energies [ 151. It recently has been proposed that the clean metal work function is the most appropriate value for this purpose [13]. Hagstrum, on the other hand, has argued that in simple chemisorption systems, the relevant quantity is the work function of the surface that is uniformly saturated with the chemisorbed molecules [4]. In this prescription for treating experimental data, it is seen that the potential of the adsorption site is approximated by the work function when the surface is saturated with adsorbate. We will adopt this prescription in the presentation of the results in this paper and will consider use of clean metal work functions for this purpose later in the discussion. The relaxation contribution to the ionization energy of an adsorbate is the simplest perturbation from the state of the free molecule and its definition rests on the choice of the vacuum potential at the adsorption site. It is possible to ascertain the dependence and sensitivity of the relaxation shift on substrates whose clean metal work function widely differ. In order to explore this question, we consider the adsorption of benzene and ethylene on transition metals. It is necessary to assure that the principal changes in ionization energies arise only from relaxation effects. We have, therefore, only considered spectra where the adsorbed hydrocarbon exhibits emission peaks which are similar in amplitude and energy spacing to those in the gas phase spectra; this occurs, for instance, when the bonding is primarily via the 77electrons and the molecule remains essentially undistorted, Only the highest occupied 7~level in ethylene and benzene molecule should experience changes in amplitude and energy position other than a uniform relaxation shift which we wish to isolate. Although a considerable amount of UPS data exists concerning acetylene adsorption, the latter is an unsuitable model gas for this analysis since it is known to undergo considerable distortion and changes in hybridization state [3,20&27]. Recent high resolution energy
S.R. Kelemen,
T.E. Fischer/Benzene
and ethylene on transition metals
41
loss measurements [28-301, and photoemission [25] results indicate adsorbed acetylene undergoes considerable distortion upon adsorption even on surfaces where this was not previously believed to occur 13 11. These results suggest reconsideration of other acetylene adsorption systems previously reported and weakens the conclusion of undistorted acetylene geometries based on early photoemission results [27,32]. Energy loss measurements for benzene on Ni(ll1) 133,341 and Pt(ll1) [33] provide evidence that the molecule does not rehybridize and is predominantly bonded with the ring parallel to the surface. The same conclusion can be reached from photoemission results of adsorbed benzene due to the visibility ofnumerous u derived levels [ 11. Photoemission results for ethylene adsorption on Ni(l11) at 80 K and Cu(ll1) surfaces indicate minimal molecular distortions upon adsorption [25]. This is in contrast to surfaces where molecuar distortions have been reported for ethylene [35,20.25,27] , consequently we must exclude distorted molecular geometries of ethylene from consideration in our analysis. Reexamination [41] of the UPS results of Demuth [2S] for ethylene adsorption on Ni(ll1) which was stimulated by high resolution electron energy loss measurements [42], considers the effect of slight (i.e. rotational) distortions. Felter and Weinberg [41] propose a relaxation shift of 1.6 eV upon taking the deformation into account; this value is consistant with conclusions to be reported in this work. The data reviewed and presented are the energy distributions of electrons that are emitted under irradiation with monochromatic light of photon energy hv (USUally hv = 21.2 eV from the He1 resonance or 40.8 eV from the He11 resonance). The work function, defined as the binding energy of electrons excited from the Fermi level, is obtained from the overall width AZ? of the energy distribution: cp= hu - AE + f A.&._ where 4 is the work function, hv the photon energy, AE the width measured from the energy at which the distribution at the Fermi edge has reached half its maximum height to the low-energy cut-off of the distribution and A&, is the energy broadening due to the finite resolution of the instrument. A.&,, can be obtained by comparing the experimental shape of the Fermi edge with its theoretical value. The UPS spectra reported from this laboratory were collected with a double pass cylindrical mirror analyser with no bias voltage applied to the sample. The photon beam is 75’ off the axis of the analyser and the sample normal between 20” to 30” from the analyser axis so as to collect electrons over a wide range of emission angles. The work function measured for the clean silver (110) surface agrees favorably with established values for silver (401. 2. Results Fig. 1 is the modification of the energy distributions of photoelectrons, AfV(E), obtained for a saturation exposure of C2H4 to Ag(l IO). We obtain a saturation
48
S.R. Kelemerz,
T.E. Fischer
I
I
/ Benzene I
and ethylene I
on transition
I
metals
I
hJ = 21 .2 (eV)
-14
I
-12
_l’O
_‘8
_‘6
_h
_$
I F
E-EF
(eV)
Fig. 1. Change in the photoelectron energy distribution upon adsorption, following a (20 L) exposure of ethylene to a clean Ag(l10) surface held at 170 K. The dashed curve represents clean silver attenuation.
value for the work function of 4.1 eV. This results in ionization energies for the ethylene orbitals that are smaller than those in the free molecule by the relaxation shift of 1.9 eV, as determined by eq. (2) using the work function of the surface saturated with ethylene. The remnant of the 77level of ethylene occurs over the emission from the silver d band. A replica of the negative of the silver emission allows us to determine a peak due to the 71level at -5.5 eV below the Fermi level. This corresponds to a 0.4 eV chemical shift toward higher binding energy for the 7-rlevel. Fig. 2 is a plot of the position of the second highest occupied level in ethylene, the saturation value of the work function and the relaxation shift for ethylene adsorbed on Ag(l lo), Cu foil [ 171, Fe foil [ 171 and Ni( 111) [l] plotted against the work function of the clean material. Although the position of the molecular orbital peak(s) with respect to the Fermi level and the work function change from metal to metal, the relaxation shift obtained from the sum of these two values remains constant. Fig. 3 contains the change in the photoelectron energy distribution resulting from benzene adsorption on Ru(OO1). We obtain a saturation coverage work function of 4.0 for Ru(OO1). Analysis of peak position indicate a 1.9 eV relaxation shift for benzene on Ru(001) and a chemical shift of 1.3 eV for the 77level. Fig. 4. is the change in the photoelectron distribution, AA$?‘), following a satu-
S.R. Kelemen,
T.E. Fischer/Benzene
and ethylene on transition metals
i
49
I
7.t I-
-I
\Ni 6 .( I-
5.t l-
2 ;
4.t IETHYLENE ADSORPTION 0 Saturation Work Function l Relaxation Shift A Peak Position (EF-E)
z s w
*
l
n
2 .( I-
n
1 .( Ii
I 4.0 CLEAN
-
I 5.0 METAL
WORK FUNCTION
-
(eV)
Fig. 2. A plot versus clean metal work function of the saturation value of the work function, the peak position of the second highest occupied molecular orbital of ethylene and the relaxation shift obtained using the saturation value of the work function.
ration exposure to benzene of a clean Ag(ll0) surface. Included in this figure is a replica of the negative of the clean silver emission. The work function at saturation is 3.8 eV resulting in a 1.7 eV relaxation shift. The peak near -3.5 eV has been observed for ethylene chemisorbed on Ag(ll0) and recently, Ag(ll1) using He11 radiation [36] as well as for pyridine chemisorbed on Ag(ll0) [37]. Since this peak has been observed with different hydrocarbons and in the case of ethylene different crystallographic orientation it is reasonable to search for a qualitative understanding of its origin in the electronic structure of silver. SCF-X SW calculations of four and
h9 = 21 .2 (eV1
Ru (OOl! 3OOK
207,
I
‘,
I
-10
\
I
-8
I -4
-6 E-EF
I -2
EF
(eV)
Fig. 3. Change in the photoelectron energy distribution a saturation exposure of benzene to a clean Ru(001) represents clean rut~eni~ln attenuation.
upon adsorption, UPS AN(E), following surface held at 300 K. The dashed curve
-hq = 21.2
(eV?
BENZENE Ag (110)
I
-12
I -10
1 -6
I -8 E-EF
Fig. 4. Change in the photoelectron a saturation exposure of benzene represents clean sitver attenuation.
I
-4
I -2
EF
eV
energy distribution to a clean, Ag(ll0)
upon adsorption, UPS AN(E), following surface held at 200 K. The dashed curve
S.R. Kelemen, T.E. Fischer /Benzene and ethylene on transition metals
51
six atom silver clusters are in satisfactory agreement with bulk energy band calculations and photoemission measurements [38]. Both cluster models exhibit small s and p derived orbital populations occurring at the Fermi energy and slightly above the large number of d-like states. It is possible that the levels immediately above the d-like states interact with the unoccupied TT*level of the hydrocarbon upon chemisorption [39]. The presence of the increased emission at -3.5 eV in the UPS difference curves may arise from this interaction and in this sense the peak has its origins in the modification of silver derived levels. We, therefore, hesitate to attribute this feature to the remnant of the TTlevel of benzene. We can identify an additional po-
A4
8.0
-
7.0 i
BENZENE ADSORPTION 0 Saturation Work Function H Relaxation Shift A Peak Position (EF-E)
5.0 5 5 CG “w 4.0 z w 3.0
I-
-A
2.0
1 .o I
I
4.0
5.0
CLEAN
METAL
WORK
I 6.0 FUNCTION
(eV)
Fig. 5. A plot versus clean metal work function of the saturation value of the work function, the peak position of the second highest occupied molecular orbital of benzene and the relaxation shift obtained using the saturation value of the work function.
52
S.R. Kelemen, T.E. Fischer /Benzene and ethylene on transition metals
sitive emission relative to the uniform attenuation of the silver emission near -4.0 eV. If we attribute this to the remnant of the TIlevel of benzene, we obtain a small chemical shift of approximately 0.2 eV. Fig. 5 contains a plot of the position with respect to the Fermi level of the third highest occupied benzene orbital, the saturation value of the work function, and the corresponding relaxation shift, determined using the saturation work function, versus the clean work function of the substrate for Ru(OOl), Ag(1 lo), Ir(lOO) [27], Ni(ll1) [l] andPt(lOO) [21]. Again, the position of the molecular orbital peak(s) with respect to the Fermi level and work function at saturation change from metal to metal, yet the relaxation shift remains constant. We observe that the saturation value of the work function for both chemisorbed ethylene and benzene tends to decrease with decreasing initial work function of the clean metal. Correspondingly, the peak movement of the non-interacting adsorbate levels tend to lower binding energy so as to keep constant the relaxation shift.
3. Discussion We have carefully chosen benzene and ethylene adsorption systems where the molecule is adsorbed with minimal molecular distortions in an attempt to ascertain the dependence and sensitivity of the measured relaxation shift using the value of the work function when the surface is saturated with adsorbate. We observe that the relaxation shift is essentially independent of the metal. The fact that the relaxation shift from the orbitals not involved in the chemisorption bond is essentially independent of the substrate can be rationalized. The existence of a relaxation shift indicates that the electric field acting on the emitted electron is modified by the polarization of the substrate metal. As stated by Lang and Williams [43] in the context of core ionization of chemisorbed atoms, “If the ionized atom is coupled sufficiently weakly to the metal and the passage of the screening charge from the metal is therefore very slow, significant spectral weight in the photoemission spectrum may be given to relaxation energies corresponding to an image-like distribution of screening charge while it is still in the metal”. Since we have limited our analysis to the case of molecules that are weakly coupled to the surface we approach conditions where image charge screening is predominant [9,10,44,45]. In the limit in which the hole surface separation is large compared to a chararacteristic screening length of the metal, the relaxation energy or screening energy is thought to arise from an image potential screening mechanism [9,10], which is independent of the nature of the metal at these distances. The leading term for the relaxation energy equals 3.6/Z in eV where 2 is the vertical distance in A of the hole from the imaging plane [9]. In this context the constancy of the relaxation shift for the adsorbate orbitals determined from the use of the adsorbate saturated work function conveys that the
S.R. Kelemen, T.E. Fischer/Benzene
and ethylene on transition metals
53
vertical distance of the adsorbate does not change appreciably (+0.2 A) from metal to metal. This result may have been anticipated since the type of binding of the adsorbate with the metal under the condition chosen is quite similar, namely, primary interaction via the 1~electrons with minimal molecular distortions. Figs. 2 and 5 show an average relaxation shift of 2.0 eV for ethylene and 1.8 eV shift for benzene implying comparable vertical distances for these adsorbates with an average vertical distance of 1.8 A for ethylene and 2.0 A for benzene. A very qualitative comparison shows these values to be remarkably similar to average carbon to metal distances in organometallic oletin [46] and arene [47] complexes based on X-ray crystallographic data. The olefin complexes based on forty compounds averaged a carbon metal distance of 2.1 A with a range between 1.9 and 2.2 A. Values from the few reported arene complexes indicate comparable bond lengths. The sole dynamical LEED calculation for an adsorbed hydrocarbon on a metal surface [48] is also generally supportive of the magnitude of these values. This entire avenue of analysis suggests that molecular relaxation processes are quite similar for gaseous ethylene and benzene as compared to the adsorbed case. The extramolecular relaxation energy caused upon adsorption can be principally accounted for based on a final state image charge screening mechanism. A very qualitative inspection of the geometric consequences of using the clean metal work function as the approximation to the adsorption site potential argues against its use in these adsorption systems. It is known for example that the interaction of ethylene and benzene with silver and copper is weaker than with nickel or platinum. We might anticipate as a consequence a tendency toward slightly shorter vertical distances for adsorption on nickel and platinum relative to1 silver and copper. If we assume that image charge screening relaxation is measured by the relaxation shift and that vertical distances can be approximated by knowing the relaxation shift from application of the classical image potential formula, calculations based on relaxation shifts using the clean metal work function predict larger vertical distances by about 1 .O A for the more strongly interacting systems. We see that the choice of the work function of the adsorbate saturated surface gives a reasonable result of comparable vertical distance. We, therefore, favor the use of the adsorbate saturated work function in the determination of relaxation shifts and ionization energies in these adsorbate systems.
4. Summary We observe that for benzene and ethylene adsorption on transition metals when the molecule remains undistorted and bonding involves mainly the highest rr levels, the relaxation shift is essentially independent of the metal when the work function of the surface saturated with adsorbate is used to approximate the adsorption site potential. Analysis in terms of screening via classical image potentials indicates that the vertical distance of the adsorbate does not change appreciably from metal to metal.
54
S.R. Kelemen,
T.E. Fischer/Benzene
and ethylene
on transition
metals
Acknowledgements The authors wish to thank A. Kaldor and I.E. Wachs for stimulating
discussions.
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
[lo] [ll] [12] [13]
[ 141 [ 151 [16] [17] [ 181 [ 191 [20] [21] [22] [23] [24] [25] [26] 1271 [28] [29] [ 301 [31] [32] [33] [34] [35] [36] [37] [38] [ 391 [40]
J.E. Demuth and D.E. Eastman, Phys. Rev. Letters 32 (1974) 1123. J.E. Demuth and D.E. Eastman, Phys. Rev. B13 (1976) 1523. J.E. Demuth, Chem. Phys. Letters 45 (1977) 12. H.D. Hagstrum, Surface Sci. 54 (1976) 197. A.M. Bradshaw, L.S. Cederbaum and W. Domcke, Struct. Bonding 24 (1975) 133. C.R. Brundle, Surface Sci. 48 (1975) 99. D.A. Shirley,Chem. Phys. Letters 16 (1972) 220. P.H. Citrin and T.D. Thomas, J. Chem. Phys. 57 (1972) 4466. J.W. Gadzuk, J. Vacuum Sci. Technol. 12 (1975) 289. J.W. Gadzuk, Phys. Rev. B14 (1976) 2267. A.M. Bradshaw and D. Menzel, Topics in Surface Chemistry (Plenum, New York, 1978). S. Evans, in: Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy (Heyden, London, 1978). J.Q. Broughton and D.L. Perry, Surface Sci. 74 (1978) 307. J. Kuppers, K. Wandelt and G. Ertl, Phys. Rev. Letters 43 (1979) 928. See discussion comments, in: Faraday Disc. Chem. Sot. 58 (1974). H. Stoh and A.B. Kunz, Chem. Phys. Letters 66 (1979) 531. K.Y. Yu, W.E. Spicer, I. Lindau, P. Pianetta and S.F. Lin, Surface Sci. 57 (1976) 157. K. Horn and A.M. Bradshaw, Solid State Commun. 30 (1979) 545. E.W. Plummer, in: The Physical Basis for Heterogeneous Catalysis, Eds. E. Drauglis and R.I. Jaffe (Plenum, New York, 1975). G. Broden and T.N. Rodin, Chem. Phys. Letters 40 (1976) 247. T.E. Fischer, S.R. Kelemen and H.P. Bonzel, Surface Sci. 64 (1977) 157. T.V. Vorburger, B.J. Waclawski and E.W. Plummer, Chem. Phys. Letters 46 (1977) 42. C. Brucker and T. Rhodin, Catalysis 47 (1977) 47. T.E. Fischer and S.R. Kelemen, Surface Sci. 74 (1978) 4j. J.E. Demuth, Surface Sci. 84 (1979) 315. E.W. Plummer, B.J. Waclawski and T.V. Vorburger, Chem. Phys. Letters 28 (1974) 510. G. Broden, T. Rhodin and W. Capehart, Surface Sci. 61 (1976) 143. J.E. Demuth and H. Ibach, Surface Sci. 85 (1979) 365. H. Ibach and S. Lehwald, J. Vacuum Sci. Technol. 15 (1978) 407. C. Backx and R.F. Willis, Chem. Phys. Letters 53 (1978) 471. J.E. Demuth, IBM J. Res. Develop. 22 (1978) 265. G. Broden, G. Gafner and H.P. Bonzel, Appl. Phys. 13 (1977) 333. S. Lehwald, 1-i. Ibach and J.E. Demuth, Surface Sci. 78 (1978) 577. J.C. Bertolini, G. Dalmai-Imelik and J. Rousseau, Surface Sci. 67 (1977) 478. T.E. Fischer and S.R. Kelemen, Surface Sci. 69 (1977) 485. T.E. Felter and W.H. Weinberg, P.A. Zhdan and G.K. Boreskov, private communication. S.R. Kelemen and A. Kaldor, Chem. Phys. Letters 73 (1980) 205. N. Rosch and D. Menzel, Chem. Phys. 13 (1976) 243. K.H. Johnson, private communication. J.A. Connor, M. Considine, I.H. Hillier and D. Briggs, J. Electron Spectrosc. Related Phenomena 12 (1977) 143, and references therein.
S.R. Kelemen, T.E. Fischer/Benzene [41] TX. Felter and W.H. Weinberg, [42] [43] 144) [45] [46] 1471 [48]
and ethylene on transition metals
Surface Sci., submitted. J.E. Demuth, Il. Iback and S. Lehwald, Phys. Rev. Letters 40 (1978) 1044. N.D. Lang and A.R. Williams Phys. Rev. B16 (1977) 2408. G.E. Laramore and W.J. Camp, Phys. Rev. B9 (19874) 3270. B. Gumhalter and D.M. Newns, Phys. Letters A57 (1976) 423. S.D. Itril and J.A. Ibers, Advan. Organometal. Chem. 14 (1976) 33. W.E. Silverthorn, Advan. Organometal. Chem. 13 (1975) 47. L.L. Kesmodel, R.C. Baetzold and G.A. Somorjai, Surface Sci. 66 (1977) 299.
55
102 (1981) Publishing
45-55 Company
BENZENE AND ETHYLENE CHEMISORBED ON TRANSITION METALS; THE MEASUREMENT OF ENERGY LEVEL SHIFTS WHICH ACCOMPANY CHEMISORPTION S.R. KELEMEN and T.E. FISCHER Exxon Research and Engineering New Jersey 07036, USA
Company, Corporate Research Science Laboratories,
Received
for publication
8 April 1980; accepted
Linden,
8 July 1980
Ultraviolet photoemission spectra for ethylene chemisorbed on Ag(ll0) and benzene on Ag(ll0) and Ru(001) indicate adsorption with minimal molecular distortions. A survey of the photoemission data for ethylene and benzene similarly adsorbed on transition metals indicate that the relaxation shift in the binding energy of molecular orbitals not contributing to the chemisorption bond is essentially independent of the metal when obtained using the value of the work function when the surface is saturated with adsorbate. This observation is in agreement with a theoretical analysis where final state image charge screening is the dominant relaxation mechanism. These observations support the choice of the work function of the surface saturated with adsorbate as the approximation of the adsorption site potential.
1. Introduction When as gas adsorbs onto a metal surface, peaks are observed in the photo emitted electron energy distributions which can be identified with electrons ionized from orbitals of the adsorbed gas. The extent to which these energy levels are modifield with respect to the free molecule ionizations gives an indication of the state of the adsorbed molecule. The ionization energies or binding energies for these orbitals, E,,, are given by the Einstein relation: Eb = hv -&kin,
(1)
where hv is the photon energy and Ekin the kinetic energy of the emitted electrons. The binding energy is rigorously defined as the difference between the ionized and the neutral system and in the case of adsorbed gases on metals as Eb = (E - EF) + @ sample,
(2)
where (E - EF) is the energy difference between an observed peak and the Fermi level and @ the work function. In general, these ionization energies are smaller for 45
46
S.R. Kelernen, T.E. Fischer /Benzene and ethylene on transition metals
an adsorbed molecule than for a free molecule in the gas phase. The difference is called the relaxation shift, which reflects differences in the initial and final state energies of the gaseous and adsorbed molecule. In addition, one or several of the highest occupied orbitals of the adsorbed molecule can be displaced to a relative higher biding energy. This displacement is labelled a chemical shift and accompanies the combination of these orbitals with substrate orbitals and is partly responsible for the chemisorption bond [I]. Distortions in molecular geometry give rise to additional displacements [2,3]. The delineation of contributing factors which govern the observed peak positions is a topic of vital interest and has received considerable attention [4-l 91. It is usually observed that adsorbate peaks that are not directly involved in the chemisorption bond appear at fixed energy with respect to the Fermi level in the whole range of adsorbate density from zero to a monolayer even though the measured work function may vary. Use of the work function at the partial coverage corresponding to the measured spectrum results in an apparent ionization potential and relaxation shift that vary with coverage. This has caused controversy concerning a proper experimental prescription for approximation of the adsorption site potential used in reporting ionization energies [ 151. It recently has been proposed that the clean metal work function is the most appropriate value for this purpose [13]. Hagstrum, on the other hand, has argued that in simple chemisorption systems, the relevant quantity is the work function of the surface that is uniformly saturated with the chemisorbed molecules [4]. In this prescription for treating experimental data, it is seen that the potential of the adsorption site is approximated by the work function when the surface is saturated with adsorbate. We will adopt this prescription in the presentation of the results in this paper and will consider use of clean metal work functions for this purpose later in the discussion. The relaxation contribution to the ionization energy of an adsorbate is the simplest perturbation from the state of the free molecule and its definition rests on the choice of the vacuum potential at the adsorption site. It is possible to ascertain the dependence and sensitivity of the relaxation shift on substrates whose clean metal work function widely differ. In order to explore this question, we consider the adsorption of benzene and ethylene on transition metals. It is necessary to assure that the principal changes in ionization energies arise only from relaxation effects. We have, therefore, only considered spectra where the adsorbed hydrocarbon exhibits emission peaks which are similar in amplitude and energy spacing to those in the gas phase spectra; this occurs, for instance, when the bonding is primarily via the 77electrons and the molecule remains essentially undistorted, Only the highest occupied 7~level in ethylene and benzene molecule should experience changes in amplitude and energy position other than a uniform relaxation shift which we wish to isolate. Although a considerable amount of UPS data exists concerning acetylene adsorption, the latter is an unsuitable model gas for this analysis since it is known to undergo considerable distortion and changes in hybridization state [3,20&27]. Recent high resolution energy
S.R. Kelemen,
T.E. Fischer/Benzene
and ethylene on transition metals
41
loss measurements [28-301, and photoemission [25] results indicate adsorbed acetylene undergoes considerable distortion upon adsorption even on surfaces where this was not previously believed to occur 13 11. These results suggest reconsideration of other acetylene adsorption systems previously reported and weakens the conclusion of undistorted acetylene geometries based on early photoemission results [27,32]. Energy loss measurements for benzene on Ni(ll1) 133,341 and Pt(ll1) [33] provide evidence that the molecule does not rehybridize and is predominantly bonded with the ring parallel to the surface. The same conclusion can be reached from photoemission results of adsorbed benzene due to the visibility ofnumerous u derived levels [ 11. Photoemission results for ethylene adsorption on Ni(l11) at 80 K and Cu(ll1) surfaces indicate minimal molecular distortions upon adsorption [25]. This is in contrast to surfaces where molecuar distortions have been reported for ethylene [35,20.25,27] , consequently we must exclude distorted molecular geometries of ethylene from consideration in our analysis. Reexamination [41] of the UPS results of Demuth [2S] for ethylene adsorption on Ni(ll1) which was stimulated by high resolution electron energy loss measurements [42], considers the effect of slight (i.e. rotational) distortions. Felter and Weinberg [41] propose a relaxation shift of 1.6 eV upon taking the deformation into account; this value is consistant with conclusions to be reported in this work. The data reviewed and presented are the energy distributions of electrons that are emitted under irradiation with monochromatic light of photon energy hv (USUally hv = 21.2 eV from the He1 resonance or 40.8 eV from the He11 resonance). The work function, defined as the binding energy of electrons excited from the Fermi level, is obtained from the overall width AZ? of the energy distribution: cp= hu - AE + f A.&._ where 4 is the work function, hv the photon energy, AE the width measured from the energy at which the distribution at the Fermi edge has reached half its maximum height to the low-energy cut-off of the distribution and A&, is the energy broadening due to the finite resolution of the instrument. A.&,, can be obtained by comparing the experimental shape of the Fermi edge with its theoretical value. The UPS spectra reported from this laboratory were collected with a double pass cylindrical mirror analyser with no bias voltage applied to the sample. The photon beam is 75’ off the axis of the analyser and the sample normal between 20” to 30” from the analyser axis so as to collect electrons over a wide range of emission angles. The work function measured for the clean silver (110) surface agrees favorably with established values for silver (401. 2. Results Fig. 1 is the modification of the energy distributions of photoelectrons, AfV(E), obtained for a saturation exposure of C2H4 to Ag(l IO). We obtain a saturation
48
S.R. Kelemerz,
T.E. Fischer
I
I
/ Benzene I
and ethylene I
on transition
I
metals
I
hJ = 21 .2 (eV)
-14
I
-12
_l’O
_‘8
_‘6
_h
_$
I F
E-EF
(eV)
Fig. 1. Change in the photoelectron energy distribution upon adsorption, following a (20 L) exposure of ethylene to a clean Ag(l10) surface held at 170 K. The dashed curve represents clean silver attenuation.
value for the work function of 4.1 eV. This results in ionization energies for the ethylene orbitals that are smaller than those in the free molecule by the relaxation shift of 1.9 eV, as determined by eq. (2) using the work function of the surface saturated with ethylene. The remnant of the 77level of ethylene occurs over the emission from the silver d band. A replica of the negative of the silver emission allows us to determine a peak due to the 71level at -5.5 eV below the Fermi level. This corresponds to a 0.4 eV chemical shift toward higher binding energy for the 7-rlevel. Fig. 2 is a plot of the position of the second highest occupied level in ethylene, the saturation value of the work function and the relaxation shift for ethylene adsorbed on Ag(l lo), Cu foil [ 171, Fe foil [ 171 and Ni( 111) [l] plotted against the work function of the clean material. Although the position of the molecular orbital peak(s) with respect to the Fermi level and the work function change from metal to metal, the relaxation shift obtained from the sum of these two values remains constant. Fig. 3 contains the change in the photoelectron energy distribution resulting from benzene adsorption on Ru(OO1). We obtain a saturation coverage work function of 4.0 for Ru(OO1). Analysis of peak position indicate a 1.9 eV relaxation shift for benzene on Ru(001) and a chemical shift of 1.3 eV for the 77level. Fig. 4. is the change in the photoelectron distribution, AA$?‘), following a satu-
S.R. Kelemen,
T.E. Fischer/Benzene
and ethylene on transition metals
i
49
I
7.t I-
-I
\Ni 6 .( I-
5.t l-
2 ;
4.t IETHYLENE ADSORPTION 0 Saturation Work Function l Relaxation Shift A Peak Position (EF-E)
z s w
*
l
n
2 .( I-
n
1 .( Ii
I 4.0 CLEAN
-
I 5.0 METAL
WORK FUNCTION
-
(eV)
Fig. 2. A plot versus clean metal work function of the saturation value of the work function, the peak position of the second highest occupied molecular orbital of ethylene and the relaxation shift obtained using the saturation value of the work function.
ration exposure to benzene of a clean Ag(ll0) surface. Included in this figure is a replica of the negative of the clean silver emission. The work function at saturation is 3.8 eV resulting in a 1.7 eV relaxation shift. The peak near -3.5 eV has been observed for ethylene chemisorbed on Ag(ll0) and recently, Ag(ll1) using He11 radiation [36] as well as for pyridine chemisorbed on Ag(ll0) [37]. Since this peak has been observed with different hydrocarbons and in the case of ethylene different crystallographic orientation it is reasonable to search for a qualitative understanding of its origin in the electronic structure of silver. SCF-X SW calculations of four and
h9 = 21 .2 (eV1
Ru (OOl! 3OOK
207,
I
‘,
I
-10
\
I
-8
I -4
-6 E-EF
I -2
EF
(eV)
Fig. 3. Change in the photoelectron energy distribution a saturation exposure of benzene to a clean Ru(001) represents clean rut~eni~ln attenuation.
upon adsorption, UPS AN(E), following surface held at 300 K. The dashed curve
-hq = 21.2
(eV?
BENZENE Ag (110)
I
-12
I -10
1 -6
I -8 E-EF
Fig. 4. Change in the photoelectron a saturation exposure of benzene represents clean sitver attenuation.
I
-4
I -2
EF
eV
energy distribution to a clean, Ag(ll0)
upon adsorption, UPS AN(E), following surface held at 200 K. The dashed curve
S.R. Kelemen, T.E. Fischer /Benzene and ethylene on transition metals
51
six atom silver clusters are in satisfactory agreement with bulk energy band calculations and photoemission measurements [38]. Both cluster models exhibit small s and p derived orbital populations occurring at the Fermi energy and slightly above the large number of d-like states. It is possible that the levels immediately above the d-like states interact with the unoccupied TT*level of the hydrocarbon upon chemisorption [39]. The presence of the increased emission at -3.5 eV in the UPS difference curves may arise from this interaction and in this sense the peak has its origins in the modification of silver derived levels. We, therefore, hesitate to attribute this feature to the remnant of the TTlevel of benzene. We can identify an additional po-
A4
8.0
-
7.0 i
BENZENE ADSORPTION 0 Saturation Work Function H Relaxation Shift A Peak Position (EF-E)
5.0 5 5 CG “w 4.0 z w 3.0
I-
-A
2.0
1 .o I
I
4.0
5.0
CLEAN
METAL
WORK
I 6.0 FUNCTION
(eV)
Fig. 5. A plot versus clean metal work function of the saturation value of the work function, the peak position of the second highest occupied molecular orbital of benzene and the relaxation shift obtained using the saturation value of the work function.
52
S.R. Kelemen, T.E. Fischer /Benzene and ethylene on transition metals
sitive emission relative to the uniform attenuation of the silver emission near -4.0 eV. If we attribute this to the remnant of the TIlevel of benzene, we obtain a small chemical shift of approximately 0.2 eV. Fig. 5 contains a plot of the position with respect to the Fermi level of the third highest occupied benzene orbital, the saturation value of the work function, and the corresponding relaxation shift, determined using the saturation work function, versus the clean work function of the substrate for Ru(OOl), Ag(1 lo), Ir(lOO) [27], Ni(ll1) [l] andPt(lOO) [21]. Again, the position of the molecular orbital peak(s) with respect to the Fermi level and work function at saturation change from metal to metal, yet the relaxation shift remains constant. We observe that the saturation value of the work function for both chemisorbed ethylene and benzene tends to decrease with decreasing initial work function of the clean metal. Correspondingly, the peak movement of the non-interacting adsorbate levels tend to lower binding energy so as to keep constant the relaxation shift.
3. Discussion We have carefully chosen benzene and ethylene adsorption systems where the molecule is adsorbed with minimal molecular distortions in an attempt to ascertain the dependence and sensitivity of the measured relaxation shift using the value of the work function when the surface is saturated with adsorbate. We observe that the relaxation shift is essentially independent of the metal. The fact that the relaxation shift from the orbitals not involved in the chemisorption bond is essentially independent of the substrate can be rationalized. The existence of a relaxation shift indicates that the electric field acting on the emitted electron is modified by the polarization of the substrate metal. As stated by Lang and Williams [43] in the context of core ionization of chemisorbed atoms, “If the ionized atom is coupled sufficiently weakly to the metal and the passage of the screening charge from the metal is therefore very slow, significant spectral weight in the photoemission spectrum may be given to relaxation energies corresponding to an image-like distribution of screening charge while it is still in the metal”. Since we have limited our analysis to the case of molecules that are weakly coupled to the surface we approach conditions where image charge screening is predominant [9,10,44,45]. In the limit in which the hole surface separation is large compared to a chararacteristic screening length of the metal, the relaxation energy or screening energy is thought to arise from an image potential screening mechanism [9,10], which is independent of the nature of the metal at these distances. The leading term for the relaxation energy equals 3.6/Z in eV where 2 is the vertical distance in A of the hole from the imaging plane [9]. In this context the constancy of the relaxation shift for the adsorbate orbitals determined from the use of the adsorbate saturated work function conveys that the
S.R. Kelemen, T.E. Fischer/Benzene
and ethylene on transition metals
53
vertical distance of the adsorbate does not change appreciably (+0.2 A) from metal to metal. This result may have been anticipated since the type of binding of the adsorbate with the metal under the condition chosen is quite similar, namely, primary interaction via the 1~electrons with minimal molecular distortions. Figs. 2 and 5 show an average relaxation shift of 2.0 eV for ethylene and 1.8 eV shift for benzene implying comparable vertical distances for these adsorbates with an average vertical distance of 1.8 A for ethylene and 2.0 A for benzene. A very qualitative comparison shows these values to be remarkably similar to average carbon to metal distances in organometallic oletin [46] and arene [47] complexes based on X-ray crystallographic data. The olefin complexes based on forty compounds averaged a carbon metal distance of 2.1 A with a range between 1.9 and 2.2 A. Values from the few reported arene complexes indicate comparable bond lengths. The sole dynamical LEED calculation for an adsorbed hydrocarbon on a metal surface [48] is also generally supportive of the magnitude of these values. This entire avenue of analysis suggests that molecular relaxation processes are quite similar for gaseous ethylene and benzene as compared to the adsorbed case. The extramolecular relaxation energy caused upon adsorption can be principally accounted for based on a final state image charge screening mechanism. A very qualitative inspection of the geometric consequences of using the clean metal work function as the approximation to the adsorption site potential argues against its use in these adsorption systems. It is known for example that the interaction of ethylene and benzene with silver and copper is weaker than with nickel or platinum. We might anticipate as a consequence a tendency toward slightly shorter vertical distances for adsorption on nickel and platinum relative to1 silver and copper. If we assume that image charge screening relaxation is measured by the relaxation shift and that vertical distances can be approximated by knowing the relaxation shift from application of the classical image potential formula, calculations based on relaxation shifts using the clean metal work function predict larger vertical distances by about 1 .O A for the more strongly interacting systems. We see that the choice of the work function of the adsorbate saturated surface gives a reasonable result of comparable vertical distance. We, therefore, favor the use of the adsorbate saturated work function in the determination of relaxation shifts and ionization energies in these adsorbate systems.
4. Summary We observe that for benzene and ethylene adsorption on transition metals when the molecule remains undistorted and bonding involves mainly the highest rr levels, the relaxation shift is essentially independent of the metal when the work function of the surface saturated with adsorbate is used to approximate the adsorption site potential. Analysis in terms of screening via classical image potentials indicates that the vertical distance of the adsorbate does not change appreciably from metal to metal.
54
S.R. Kelemen,
T.E. Fischer/Benzene
and ethylene
on transition
metals
Acknowledgements The authors wish to thank A. Kaldor and I.E. Wachs for stimulating
discussions.
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55