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Orbital resonances in the chemisorption of oxygen on an aluminum (100) surface
Volume 49, number 1
ORBlTAL
CHEMICAL PHYSICS LETTERS
RESONANCES
ON AN ALUMINUM
(100)
IN THE CHEMISORPTION
13uly 1977
OF OXYGEN
SURFACE
R.P. MESSMER and D.R. SALAHUB* Genera1 Electrìc Corporate Research and Development,
Schenectad_v, New York 12301, USA
Received 22 April 1977
Results of extensive Xar cluster model calculations are presented. Clusters of 5.9 and 25 atoms representing a (100) surface of aluminum have been used to study the chemisorption of oxygen atoms. Tor oxygen atoms incorporated in the surface layer, the present calculations predict structure in the electronic density of states, attributable to 0 2p character, at a 3.5,7.0 and 9.5 eV below the Fermi level. Comparisons with other theoretical work as wel1 as recent photoemission experiments are made.
The choice of aluminum as a substrate for the theoretical investigation of chernisorption was motivated by four factors. First, as aluminum is traditionally thought of as a prime example of a free electron metal, it provides a stringent test of the cluster approach employed here. Second, a comparison with prevïous theoretical work [1,2], some of which has employed a solid state viewpoint - namely the atom-jellium model [l], can be made. Third, unlike the transition metals, only s and p electrons need to be considered for aluminum, hence larger clusters can be conveniently treated and the convergente of the results as a function of cluster size can be studied. Fourth, recent and on-going photoemission work for aluminum and for oxygen chemisorption on aiuminum [3,4] allows a comparïson with experiment and the possibility of a fruïtful interplay between theory and experiment. The theoretical method used in the present study to obtain the electronic structures of the clusters is the SCF-Xa scattered wave method [5,6 J_ Tlüs method has previously been used to investigate the electronie structures of clusters of metal atoms and adsorbates ïnteracting with metal clusters [7-101; however, the present calculations consider the largest such clusters studied to date. The calculations were carried out in the muffm-tin approximation with the alumí-
* Present address: Departement de Chimie, Université de Montreal, Québec H3C 3Vl, Canada.
num atomic sphere radii taken as half the Al-AI nearest neighbor distance of bulk aluminum and with the a’s of the Slater statistical exchange approximation taken from the tabulation of Schwarz [ 113. The clusters of 5,9 and 25 Al atoms used to represent the (100) surface of the metal exhibit Cav symmetry. The 25 atom cluster includes up to ttird neaiest neighbors of an adsorbate atom situated in the central4-fold site of the first surface plane. It consists of 12 atoms in tlüs plane, nine in the second plane and four in the third plane. For the five and nine atom clusters, calculations were carried out for an oxygen atom above the 4-fold hole site at three distances from the fïrst atom plane, namely at ZO = O.O.2.0 and 4.0 bohr. For the 25 atom cluster, calculations for Zo = 0.0 and 2.0 bohr were performed for the 4-fold site. The Zo = 0.0 calcuiations represent a mot!el for the incorporation of oxygen into the fïrst atomic layer of aluminum. Detailed results of these calculations as wel1 as for other aluminum clusters and for hydrogen chemisorption on aluminum wil1 be presented elsewhere [13]. In the present letter we restrict ourselves to reporting some key results for the aluminumoxygen system. The convergente of the electronic structure of the aluminum clusters as a function of cluster size wil1 not be discussed here as it has been taken up in detail elsewhere [12,13]. Suffice it to say that in so far as the distribution of energy levels and character of orbitals 59
Volume 49, number 1
CHEMICAL PHYSICS LETTERS
is concerned, the cluster of 25 atoins represents a very reasonable description of the local electronic structure of the substrate. In order to compare results obtained from cluster calculations with those obtained by solid state approaches or with experiment, it is convenient to construct density of states (DOS) curves of various types for the clusters, as previously described [10,12], by replacing each discrete energy leve1 of the cluster by a
gaussian. For al1 of the aluminum calculations the gaussian width parameter Ras been taken as o = 0.05 rydberg. Curves whïch display the changes in density of states (ADOS) upon chemisorption are quite instructive and are constructed by performing a point by point subtraction of the DOS curves obtained for the individbal clusters, i.e. Al?*+ 0 and Al,, , aiter aligning the Fermi levels (energies of highest occupied orbitals). In fig. 1, ADOS curves are presented for the case of an oxygen atom situated at three distances (Zo = 4.0, 2.0 and 0.0 bohr) above the four-fold symmetrie hole site of a cluster of five aluminum atoms. The structure in the ADOS curves arises mainly from t:he 2p orbitals
1 July 1977
of oxygen as a result of their interaction with the aluminum. Adsorbate atomic levels broadened by the adsorbate-substrate interaction are commonly referred to as “orbital resonances” [1,2]. At Z, = 4.0 bohr a rather welldefïned orbital resonance is found at 2 2 eV below the Fermi leve1 (Ef). This result is in substantial agreement with the ADOS curve derived from the atom-jellium model [l] as wel1 as the result deduced for Zo = 3.0 from a previous cluster calculation for oxygen on aluminum [2]. A similar orbital resonance is also observed for ZO = 4.0 on the 9-atom cluster. Hence, for large adsorbate-substrate separations al1 of the models mentioned above give essentially the same result which is quite insensitive to the detailed nature of the substrate. For short distances however, and particularly for Zo = 0.0 not considered by the investigators in refs. [1,2], increased structure in the ADOS occurs and a simple orbital resonance no longer results. Also for shorter distances the ADOS curves are quite sensitive to the substrate model chosen. Thus it is important to use a reasonably large cluster; in the present case, 25 atoms. It should be pointed out that the results in fig. 1 are based on X~Yorbital energïes. However, the Slater transition state energies [5] which should be used in a comparison with photoemïssion results have been calculated for al1 the orbitals and the ADOS curves derived therefrom
are almost identical to those of fig. 1.
Peaks in the ADOS curves derived from the transition states exhibit a ma‘rimum shift of 0.3 eV (with respect to Ef) from those shown in fig. 1. Therefore, in the following, only the results based on orbital energies need be discussed. The ADOS curves for the Al,, cluster with an oxygen atom at Zo - 2.0 and 0.0 bohr above the center of the (100) face are presented in fig. 2. The ADOS curve for Zo = 2.0 bohr is very similar to the corresponding curve for the 5-atom cluster shown in fig. 1. The curves for 2, = 0.0 bohr are notably different however, especially as regards the appearance of a thïrd peak at eg.5
-5 EWERGY (eV)
Fig. 1. ADOS curves for Als cluster with an oxygen atom at and 0.0 bah; abcve the midpoint of the plane dete:minïng the (100) face.
zc = 4.0.2.0
60
eV below Ef for the Alz5 + 0 case. The 5-atom
cluster is incapable of reproducing this structure at -9.5 eV since it yields a band width which is too narrow [12,13] and thus contributes no states in this region with which the oxygen atom can interact. Thus the five atom cluster mïght be adequate to roughly represent a situation where the oxygen atom is at Zo = 2.0 bohr, but it is inadequate to represent the situa-
Volume 49, number 1
AOOS (Al,,
CHEMICAL PHYSICS LETTERS
+ 0 1
-13
ENEi-&aVl
0
EF Fig. 3. (a) Photoemission spectra for oxygen chemisorbed on -10
0 EF
-5 ENERBY (rV)
Fig. 2. ADOS curves for Al25 cluster with an oxygen atom at ZO = 2.0 and 0.0 bohr above the (100) face of the cluster.
tion where the atom penetrates the lattice (i_e_ 2, = bohr). This conclusion is important because recent experimental evidente 13,141 suggests rather strongly that oxygen atoms are incorporated into the aluminum lattice, even at low exposures. Fortunately, rather recently some photoemission results have been published for oxygen chemisorption on aluminum [3,4]. Although these experiments have 0.0
been for evaporated polycrystalline films rather than Íor single crystal faces, they do allow some point of comparison between theory and experiment. In fig. 3a the results of Flodstr8m et al. using He II(40.8 eV) radiation are shown for two different exposures, 1 L and 5 L (1 L = 1 Langmuir = 10e6 torr s). Notice the increase in the intensities of the two peaks at z= 7 eV and = 9-5 eV below Ef with increased exposure to oxygen. Photoemission intensity differente curves whïch could be compared
to our calculated
curves were not generated
by the experimenters.
ADOS Thus
aluminum at two different oxygen exposures. The intensity of the curve for the 5 L exposure has been reduced by 112 to aid in displaying the curves. Data is from ref. [ 3 ]_ (b) Calculated density of states (DOS) curve for a cluster of Al25 wïth an
oxygen atom incorporated in the surface plane.
effects can modify dramatically the intensities based on the simple DOS curve. These effects have recently been shown to be accessible withïn the XaSW framework [15], and wil1 undoubtedly be included in future calculations. However, in the spirit of previous theoretical work which has also neglected these effects, we present the comparison in fig. 3. The similarity between the calculated curve and experimental curves, especially the lower coverage result, is quite striking. The fact that the only calculated curves wlüch exhibit
the two peaks at -9.5 eV and -7.0 eV are those for which the oxygen is incorporated in the lattice, is strong support for the oxygen penetratiag the lattice. Besides the result shown in fig. 3b for oxygen at 2, = 0.0, similar results showing these two peaks have been obtained for fïve oxygen atoms at 2, = 0.0 and for four oxygen atoms at octahedral sites inside the Al,, cluster 1131 corresponding to incorporation below the
the calculated shown in fig. 3b. An Xoz-DOS curve, even though based
surface [ 14]_ It should be mentioned that the atom-jellium model [l] bas been approximately corrected in order to investigate lattice penetration [17]. The chief result is
on transition state energies which take account of certain reiaxation effects attentant to the photoemission process, is not strictly comparable to an experimental photoemission curve. This is because matrix element
that as the oxygen atom approaches the surface and is moved into the metal, the 0 2p resonance quickly drops from 2 eV below Ef (previous result [l] ) to an interior limit of = 10 eV below Ep Evïdently no struc-
in order to provïde a comparison
with experiment, DO3 curve for A12,0 (Zo = 0.0)is
61
ture in the orbital resonance is found, which can accomrnodate the structure found experimentally in the region 5-10 eV below Ef. We have also calculated the Al 2p leve1 shifts
which take place on interaction with an oxygen atom, using the AIz clusters. The results are shown ín table 1_ Aga& the results for lattice incorporation (2, = 0.0) are consistent with experimental fìndings 1161, for less than a monolayer coverage, In our model each aiuminurn atom Iisted in table 1 has a single oxygen atom as a nearest neighbor. As the number of oxygen atoms increases the core shifts will also increase as we have found for the rnodels involvhrg four or five oxygen atoms mentioned above. The experimental shift of 1.3 eV is for intermediate exposure of oxygen (much less than a monolayer) and so it is reasonable to assume that most of the surface aluminum atoms have only one adjacent oxygen and hence the comparison made is valid. At higher exposures a sbift of 2.6 eV is observed which corresponds to an ahrmi-
num oxïde type surface. Our discussion thus far has centered on the caleulated peaks at w 7 eV and 9.5 eV below Ef- However, +thereis a third peak at -3.5 eV in the ADOS of fig. 2 which also appears in projected DOS curves in which 0 2p character only is projected out of the total DOS of fig. 3b. Hence this third peak arises from 0 2p character according to the calculations. In fig. 3a there appears to be some change in the photoemission intensity in this energy region as a function of oxygen exposure. The change is however niuch less dramatic than that observed below -5 ev. Moreover further re-
cent photoemission studies have appeared [16-183, which havo studied much higher exposures. Even at 100 L it appears that a complete monolayer has not íormed. At these hïgher exposures the oxygen strucTable 1 Calcufated Al 2p leve1sbifts due to chemisorption of oxygen (in ev) AtOm
a,
AI (1) Al (4)
20
=
0.30 0.65
2.0
zo = 0.0
1.21 1.05
Expt. b) 1.30
atom just beneath the oxygen atom; Al (4) - one of the four atoms on the surfacc plane closest to the oxygen
a) Al (11 --
atom. b) Ref. [ 161.
62
1 July 1977
CHEMICAL PHYSICS LETTERS
Volume 49, number 1
ture below -5 eV is cïearfy seen above a very small Al background and there is no indication of oxygen related structure above -5 ev.
It should be emphasized however, that the experimental results have been obtained using a rather limited number of photon energies and also a very hmited number of incident Iight and efectron cohection angles. There is ample theoretical [í5] and experimental [lg] evidente in the literature for the extreme sensitivity of surface photoemission results to aIl of these parameters_ There are thus two possible situations which may be present for the part of the spectrum above -5 ev. First, there may be no (or very little) oxygen related structure in this region, in which case our theoretical model is inadequate in this respect. Second, as systematic angle and photon energy dëpendent photoe~ssion studies have not yet been carried out, the structure may not yet have been detected in a convincing manner. The choice between these two possibilities clearly requires further study both experi-
mentally
and theoretically. One thing is clear, the aluminum f oxygen system
is not so simple as it had at first appeared. We believe that a close interaction between theory and experiment will be necessary in order to fully understand this system.
References [ 11 N.D. Lang and A.R. WiJIiams,Phys. Rev. Letters 34 (1975) 531. 121 J. Hsrris and G.S. Painter, Phys. Rev. Letters 36 (1976) 151. [3] S.A. Flodström, L.G. Petersson and S.B.M. Hagström, J. Vacuurii Sci. TechnoI. 13 (1976)
280.
[4] S.A. Flodström, L.G. Petersson and S.B.Pol.Hagström, SoIid State Commun. 19 (1976) 257. [SI J.C. Slater, In: Advances In quantum chemistry, Vol. 6,ed. P_-O,Löwdin (Acsdemic Press, New York, 1972) p.1. [61 K.H. Johnson, in: Advances in quantum chemistrv, Vol. 7, ed. P._O. Löwdin (Aeademic Press, New York, 1973) p- 143. [7] K.H. Johnson and R.F. Messmer, J. Vacuum Sci. Technol. 11 (1974) 236. [8] I.P. Batra and 0. Robaux. 3. Vacuum Sci. Techm% 11 (1974) 236. 191 S.J. Niemczyk, J_ Vacmrm SeI_TeehnoL 12 (1975) 246. [IOj RP. Messmer,S.K. Rnudson,K.H. Johnsan, J.B. Diamondand C.Y. Yang, Phys Rev. 813 (1976) 1396. [! 11 K. Schwarz, Phys. Rev. BS (1972) 2466.
Volume 49, number 1
CHEMICAL PHYSICS LETTERS
R.P. Mesrmer and D.R. Salahb. Intern. J. Quantum Chem. 10s (1976) 147. [ 131 D.R. Salahub and R.P. Messmer, to be publïshed. [ 141 P.H. Dawson, Intern. J. Mass Spectrom. Ion Phys. 16 (1975) 269; Swface Sci. 57 (1976) 229. [15] J.W. Davenport, Phys. Rev. Letters 36 (1976) 945; Ph. D. Thesis, Physics Department, University of Pennsylvanïa (1976). [12]
1 July 1977
[16] S.A. Flodström, R.Z. Bachrach, R.S. Bauer and S.B.M. Hagström, Phys. Rev. Letters 37 (1976) 1282. 1171 K.Y. Yu, J.N. Miller, P. Chye, W.E. Spicer, N.D. Lang and A.R. Williams, Phys. Rev. Bl4 (1976) 1446. [ 181 C. Martinsson. L.G. Petersson, S.A. Flodström and S.B.M. Hagstrom, to be published. [ 191 B. Feuerbacher and R.F. Willis, Phys. Rev. Letters 36 (1976) 1339.
63
ORBlTAL
CHEMICAL PHYSICS LETTERS
RESONANCES
ON AN ALUMINUM
(100)
IN THE CHEMISORPTION
13uly 1977
OF OXYGEN
SURFACE
R.P. MESSMER and D.R. SALAHUB* Genera1 Electrìc Corporate Research and Development,
Schenectad_v, New York 12301, USA
Received 22 April 1977
Results of extensive Xar cluster model calculations are presented. Clusters of 5.9 and 25 atoms representing a (100) surface of aluminum have been used to study the chemisorption of oxygen atoms. Tor oxygen atoms incorporated in the surface layer, the present calculations predict structure in the electronic density of states, attributable to 0 2p character, at a 3.5,7.0 and 9.5 eV below the Fermi level. Comparisons with other theoretical work as wel1 as recent photoemission experiments are made.
The choice of aluminum as a substrate for the theoretical investigation of chernisorption was motivated by four factors. First, as aluminum is traditionally thought of as a prime example of a free electron metal, it provides a stringent test of the cluster approach employed here. Second, a comparison with prevïous theoretical work [1,2], some of which has employed a solid state viewpoint - namely the atom-jellium model [l], can be made. Third, unlike the transition metals, only s and p electrons need to be considered for aluminum, hence larger clusters can be conveniently treated and the convergente of the results as a function of cluster size can be studied. Fourth, recent and on-going photoemission work for aluminum and for oxygen chemisorption on aiuminum [3,4] allows a comparïson with experiment and the possibility of a fruïtful interplay between theory and experiment. The theoretical method used in the present study to obtain the electronic structures of the clusters is the SCF-Xa scattered wave method [5,6 J_ Tlüs method has previously been used to investigate the electronie structures of clusters of metal atoms and adsorbates ïnteracting with metal clusters [7-101; however, the present calculations consider the largest such clusters studied to date. The calculations were carried out in the muffm-tin approximation with the alumí-
* Present address: Departement de Chimie, Université de Montreal, Québec H3C 3Vl, Canada.
num atomic sphere radii taken as half the Al-AI nearest neighbor distance of bulk aluminum and with the a’s of the Slater statistical exchange approximation taken from the tabulation of Schwarz [ 113. The clusters of 5,9 and 25 Al atoms used to represent the (100) surface of the metal exhibit Cav symmetry. The 25 atom cluster includes up to ttird neaiest neighbors of an adsorbate atom situated in the central4-fold site of the first surface plane. It consists of 12 atoms in tlüs plane, nine in the second plane and four in the third plane. For the five and nine atom clusters, calculations were carried out for an oxygen atom above the 4-fold hole site at three distances from the fïrst atom plane, namely at ZO = O.O.2.0 and 4.0 bohr. For the 25 atom cluster, calculations for Zo = 0.0 and 2.0 bohr were performed for the 4-fold site. The Zo = 0.0 calcuiations represent a mot!el for the incorporation of oxygen into the fïrst atomic layer of aluminum. Detailed results of these calculations as wel1 as for other aluminum clusters and for hydrogen chemisorption on aluminum wil1 be presented elsewhere [13]. In the present letter we restrict ourselves to reporting some key results for the aluminumoxygen system. The convergente of the electronic structure of the aluminum clusters as a function of cluster size wil1 not be discussed here as it has been taken up in detail elsewhere [12,13]. Suffice it to say that in so far as the distribution of energy levels and character of orbitals 59
Volume 49, number 1
CHEMICAL PHYSICS LETTERS
is concerned, the cluster of 25 atoins represents a very reasonable description of the local electronic structure of the substrate. In order to compare results obtained from cluster calculations with those obtained by solid state approaches or with experiment, it is convenient to construct density of states (DOS) curves of various types for the clusters, as previously described [10,12], by replacing each discrete energy leve1 of the cluster by a
gaussian. For al1 of the aluminum calculations the gaussian width parameter Ras been taken as o = 0.05 rydberg. Curves whïch display the changes in density of states (ADOS) upon chemisorption are quite instructive and are constructed by performing a point by point subtraction of the DOS curves obtained for the individbal clusters, i.e. Al?*+ 0 and Al,, , aiter aligning the Fermi levels (energies of highest occupied orbitals). In fig. 1, ADOS curves are presented for the case of an oxygen atom situated at three distances (Zo = 4.0, 2.0 and 0.0 bohr) above the four-fold symmetrie hole site of a cluster of five aluminum atoms. The structure in the ADOS curves arises mainly from t:he 2p orbitals
1 July 1977
of oxygen as a result of their interaction with the aluminum. Adsorbate atomic levels broadened by the adsorbate-substrate interaction are commonly referred to as “orbital resonances” [1,2]. At Z, = 4.0 bohr a rather welldefïned orbital resonance is found at 2 2 eV below the Fermi leve1 (Ef). This result is in substantial agreement with the ADOS curve derived from the atom-jellium model [l] as wel1 as the result deduced for Zo = 3.0 from a previous cluster calculation for oxygen on aluminum [2]. A similar orbital resonance is also observed for ZO = 4.0 on the 9-atom cluster. Hence, for large adsorbate-substrate separations al1 of the models mentioned above give essentially the same result which is quite insensitive to the detailed nature of the substrate. For short distances however, and particularly for Zo = 0.0 not considered by the investigators in refs. [1,2], increased structure in the ADOS occurs and a simple orbital resonance no longer results. Also for shorter distances the ADOS curves are quite sensitive to the substrate model chosen. Thus it is important to use a reasonably large cluster; in the present case, 25 atoms. It should be pointed out that the results in fig. 1 are based on X~Yorbital energïes. However, the Slater transition state energies [5] which should be used in a comparison with photoemïssion results have been calculated for al1 the orbitals and the ADOS curves derived therefrom
are almost identical to those of fig. 1.
Peaks in the ADOS curves derived from the transition states exhibit a ma‘rimum shift of 0.3 eV (with respect to Ef) from those shown in fig. 1. Therefore, in the following, only the results based on orbital energies need be discussed. The ADOS curves for the Al,, cluster with an oxygen atom at Zo - 2.0 and 0.0 bohr above the center of the (100) face are presented in fig. 2. The ADOS curve for Zo = 2.0 bohr is very similar to the corresponding curve for the 5-atom cluster shown in fig. 1. The curves for 2, = 0.0 bohr are notably different however, especially as regards the appearance of a thïrd peak at eg.5
-5 EWERGY (eV)
Fig. 1. ADOS curves for Als cluster with an oxygen atom at and 0.0 bah; abcve the midpoint of the plane dete:minïng the (100) face.
zc = 4.0.2.0
60
eV below Ef for the Alz5 + 0 case. The 5-atom
cluster is incapable of reproducing this structure at -9.5 eV since it yields a band width which is too narrow [12,13] and thus contributes no states in this region with which the oxygen atom can interact. Thus the five atom cluster mïght be adequate to roughly represent a situation where the oxygen atom is at Zo = 2.0 bohr, but it is inadequate to represent the situa-
Volume 49, number 1
AOOS (Al,,
CHEMICAL PHYSICS LETTERS
+ 0 1
-13
ENEi-&aVl
0
EF Fig. 3. (a) Photoemission spectra for oxygen chemisorbed on -10
0 EF
-5 ENERBY (rV)
Fig. 2. ADOS curves for Al25 cluster with an oxygen atom at ZO = 2.0 and 0.0 bohr above the (100) face of the cluster.
tion where the atom penetrates the lattice (i_e_ 2, = bohr). This conclusion is important because recent experimental evidente 13,141 suggests rather strongly that oxygen atoms are incorporated into the aluminum lattice, even at low exposures. Fortunately, rather recently some photoemission results have been published for oxygen chemisorption on aluminum [3,4]. Although these experiments have 0.0
been for evaporated polycrystalline films rather than Íor single crystal faces, they do allow some point of comparison between theory and experiment. In fig. 3a the results of Flodstr8m et al. using He II(40.8 eV) radiation are shown for two different exposures, 1 L and 5 L (1 L = 1 Langmuir = 10e6 torr s). Notice the increase in the intensities of the two peaks at z= 7 eV and = 9-5 eV below Ef with increased exposure to oxygen. Photoemission intensity differente curves whïch could be compared
to our calculated
curves were not generated
by the experimenters.
ADOS Thus
aluminum at two different oxygen exposures. The intensity of the curve for the 5 L exposure has been reduced by 112 to aid in displaying the curves. Data is from ref. [ 3 ]_ (b) Calculated density of states (DOS) curve for a cluster of Al25 wïth an
oxygen atom incorporated in the surface plane.
effects can modify dramatically the intensities based on the simple DOS curve. These effects have recently been shown to be accessible withïn the XaSW framework [15], and wil1 undoubtedly be included in future calculations. However, in the spirit of previous theoretical work which has also neglected these effects, we present the comparison in fig. 3. The similarity between the calculated curve and experimental curves, especially the lower coverage result, is quite striking. The fact that the only calculated curves wlüch exhibit
the two peaks at -9.5 eV and -7.0 eV are those for which the oxygen is incorporated in the lattice, is strong support for the oxygen penetratiag the lattice. Besides the result shown in fig. 3b for oxygen at 2, = 0.0, similar results showing these two peaks have been obtained for fïve oxygen atoms at 2, = 0.0 and for four oxygen atoms at octahedral sites inside the Al,, cluster 1131 corresponding to incorporation below the
the calculated shown in fig. 3b. An Xoz-DOS curve, even though based
surface [ 14]_ It should be mentioned that the atom-jellium model [l] bas been approximately corrected in order to investigate lattice penetration [17]. The chief result is
on transition state energies which take account of certain reiaxation effects attentant to the photoemission process, is not strictly comparable to an experimental photoemission curve. This is because matrix element
that as the oxygen atom approaches the surface and is moved into the metal, the 0 2p resonance quickly drops from 2 eV below Ef (previous result [l] ) to an interior limit of = 10 eV below Ep Evïdently no struc-
in order to provïde a comparison
with experiment, DO3 curve for A12,0 (Zo = 0.0)is
61
ture in the orbital resonance is found, which can accomrnodate the structure found experimentally in the region 5-10 eV below Ef. We have also calculated the Al 2p leve1 shifts
which take place on interaction with an oxygen atom, using the AIz clusters. The results are shown ín table 1_ Aga& the results for lattice incorporation (2, = 0.0) are consistent with experimental fìndings 1161, for less than a monolayer coverage, In our model each aiuminurn atom Iisted in table 1 has a single oxygen atom as a nearest neighbor. As the number of oxygen atoms increases the core shifts will also increase as we have found for the rnodels involvhrg four or five oxygen atoms mentioned above. The experimental shift of 1.3 eV is for intermediate exposure of oxygen (much less than a monolayer) and so it is reasonable to assume that most of the surface aluminum atoms have only one adjacent oxygen and hence the comparison made is valid. At higher exposures a sbift of 2.6 eV is observed which corresponds to an ahrmi-
num oxïde type surface. Our discussion thus far has centered on the caleulated peaks at w 7 eV and 9.5 eV below Ef- However, +thereis a third peak at -3.5 eV in the ADOS of fig. 2 which also appears in projected DOS curves in which 0 2p character only is projected out of the total DOS of fig. 3b. Hence this third peak arises from 0 2p character according to the calculations. In fig. 3a there appears to be some change in the photoemission intensity in this energy region as a function of oxygen exposure. The change is however niuch less dramatic than that observed below -5 ev. Moreover further re-
cent photoemission studies have appeared [16-183, which havo studied much higher exposures. Even at 100 L it appears that a complete monolayer has not íormed. At these hïgher exposures the oxygen strucTable 1 Calcufated Al 2p leve1sbifts due to chemisorption of oxygen (in ev) AtOm
a,
AI (1) Al (4)
20
=
0.30 0.65
2.0
zo = 0.0
1.21 1.05
Expt. b) 1.30
atom just beneath the oxygen atom; Al (4) - one of the four atoms on the surfacc plane closest to the oxygen
a) Al (11 --
atom. b) Ref. [ 161.
62
1 July 1977
CHEMICAL PHYSICS LETTERS
Volume 49, number 1
ture below -5 eV is cïearfy seen above a very small Al background and there is no indication of oxygen related structure above -5 ev.
It should be emphasized however, that the experimental results have been obtained using a rather limited number of photon energies and also a very hmited number of incident Iight and efectron cohection angles. There is ample theoretical [í5] and experimental [lg] evidente in the literature for the extreme sensitivity of surface photoemission results to aIl of these parameters_ There are thus two possible situations which may be present for the part of the spectrum above -5 ev. First, there may be no (or very little) oxygen related structure in this region, in which case our theoretical model is inadequate in this respect. Second, as systematic angle and photon energy dëpendent photoe~ssion studies have not yet been carried out, the structure may not yet have been detected in a convincing manner. The choice between these two possibilities clearly requires further study both experi-
mentally
and theoretically. One thing is clear, the aluminum f oxygen system
is not so simple as it had at first appeared. We believe that a close interaction between theory and experiment will be necessary in order to fully understand this system.
References [ 11 N.D. Lang and A.R. WiJIiams,Phys. Rev. Letters 34 (1975) 531. 121 J. Hsrris and G.S. Painter, Phys. Rev. Letters 36 (1976) 151. [3] S.A. Flodström, L.G. Petersson and S.B.M. Hagström, J. Vacuurii Sci. TechnoI. 13 (1976)
280.
[4] S.A. Flodström, L.G. Petersson and S.B.Pol.Hagström, SoIid State Commun. 19 (1976) 257. [SI J.C. Slater, In: Advances In quantum chemistry, Vol. 6,ed. P_-O,Löwdin (Acsdemic Press, New York, 1972) p.1. [61 K.H. Johnson, in: Advances in quantum chemistrv, Vol. 7, ed. P._O. Löwdin (Aeademic Press, New York, 1973) p- 143. [7] K.H. Johnson and R.F. Messmer, J. Vacuum Sci. Technol. 11 (1974) 236. [8] I.P. Batra and 0. Robaux. 3. Vacuum Sci. Techm% 11 (1974) 236. 191 S.J. Niemczyk, J_ Vacmrm SeI_TeehnoL 12 (1975) 246. [IOj RP. Messmer,S.K. Rnudson,K.H. Johnsan, J.B. Diamondand C.Y. Yang, Phys Rev. 813 (1976) 1396. [! 11 K. Schwarz, Phys. Rev. BS (1972) 2466.
Volume 49, number 1
CHEMICAL PHYSICS LETTERS
R.P. Mesrmer and D.R. Salahb. Intern. J. Quantum Chem. 10s (1976) 147. [ 131 D.R. Salahub and R.P. Messmer, to be publïshed. [ 141 P.H. Dawson, Intern. J. Mass Spectrom. Ion Phys. 16 (1975) 269; Swface Sci. 57 (1976) 229. [15] J.W. Davenport, Phys. Rev. Letters 36 (1976) 945; Ph. D. Thesis, Physics Department, University of Pennsylvanïa (1976). [12]
1 July 1977
[16] S.A. Flodström, R.Z. Bachrach, R.S. Bauer and S.B.M. Hagström, Phys. Rev. Letters 37 (1976) 1282. 1171 K.Y. Yu, J.N. Miller, P. Chye, W.E. Spicer, N.D. Lang and A.R. Williams, Phys. Rev. Bl4 (1976) 1446. [ 181 C. Martinsson. L.G. Petersson, S.A. Flodström and S.B.M. Hagstrom, to be published. [ 191 B. Feuerbacher and R.F. Willis, Phys. Rev. Letters 36 (1976) 1339.
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