Core-level excitation as a probe of molecular structure at surfaces

Journal of Ekctron Spectroscopy and Related Phenomena, 62 (1993) 33-50 0368.2048/93/$06.00 0 1993 - Elsevier Science Publishers B.V., Amsterdam

Core-level excitation at surfaced

as a probe of molecular structure

A.M. Bradshaw Fritz-Habe?‘-b&itut (Germany)

der Max-Planck-Gesellschaft,

Faradayweg 4-6, W-1000 Berlin 33

(First received 25 September 1992; in final form 9 November 1992)

Abstract Techniques of surface structural analysis based on core-level excitation in molecules are briefly reviewed. Using soft X-ray radiation the phenomena of photoionisation and photoabsorption can be used not only to determine orientation but also to measure precisely structural parameters such as nearest neighbour distances, adsorption site, intramolecular separations and bond angles.

Introduction Core-level excitation as a surface science probe has its origin in the characterisation of atomic and molecular adsorbates with X-ray photoelectron spectroscopy which began some 20 years ago. This early work culminated in the comprehensive studies by Fuggle et al. of adsorbed molecules on tungsten and ruthenium surfaces, such as in ref. 1. In photoelectron spectroscopy electrons emitted as a result of their excitation into states above the photoionisation threshold are analysed according to their kinetic energy. One of the aims of the technique in surface science has been to investigate the influence of the local electrostatic potential on the core-level eigenvalues of the adsorbate. The electrostatic potential at the site of an atom in the adsorbate layer is determined by the chemisorption bond and by the electrostatic field in the surface dipole layer. In the single particle picture, a core-level spectrum would consist of a series of discrete lines corresponding to those eigenvalues of the adsorbatesubstrate system Q, which lie more than about 20 eV below the Fermi level. Owing to relaxation effects and the change in correlation, however, the measured photoelectron binding energies are not identical with the &i. As a corollary of this the corresponding line in the photoelec1Dedicated to the memory of John C. Fuggle.

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tron spectrum may be accompanied by satellites, indicating that photoionisation can also result in excited states of the system. The probability of observing such effects depends on the time scale on which the particular relaxation, or screening, mechanism occurs. One mechanism is charge transfer screening, in which an adsorbaie level is filled, or partially filled, owing to its energy lowering in response to core hole creation, It was first detected in weak chemisorption systems by Fuggle et al. [2]. A further screening effect corresponds to the classical image charge mechanism, which is described in a quantum model by the creation of surface plasmons [3]. Schtinhammer and Gunnarsson presented an early dynamical description of both screening processes [4]. The use of core-level photoelectron spectroscopy in the “fingerprinting” of adsorbates and the study of the surface photoionisation process itself were later supplemented by extensive investigations of the surface core-level shift, i.e. the difference in binding energy of the substrate surface atoms relative to the bulk [5]. That core-level excitation can also be used as a structural probe is not immediately apparent. This type of application of photoionisation phenomena derives from the possibility of measuring total and partial cross sections using synchrotron radiation as a photon source. The total photoionisation cross section (or absorption coefficient) from the surface region is given by the fraction of absorbed photon flux, but because in practice this measurement is not feasible, the current from the subsequent Auger decay, or a part of the secondary electron current that is proportional to it, is used. This latter “partial yield” experiment then has approximately the same surface sensitivity as photoelectron spectroscopy. The (surface) extended absorption fine structure (EXAFS) (or oscillations in the absorption coefficient) which is observed above an absorption edge of an adsorbate corelevel is due to the interference between the emitted photoelectron wave and the waves backscattered from the surrounding atoms. The frequency of the oscillations as a function of photon energy (wavelength) is determined by the distances to the surrounding substrate atoms, so an appropriate analysis can provide structural information. Surface EXAFS studies on atoms and molecules adsorbed on single crystal surfaces began in 1978 at Stanford with the pioneering work of Citrin et al. [6] and Stdhr et al. [7]. In the case of adsorbed molecules the absorption spectrum just below and just above the edge is dominated by absorption resonances which may generally be identified with excitations into antibonding orbitals. In comparison with the corresponding spectra of free molecules the features below the edge (corresponding to “bound” electronic final states) are broader; Rydberg excitations normally disappear owing to a “quenching” effect by the surface. A molecule tends to have a fixed geometry on the surface and the excitations are of the electric dipole sort, so the transitions are polarised. Thus the orientation can be determined by suitably varying the direction

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35

of the electric vector of the incident radiation. The first experiment of this type was performed by Stijhr et al. in 1981 [8]. Near-edge X-ray absorption spectroscopy of adsorbed moelcules (often termed NEXAFS as the complement of surface extended absorption fine structure (SEXAFS)) has proved particularly useful in recent years - when interpreted correctly - in studying molecular orientation, In angle-resolved photoelectron spectroscopy the intensity of an adsorbate core-level peak will change when the photon energy is varied. This determination of what is actually the differential partial photoionisation cross section provides the basis for another structural tool, namely, photoelectron diffraction (PhD) which has certain similarities with SEXAFS. For a given collection angle the photoelectron wave can reach the detector either directly or after scattering from the substrate atoms. The coherent interference leads to strong (up to 50%) intensity oscillations between the various components of the wavefield and depends not only on the atomic separation but also on the real space position of the atom in the atomic or molecular layer. By performing model calculations for a given structure and emission direction and comparing with experiment, the required structural information may be obtained. (PhD effects are also observed when the emission angle is varied at constant photon energy. One then refers to a scanned angle mode.) The first experiments were performed almost simultaneously by several groups [9-U] following a suggestion by Liebsch in 1974 [12]. How does the technique of PhD relate to SEXAFS? By varying the photon energy in PhD the scattered electron intensity is redistributed between the various final state manifolds corresponding to different emission directions. If it were possible to integrate over the whole photoelectron current in 47c solid angle, the “diffraction” effects would be entirely averaged out. Only the interference between the outgoing photoelectron wave and those backscattered towards the emitter would remain, i.e. we would recover the SEXAFS. In PhD the interference between the directly emitted and scattered waves occurs at the detector and is determined by the path length differences, which in turn depend on both the separation and direction of the substrate atoms. In SEXAFS this directional information is only obtainable via the E vector (or polarisation) dependence of the modulation. Although SEXAFS modulations are invariably a factor of 10 smaller than those in PhD, SEXAFS has so far been used more widely in surface structural studies, particularly for atomic adsorbates. In contrast, however, the full potential of PhD has yet to be realised. Both techniques have the important advantage over LEED and X-ray diffraction that long range order is not required. Together with diffuse LEED [13] they will therefore play an increasingly important role in the future investigation of molecular adlayers, which in general are seldom well ordered.

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In this paper the techniques of X-ray absorption spectroscopy (both in the near edge and EXAFS regimes) and PhD are illustrated with some examples from recent studies on adsorbed molecules carried out at the Fritz-HaberInstitut. Somewhat more emphasis is placed on PhD because it has received relatively scant attention in the past but is potentially of great importance, particularly for molecules and molecular fragments. Both techniques necessitate the use of synchrotron radiation; in the examples cited here the BESSY electron storage ring in Berlin has been used. The effective exploitation of beam time and instrumentation at synchrotron radiation sources often demands collaborative efforts and a somewhat larger number of participating researchers than is the case for “normal” laboratory experiments. Several examples described here derive from a collaboration with Professor D.P. Woodruff at the University of Warwick. All participants are explicitly acknowledged by reference to the appropriate original paper and are listed again at the end of this article. X-ray

absorption

spectroscopy:

the near-edge

structure

Near-edge structure in X-ray absorption spectra of adsorbed molecules was originally interpreted in terms of excitation into unoccupied Iocalised orbitals or as scattering resonances, the polarisation dependences of which were associated with specific bond directions or aromatic rings [14]. It is now generally accepted, however, that the analysis of such data should take account of the effective point group, the symmetry of the delocalised molecular orbitals and the dipole selection rules governing transitions between initial and final states [15]. If the surface molecule possesses at least one symmetry element, the transition from a core-level into an unoccupied molecular orbital will be polarised and its intensity given by 1 cc [E * .?z

0)

where M is the Cartesian component of the electric dipole vector associated with the transition. For non-zero intensity the product of the irreducible representations corresponding to Ii>, If) and M must belong to, or contain, the totally symmetric representation of the point group. The angular dependence of an allowed transition is then given by I a [E - Ml2

=

con& x cos28

(2)

where 19is the angle subtended by the E vector and the electric dipole vector. The transition is said to be polarised when at least one of the Cartesian components of M is zero. The electric dipole is normally directed along a symmetry axis or lies in a symmetry plane, so the application to orientation determination follows. For a CO molecule with its internuclear axis perpendicular to the surface, giving rise to a C,, , C,, or C& point group,

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the la + 2x (e) transition is polarised in the surface plane and the la + 6a(a,) transition is polarised perpendicular to the surface. Thus, for a given surface molecule with at least one symmetry element, the orientation can generally be determined if the transitions in the spectrum have been properly assigned. The latter is not easy in a complex molecule where excitations and so-called substrate Rydberg transitions, multielectron resonances may also play a role. Furthermore, the possible effects of symmetry lowering and equivalent cores may have to be taken into account [15]. In a quantitative analysis, the imperfect polarisation of the incident 1 radiation, azimuthal orientation and the unknown background under the resonances may also cause difficulties. Two examples are given here to illustrate the practical application of near-edge X-ray absorption spectroscopy; the first highlights some of the physics associated with core-level excitation, the second is more chemically oriented. On a Ag{llOj surface the reaction of preadsorbed atomic oxygen with carbon dioxide gives rise to a surface carbonate species [16]. Its formation produces new half-order features in the LEED pattern, which, as we shall see below, are indicative of a surface reconstruction. Figure 1 shows the Cls and 01s photoabsorption spectra at angles of 0 = 20 and 90* between the E vector and the surface normal [17]. A very strong polarisation dependence of the resonances is apparent. From the molecular orbital (MO) energy diagram of CO:- we expect four unoccupied MOs, which are 2a;, 5a; and 5e’ under the point group symmetry DSh; 2ai is x-like, whereas 5a; is o-like; 5e’ is also a-like but doubly degenerate. Whereas at the oxygen edge all three resonances are present, the second is missing at the carbon edge. The excitation 2a; -P 5a; is thus forbidden, which immediately tells us that the effective point group remains &. Despite the influence of the surface, no symmetry lowering takes place, although formally the point group should be C,, (or lower if the discrete atomic structure of the surface is also taken into account). Why is the corresponding excitation at the oxygen edge dipole-allowed? We note that there are three equivalent oxygen atoms which, formally at least, give rise to molecular orbitals designated la; and le’; the excitation le’ + 5a; is dipole-allowed. This explanation might seem somewhat unsatisfactory, because we intuitively expect the resulting core hole to be sited at a particular oxygen atom and not in a doubly-degenerate declocalised molecular orbital derived from the 1s orbitals. The effect which actually gives rise to core hole localization is vibronic coupling [18]. Although the lal, + 5a’, transition is forbidden, the corresponding excited state of the molecule (A;) can couple to the allowed excited state (E’) via vibrational modes of appropriate symmetry (in this case also E’). This gives rise to a dynamic localisation of the core hole on one of the oxygen atoms. The effect has recently been investigated experimentally and theoretically in high resolution absorption spectra of free molecules [19]. Another way

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Photon energy (eV) 540 550 530

I

280

I

I

300 290 Photon energy (eV)

I

330

Fig. 1. Surface carbonate species adsorbed on Ag{IlO>. Near-edge X-ray absorption spectra taken at (a) the oxygen K edge and (b) carbon K edge with the polarisation vector aligned in the (110) azimuth at polar angles of B = 20 and 90’. Right: schematic representation of the missing row reconstruction showing two possible adsorption sites for the surface carbonate species; after ref. 17.

of treating this problem with group theory is to consider all three broken symmetry configurations in the final state and form linear combinations thereof such that the transition can take place within the higher symmetry of the ground state [20]. A quantitative analysis of the polarisation dependence in Fig. 1 reveals that the surface carbonate species is highly oriented with its molecular plane parallel (to within x loo) of the Ag surface; Less good agreement between experiment and the calculated polarisation dependence (assuming an isolated oriented molecule) is obtained for the 2a,”(x) resonance at high 8. This may perhaps indicate that some symmetry lowering does after all take place for the 7ctransition and that excitation with the components of the E vector parallel to the surfaces may occur to some extent. One final observation pertaining to the structure is appropriate here. The observed change in the surface unit mesh, designated (1 x 2), cannot be due to the adsorbate layer itself: flat-lying carbonate species cannot reach a packing density corresponding to the Ag-Ag separation along the atomic rows of

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PMDA - ODA monolayer on graphite Nl s edg

I

I

400

405

I

410 hwieV

I

415

I

420

Fig. 2. Nitrogen K near-edge spectra of a single monolayer 6f PMDA-ODA HOPG graphite at angles of 8 = 20, 50 and 90°; asker ref. 22.

polyimide on

the (110)surface. Thus the (1 x 2) structure must be due to a surface reconstruction, probably of the missing row type (Fig. 1 (bottom)). A somewhat different application of NEXAFS is shown in Fig. 2. Using the Langmuir-Blodgett technique it is possible to deposit polyimide layers on various substrates [21]. Polyimides have attracted considerable attention in recent years as thermally very stable polymer systems with widespread uses including packaging in microelectronics. Figure 2 shows the Nls edge spectrum of a monolayer of PMDA-ODA polyimide on HOPG graphite at three different angles of 8 [22]. (PMDA-ODA = pyromellitic dianhydrideoxydianiline; see Fig. 2 inset.) Because of the bent C-O-C bond at the ether oxygen atom the polymer has a zig-zag structure, in addition to a possible rotation about the C-N bond linking the PMDA and

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ODA moieties. The first two sharp features 402 and 404eV are due to overlapping x*-type resonances associated with both the PMDA and ODA parts. There is a strong polarisation dependence: the intensity of the X* resonances goes to zero when the E vector is in the surface plane. Conversely, the weaker resonances at higher energy due to o*-type excitations tend to be stronger when the E vector is parallel to the surface. This effect is best illustrated by the resonance at 413 eV. The features at 407 and 409eV probably also contain some n* contributions. The results thus indicate that the two moieties in the monolayer are coplanar and oriented parallel to the HOPG surface. Furthermore, there is a strong similarity between these spectra and the sum of the spectra of pyromellitic acid diimide and diphenyl ether [22] (not shown), which suggests that despite the co-planar configuration - there is little or no interaction of the two ring systems, i.e. no K* delocalisation occurs across the N atom. SEXAFS As noted above, the total photoionisation cross section of an adsorbate atom is modified at higher energies by backscattering of the emitted photoelectrons from the neighbouring substrate atoms. Interference of the emitted wave with the backscattered waves at the site of the emitting atom gives rise to modulations (fine structure) superimposed on the otherwise smooth background of the total cross section. The so-called modulation function (for initial s states) is given by

x(k) = 14(k) sin [2kri + q$(k)] i

The modulations depend on the distances ri between the emitter and the various “shells” i of the substrate atoms. These distances may be extracted from the Fourier transform of x, which does not actually peak at the Fi because of the wavevector-dependent phaseshift tji(k). The latter must be “transferred” from other systems or calculated. Whereas this analysis only gives bond lengths, information on adsorption site geometry can also be obtained from the polarisation dependence. The amplitude Ai (k) is proportional to an effective coordination number given by JJ* =

32

COS2 clij

(4)

j

where aij is the angle between the E vector and the vector rij from the emitter to the jth atom in the ith shell. The “3” is effectively a normalising factor, because (cos’aij) = lf3. The amplitude also contains terms accounting for inelastic scattering and thermal vibrations (DebyeWaller factor). Although earlier practice involved direct extraction of bond lengths via the Fourier transform, calculated curves for the various surface geometries are

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62 (1993) 33-50 hollow Q=90°

I 2

I4

6

r (4

x 2)-X0 system (left, solid lines) Fig. 3. 01s SEXAFS simulations for the system Ni(lll}c(4 for e = 90” (top) and 8 = 200 (bottom) assuming a three-fold coordinated hollow site compared with experimental (broken lines) and their Fourier transforms (right); the simulation for 20” was performed with exactly the same parameters as for 90” (best fit); after ref. 23.

now normally fitted to the data curves (or their Fourier transforms). It may also be necessary to include multiple scattering. A recent example by Becker et al. [23] shows the importance of direct structural probes, rather than relying on the more qualitative information provided, for example, by vibrational spectroscopy. In the system Ni(lll}c(4 x 2)-CO the C-O stretching frequency is indicative of adsorption on a two-fold bridge site [24,25]. The 01s SEXAFS spectra are shown in Fig. 3 for 8 = 90 and 200, together with the corresponding Fourier transforms (right). The latter are dominated by peaks A and B, which after phase shift corrections correspond to distances of about 2.7 and 3.7A, respectively. Owing to the polarisation dependence given by eqn. (4), peak A in the 0 = 20* data must correspond to the nearest neighbour (nn) distance. A simple assignment of the 8 = 90’ data is not possible: for a bridge site peak A would consist of overlapping nn and next nn distances; peak B would be the third nn distance. In contrast, for a three-fold hollow site, peaks A and B would correspond to the nn and next nn distances, respectively. The simulated spectra, also for the Cls edge, clearly favour, however, the hollow site (broken line in Fig. 3). The measured C-Ni bond length of 1.78A agrees well with LEED data for Ni{ loo>-CO. Moreover, the

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Electron Spectrosc. Relat. Phenom. 62 (1993) 3%50

Fig. 4. Model for Ni{lll}c(4 x 2tCO structure corresponding to a coverage of 0.5ML CO molecules (small filled circles) are in inequivalent (fee and hcp) hollow circles; after ref. 23.

resulting C-C distance is 1.15& in accordance with expectation. This result calls for a new interpretation of the ~(4 x 2) LEED pattern. Consideration of not only the SEXAFS data but also possible missing spots in the LEED pattern (due to glide lines) suggests the structure shown in Fig. 4. Here the CO molecules occupy a mixture of fee and hcp hollow sites in which the closest intermolecular separation is 2.88 A, i.e. substantially shorter than in the previously accepted model involving only bridge sites. It is therefore possible that strain in the overlayer is relieved by the molecules slightly tilting and/or moving off the threefold site [26]. Scanned energy photoelectron

diffraction (PlD)

Figure 5 demonstrates schematically the principle of photoelectron diffraction for an adsorbed atom or for an atom in an adsorbed molecule. As already noted, the intensity of a core-level photoelectron peak of the adsorbate is measured at a selected emission angle as a function of photon energy, and thus of photoelectron kinetic energy. In the plot of peak intensity against kinetic energy (“the photoelectron diffraction spectrum”) modulations occur as a result of interference between the primary photoelectron wave and the secondary waves elastically scattered at surrounding atoms. Assuming only single scattering events at the substrate atoms, the photoelectron diffraction intensity from an initial s state is given by I(lz) Cc ICOS X

8,

+

~

~(ej,

k)W(Bj,

lz) COS e~j/rj

exp [ - L,;22(k)] exp [ikrj(l -

(5)

COS ej)]l”

where f(
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Fig. 5. Illustration of the processes contributing to PhD: (1) direct emission; (2) indirect emission via backscattering from surrounding atoms; only two single and one multiple scattering events are indicated; after Liebsch [12],

angle ej. The first term represents the directly emitted component with 8, the angle between the E vector and the outgoing wavevector; 8, is the angle between rj and the E vector; W(ej, lz) is a DebyeWaller factor; the first exponential term accounts for inelastic scattering. The phase difference between the primary wave and the secondary wave scattered from atom j contains a contribution krj(l-cos ej) due to the additional scattering path length. The path length difference thus contains information on the distance and directions of the surrounding substrate atoms. This is extracted by comparing the experimental diffraction curves with calculated curves, for which purpose it may be necessary to take higher order scattering into account. In the adsorbate situation it is necessary to exploit so-called backscattering events (i.e. those involving scattering angles between 90 and 1800}. These only have a significant scattering amplitude if the photoelectron energy is less than about 500 eV. Two examples which illustrate quite well the importance of backscattering are shown in Fig. 6: the 01s photoelectron diffraction spectra measured in normal emission from both atomic oxygen and the formate species adsorbed on Cu{ lOO>. The oxygen overlayer is ordered and designated (,/2 x 2,/2)R45O, corresponding to approximately half a monolayer [27]. The surface formate species is formed by deprotonation of formic acid and shows no long-range order [28]. Although the spectra show fine structure (and some noise), each is characterised by a strong underlying modulation. This is due to a dominant scattering path length difference giving rise to aIternate constructive and destructive interference as the electron energy, and thus its wavelength, is changed. In each case, strong 180° backscattering from a substrate atom “behind”, or nearly “behind”, the oxygen atom is responsible for the oscillation. The broken lines in Fig. 6 are the result of very simple scattering calculations each with a single MO0 backscatter-

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Cu{ 1OO} 0 1s normal emission

(1001 I

I 100

OCu I

-(IlO) a0

I

200 Photoelectron

OC I

OH

I

300 energy (eV)

I

1 400

Fig. 6. Solidlines, experimental 01s normal emission PhD spectra from atomic oxygen and the surface formate species adsorbed on Cu(100); broken lines, results of very simple scattering calculations each with a single 180’ backscattering Cu atom at a separation of 2.0 and 2.2 A for the atomic oxygen and the oxygen atom-of the formate species, respectively; insets, side view bonding geometries for chemisorbed oxygen (left) and fonnate (right) on Cu(100). ing Cu atom at separations

of 2.2 and 2.OA for the atomic oxygen

and

oxygen atom of the formate species, respectively. The longer period of the formate oscillations is symptomatic of the shorter distance -and thus the smaller path length difference. The different frequency of oscillation and the different fine structure in these two spectra are indicative of different surface sites. In the case of the oxygen overlayer the atom is adsorbed almost coplanar in what would be the four-fold hollow site. In fact, one of the four nn coplanar Cu atoms is removed as the result of an adsorbateinduced surface reconstruction (see Fig. 6 inset). The Cu atom responsible for the strong backscattering is thus in the second layer. The oxygen atom of the formate species, in contrast, bridges two Cu atoms; the molecular plane is perpendicular to the surface and aligned in the (110) azimuth. The oxygen atoms are thus almost in on-top geometries (see Fig. 6 inset). In the case of the oxygen overlayer, LEED gave the decisive structural evidence [29]. PhD provided, however, what is now regarded to be the correct structural model for the formate species (as it, incidentally, also does for formate on the Cu{llO} surface, where the local geometry is identical). The reader is referred to the original paper [30] for the detailed

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comparison between theory and experiment, which involves a larger “cluster” of Cu atoms and a more exact treatment of the scattering problem. It sufllces at this point to note that the technique not only shows site sensitivity and is applicable to systems without long-range order, but also provides quantitative structural information. A further example is provided by the adsorption system Ni(lll)-PF, [31]. The properties of this molecule in the chemisorbed state are particularly interesting because as a ligand in coordination chemistry it forms zerovalent mononuclear complexes via the P atom with most transition metals [32]. Moreover, the bonding mechanism is similar to that of CO: a-donation occurs via the 8a, orbitals with It-backdonation from the metal into the 7e orbital [33]. PF, only bonds in a linear configuration as a ligand so we might suspect that on a Ni(lll} surface the on-top adsorption site is occupied. Previous electron-stimulated desorption ion angular distribution (ESDIAD) measurements had already indicated this bonding geometry and demonstrated an azimuthal orientation of the molecule such that the individual P-F bonds are directed over neighbouring Ni atoms. In a parallel investigation to the PhD study cited here, the Ni{lll}-PF, system was investigated with Pls near-edge absorption spectroscopy, SEXAFS and the X-ray standing wave technique [34]. The near-edge data indicated that the threefold symmetry axis is perpendicular to the surface; the other two techniques confirmed the result of the structural analysis presented below. The P2p spin-orbit split doublet of binding energy w 136eV was measured over the kinetic energy range * 8&380 eV at emission angles of 4 = 0,30 and 50° in the (110) azimuth. Comparison with multiple scattering calculations [35] revealed satisfactory agreement only for the on-top site with a Ni-P distance between 2.0 and 2.1 A (Fig. 7). Note that in this case the presence of the Ni atom directly behind the phosphorous in the norms1 emission geometry also gives rise to a strong oscillation as a result of 180” backscattering. Inclusion of the F atoms in the calculations makes no difference to the results; experience with other systems shows that the’ scattering at other light atoms of the molecule usually has very little influence. In order to facilitate objective comparison between experiment and theory two reliability factors R, [36] and R, [37] were used. These are both normalised such that the value of unity corresponds to zero correlation between theory and experiment and the value of two corresponds to complete anti-correlation. The normal emission results are shown in Fig. 8. Whereas the curves clearly show the best agreement for the on-top site with a Ni-P distance of 2.0?& they also show so-called multiple site coincidences [38], i.e. a series of minima in the R factors as the Ni-P distance is changed. As can be seen in Fig. 7, the pathlength difference at 1.7 Bi in this strong single backscattering geometry leads to a modulation frequency of the calculated curve which, although somewhat lower, still resembles that

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2.5

J-k/L-a

A

2.3A

dm 0

300 200 Kinelicenergy(ev>

Fig. 7. Ni{lll}-PF,: comparison of the experimental normal emission P2p PhD spectrum (heavy line) with calculated spectra at different Ni-P separations in the on-top site; after ref. 31.

of the experiment. The off-normal data also indicate that the on-top site is occupied with a Ni-P distance of 2.07 A, but here the successive minima due to the “coincident” site occur at different separations. The R factor values for the off-normal data are higher (i.e. there is less good agreement with experiment), which is due to the lower amplitude of the modulations than in normal emission. This is a consequence of the external adsorbate vibrational modes parallel to the surface (“frustrated” translations) which are softer than those perpendicular. Vibrations produce an uncertainty in the atomic position and hence in the path differences; the anisotropy in the vibrational amplitude means that the off-normal data is more strongly affected. The first evidence of this effect was a PhD study of NH, on Ni(ll1) at a fixed temperature 1371.Very recently, temperature-dependent measure-

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1:5

210

47

2.5

P-Ni layer spacing (A)

PhD reliability factors (see text) as a function of the Ni-P first layer Fig. 8. Ni{lll)-PF,: separation for the on-top, hcp hollow, bridge and fee hollow sites; the solid lines shown are cubic spline fits to the measured R factor values (points); after ref. 31,

ments on the Ni(lll)-PF, system have confirmed that these dynamic effects are indeed important, The last example demonstrates the feasibility of chemical shift PhD. In the case of larger molecules on surfaces, for example hydrocarbons, atoms of the same element may be chemically distinct (i.e. different surface site and/or separation from the surface) and thus have different core-level binding energies. If the corresponding features can be separated in the photoelectron spectrum, then the PhD spectrum for each atom can be measured separately and, by comparison with calculation, the surface coordinates determined. The surface acetate species is such a molecule containing two non-equivalent carbon atoms [39]. In an analogous way to the formate species it can be prepared on the Cu(ll0) surface by the decomposition of acetic acid. NEXAFS shows that the molecular plane is aligned in the (llO> azimuth, again in a similar way to the formate species (see Fig. 9 inset). The 1s peaks from the carboxyl carbon atom and the methyl carbon atom in the photoelectron spectrum are separated by

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Cls photoemission spectrum (hv=390eV)

I 90

I 105

1 100

I 95

Kinetic energy (eV) Fig. 9. Cls photoemission spectrum of the surface acetate species on Cu{llO} at hv = 390eV; inset, structural model assuming adsorption in the aligned bridge site; after ref. 39.

x 3.6 eV (Fig. 9). The PhD spectra at a polar angle of emission of 8 = 20’ in the
1.. _

Acetate/G ( 110) OPD 9 = 20”

methyl exp. th.

-A

carboxyl exp.

,.” ‘. ,’ :

I

,.-. ‘._.’

t 50

I 100

,*

‘.

,’

l..’

’

‘. ‘.__.-S

l.

I

I

I

150

200

250

th.

Kinetic energy (eV) Fig. 10. Compatiison of experimental Cls photoelectron diffraction spectra from the carboxyl and methyl carbon atoma in the surface acetate species on Cu(ll0); after ref. 39.

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49

(e.g. dashed curve in Fig. 10 for 8 = 2OO).The carboxyl

carbon atom and the methyl carbon atom are found at 2.44 and 3.98& respectively, above the plane of outermost Cu atoms. Similarly, the 01s data give a separation for the oxygen atom of 1.9OA and a O-O distance of 2.25A. The values give 0-Cu, GO and C-C bond lengths of 1.91,1.25 and 1.54& respectively, and a O-C-O bond angle of 129O, which is very similar to those found in inorganic acetates. The precision is estimated to be + 0.04A. Conclusions Surface structural techniques based on core level excitation phenomena already play a leading role in understanding the physics and chemistry of surfaces. In particular, they can be applied readily to the study of the structure of molecules and molecular fragments adsorbed on metal surfaces_ Such systems are rarely characterised by long range order and the application of conventional diffraction techniques is thus excluded. The development of the next generation of soft X-ray synchrotron radiation sources is almost certain to produce an increase in both number and quality of such studies. Acknowledgments The author gratefully acknowledges

the role of many collaborators

and

colleagues in the work described above, in particular M.C. Asensio, J.C. Conesa, R. Dippel, V. Fritzsche, P. Gardner, A. Gonzalez-Elipe, J. Haase, M. Keil, A.L.D. Kilcoyne, Th. Linder, D.E. Ricken, A.W. Robinson, Th. Schedel-Niedrig, K.-M. Schindler, J. Somers, H. Sotobayashi, K.U. WeiD and D.P. Woodruff. He also thanks the German Federal Ministry for Research and Technology and the EC for financial support. References 1

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