Elementary surface excitations and electron energy loss spectroscopy

Ultramicroscopy North-Holland

179

11 (1983) 179-186 Publishing Company

ELEMENTARY SURFACE EXCITATIONS SPECTROSCOPY

AND ELECTRON

ENERGY LOSS

H. IBACH Institut fiir Grenzfliichenforschung German) Received

6 January

und Vakuumphysrk.

1983; presented

at Workshop

Kernfurschungsanlage

January

Jiilich, Postfuch 1913, D 5170 Jiilich, Fed. Rep.

of

1983

Energy loss spectroscopy of electrons reflected from solid surfaces has become ‘a major source of information about a variety of elementary excitations of the surface. These excitations comprise plasmons of free carriers in metals and semi-conductors, interband transitions, electronic excitations of adsorbed molecules and, with high resolution spectroscopy, vibrations of adsorbates and the substrate. Our understanding of surface chemistry is greatly improved by the spectroscopy of adsorbate vibrations since decomposition reactions of adsorbed molecules on surfaces are easily analysed by the characteristic eigenmodes of the products. In many cases new and unexpected surface species have been identified. One of the advantages of electron energy loss spectroscopy as opposed to other techniques is that different scattering mechanisms may be employed which have different selection rules with respect to the symmetry elements of adsorbate complexes. Recently even surface phonon spectroscopy has become possible, and the dispersion of Rayleigh waves has been measured throughout the Brillouin zone.

1. Introduction Electron energy loss spectroscopy has become one of the prime tools in surface physics and chemistry. Over the past decade substantial technical improvements were made and spectrometers have become commercially available. Scientists were attracted by the method, mainly in connection with using energy loss spectroscopy for the analysis of vibrations of adsorbed molecules. When used this way the method, unlike most others, probes the chemical bond between surface atoms. Therefore, in connection with other techniques for surface vibration analysis, electron energy loss spectroscopy has made a very significant contribution to our current understanding of surface chemistry. For example, the absence or presence of a certain bond is easily seen just by inspection of the spectrum and comparison to the vibration spectra of free molecules. Hence simple chemical reactions on surfaces are easily investigated. Also new unexpected surface species as reaction intermediates have been detected and analyzed. A major ad0304-3991/83/0000~0000/$03.00

0 1983 North-Holland

vantage of electron energy loss spectroscopy is the possibility of exploiting different scattering mechanisms depending on kinetic energy of the electron and the scattering geometry. The different scattering mechanisms lead to different selection rules which, in turn, can be used to obtain information on the symmetry of adsorbed molecular complexes. In that sense electron energy loss spectroscopy has also become a tool for structure information. The ease by which such qualitative mformation on structure, symmetry, and bonds is obtained mainly by inspection of the spectra has been very attractive to the surface chemists, and at this time probably several hundred publications dealing with this aspect have appeared. More recently, we have found yet another mode of operation which allows performance of a complete vibrational analysis of two-dimensionally-ordered systems. There electron energy losses are measured as a function of momentum parallel to the surface and one maps out complete two-dimensional dispersion curves. For well ordered and well prepared surface systems one thus moves from qualitative to

180

H. Ihach / Elemenmry

quantitative information. The impact on surface physics will be comparable to inelastic neutron scattering on phonon spectroscopy in bulk solid state physics. While surface vibration spectroscopy has been the focus of interest in the application of electron energy loss spectroscopy, other surface elementary excitations such as interband transitions [l], plasmons [2], and electronic surface states [1,3] have been studied also. The comprehensive account of the principles of electron energy loss spectroscopy in the experimental and theoretical work up to 1982 has been given by Mills and this author [4], and the reader is referred to this volume for more complete information. In this paper I intend to give a brief summary of issues and an update of the current developments. The paper is organized as follows. In section 2 the experimental technique is described briefly and compared to other techniques which can be used to probe surface vibrations. The different scattering mechanisms in which electrons can engage themselves are outlined, and the resulting selection rules are discussed. The third section addresses the application of electron energy loss spectroscopy for simple chemical reactions and the identification of new surface species. The final section explores the measurements of surface phonon dispersion relations, and the first example of such a study will be presented.

2. Experimental An electron energy loss spectrometer consists of an electron source, a monochromator, a lens system to focus the beam onto the sample with selected impact energy, a retarding lens system, an analyzer and finally the detector. The optimization of the individual components and their match to each other requires subtle considerations. They have been described to some detail in Ch. 2 of ref. [4]. The limiting factor in energy loss spectroscopy is the available monochromatic current which is limited by the space charge in dense electron beams, and also the resolution. Typical resolutions nowadays are about AE = 5 MeV (A 40 cm- ‘) measured as full width at half maximum. Best reported resolutions range about 3 meV. As the

surface excitatmns und EELS space charge limited monochromatic current is a (AE)2 and also the acceptance angle for electrons inelastically scattered from the sample is a AE the limiting resolution is mostly dictated by the signal available for a specific adsorption system and the time allotted for sampling the data. Improvements up to an order of magnitude in signal intensity and a factor of two in resolution may be reachable within a few years. When applied to vibration spectroscopy on single crystal surfaces electron energy loss spectroscopy has proved to be very sensitive to submonolayer quantities of adsorbates. Sensitivities of lo-’ monolayers have been reported in favorable cases. The intensity of energy losses depends of course on the scattering mechanisms to be described shortly. However, once by proper choice of impact energy and scattering geometry a certain mechanism is employed the signal from adsorbates does not depend greatly on the specific treatment to which a surface has been subjected. Therefore if, for example, by virtue of some catalytic reaction the adsorbate has turned into a new surface species one can be assured that the new species will display characteristic vibration features in the spectrum and does not become invisible for some mysterious reason. Furthermore, as one always scans the entire energy range of vibrations a species will not be disguised by displaying vibrational features in a frequency regime not accessible to the spectroscopy. These two virtues are of overwhelming importance in surface chemistry. As other surface vibration spectroscopies do not hold to the same virtues it is not surprising that electron energy loss spectroscopy has found the widest application of all. We now turn to the discussion of the different inelastic scattering processes in which electrons may engage when interacting with surfaces. A brief summary of scattering mechanisms and their most important properties are displayed in table 1. In a way which bears considerable similarities to optical absorption phenomena the electron may couple with the dipole moment of adsorbate vibrations via Coulomb interaction. The dipole field is of long range compared to the diameter of atoms. Therefore, because of the reciprocity of the scales in real space and (Fourier-transformed) k-space

H. Ibach / Elementary

surface excitations

and EELS

181

Table 1 Listing of the inelastic scattering processes through which electrons may lose or gain vibration quanta; dipole and impact scattering are prevailing in most cases; the selection rule for resonance scattering (shape resonances) reads: the vibration must belong to the irreducible representation of the direct product of the representation of the negative ion state with itself Mechanism

Primary

d Distribution

Selection rule

Examples

Dipole scattering

Preferably o-E-’

< 10 eV,

Near specular

Totally symmetric (A, ) modes

CO,NO,O,N,C.S

,...

Impact

Preferably 0-E

> 50 eV,

Broad

Forbidden if (k’s’_k”‘).u=

Surface phonon

dispersion

scattering

Resonance

scattering

energy

Narrow energy range, E < 20 eV

Broad

the inelastic scattering process is in the forward direction. Additional elastic scattering from the surface turns the electrons around, as if they were reflected from a mirror. Inelastic events are therefore detected within a narrow angular cone of a few degrees around the elastic specular beam. The cross-section essentially increases with smaller electron energies. Since on a metal surface (and on surfaces of substrates with a sufficiently high dielectric constant) the dipole moment parallel to the surface is screened one sees only modes which bear a perpendicular dipole moment. In the language of group theory these are the modes which belong to the totally symmetric representation of the point group of the adsorbate complex. These modes may or may not be active in infrared absorption when the molecule is in the gas phase. For example, a homonuclear diatomic molecule always becomes dipole-active when adsorbed on the surface. Dipole scattering has been most widely employed so far since the differential cross-sections tend to be high in many cases. For the electron to be inelastically scattered, the dipole moment may arise from any elementary excitation. Ionic surface waves [5], surface plasmons [2,3] and interband transitions are classical examples. In most cases electronic excitations are well separated in energy from vibrational excitations. Occasionally, however, the two may interfere [2] which gives rise to effects which are interesting in themselves. The second scattering process in table 1 is labeled as impact scattering. By this term one means that the electron is scattered locally from

w E N(O@O)

t) Shape resonance CO, O,,

of physisorbed

N,

the electron shell around the nuclei. As an elastic scattering process the result is electron diffraction. As the atoms also move about their equilibrium position by their zero point motion or by virtue of vibrations being thermally excited, the scattering process depends parametrically on the momentary positions of the atoms which gives rise to the inelastic processes as vibrations quantum losses and gains. For two-dimensional periodic surfaces and adsorbate layers a wave vector conservation rule between the wavevector of the incident electron k(I), the scattered electron kc’) and the wavevector of the vibration q holds for components of the vectors parallel to the surface. When, as a result of lateral interactions between surface atoms or adsorbates the frequency o of a surface vibration depends on the wavevector q,, , the dispersion relation o(q,,) can be probed. While for a long time this possibility had existed only in principle, a major breakthrough was achieved quite recently. Selection rules can also be formulated for impact scattering [7] by making use of time reversal symmetry. In a crude way the selection rule may be expressed by saying that the product (k(‘) - k(I)). u with u the vibration amplitude vector must not be zero. The theory of impact scattering is much less developed than the theory of dipole scattering. Quantitative consideration must be paid to multiple scattering events [6,7]. Heavy computing with the vibration amplitudes as input parameters is involved here if one attempts to calculate intensities. While the two scattering processes described

above occur always with surfaces. save for scattering conditions under which they are forbidden by symmetry or for situations where the matrix element is small, the last entry in table 1 applies to certain cases only. From electron-molecule scattering with the molecules being in the gas phase one knows that the electronmolecule system can form short-lived negative ion states, called resonances [S]. Such resonances may survive when the molecule couples weakly to the surface, as for example in physisorption [9]. Selection rules for resonance scattering (shape resonances) have been formulated [S,lO]. As in the forthcoming examples only dipole and impact scattering was used, we leave the subject of resonance scattering with this brief comment.

3. Application in surface chemistry The most interesting and rewarding application of vibration spectroscopy in surface chemistry is in studies where the adsorbed species transforms into new species by virtue of the catalytic activity of the surface. Here the analytic potential of vibration spectroscopy is unmatched by any other surface technique. In the following we discuss reactions involving cyclohexane, methanol and ethylene on platinum and nickel respectively. The examples were selected since they have a model character. and similar reactions and reaction products have been found on other surfaces. We begin our presentation with cyclohexane adsorbed on Pt( 111) at a temperature of 200 K or lower (fig. 1). Save for the broad feature around 2600 cm-’ the spectrum can be nicely interpreted with the vibrations of gaseous C,H ,z when the surface symmetry is assumed to be broken down to C, (see structure model in fig. 1). The broad feature with a large intensity is attributed to the CH bonds pointing towards the surface to form a type of carbon-to-metal hydrogen bonding. It is assumed that these bonds break first when dehydrogenation begins at higher temperatures. The product stable enough for spectroscopy is then benzene which is observed after annealing the surface to 300 K or exposing the surface to cyclohexane directly at 300 K (fig. 1). Again the

Ftlllll+C,H,, T=ZOOK

0

1000 ENERGY

w h *z"Vi

2OW

Moo

LOSS km-'1

Fig. I. The upper trace 1s the spectrum of cyclohexane, hydrogen-bonded to the platinum surface in a geometry as depicted on the right hand side. After annealing to room temperature cyclohewane dehydrogenates to benzene. The benzene molecule by v bonding to the (I I I) is distorted to a C,, symmetry surface with threefold symmetry. This distortion makes the A,, and Bz,, modes dipole active. The relative intensity of the modes designated as Azu (CH bending) depends on the coverxge and temperature. Both are considerably shifted from the gas phase value because of the YTbonding established with the surface, with the magmtude of the shift pending on the binding site [I I].

symmetry can be deduced from the number of dipole-active modes. The two A,, modes (CH bending) indicate two different binding sites [ 111. Our second example is the stepwise decomposition of methanol on Ni(ll1). With methanol adsorbed at 150 K or below all vibrational features can be assigned to the vibrations of methanol except for the modes at 320 and 470 cm-’ which are associated with the bonding of the methanol through the lone-pair orbital of oxygen. The frequency of the eon mode is shifted downwards as in liquid methanol, again probably because of H bonding. Upon annealing, methanol decomposes stepwise, the first intermediate being a methoxy group. Note that the OH mode has disappeared from the spectrum (fig. 2). After annealing to room temperature mostly CO remains on the surface with possibly some hydrogen which is un-

H. Ibach / Elementary

Ndllll

surface excitations

and EELS

183

Pt(lll)+C,H,

+ 3 L CH,OH

exposed to 92 annealed

1000

2000 ENERGY

ENERGY

LOSS

km’1

Fig. 2. The decomposition of methanol on Ni( 111).The intermediate spectrum is spectrowopic widence for a methoxy intermediate.

detectable on this particular surface because of the small dipole moment. Our final example is concerned with a genuinely new surface species which forms on platinum and rhodium [ 121 when exposed to ethylene at room temperature or when exposed to ethylene at low temperature after annealing to - 400 K. The identification of the surface species required a considerable effort of several groups [ 131. The most significant contribution came from the analysis of the vibration spectrum of CH,CCo(CO), [14]. The frequencies of the surface species make an almost perfect match with the normal modes of CH,CCo(CO),. Also, the symmetry assignment is appropriate since the modes designated as as, 8, or p disappear when the spectrum is observed with dipole scattering. (For the spectrum in fig. 3 impact scattering is used with the analyzer set to a position off the specular beam.) With the close correspondence of the vibrations of the surface species and CH,CCo(CO), the latter is identified as CH,C (ethylidine) in the threefold hollow sites

K to 415 K

LOSS

xxx) km?

Fig. 3. Spectrum of a new surface species formed by the decomposition of ethylene on platinum [ 151. The spectrum is recorded using 2 eV impact energy at 8” off the specular position in order to bring out the features of the vibrational modes which are forbidden by the dipole selection rule in specular reflection (table 1). The surface species is identified as ethylidine (CH,C=) by comparing to tricobalt-ethylidinenonacarbonyl (CH,CCo,(CO),).

provided by the (111) surface. The hydrogen mass balance also supports this conclusion. The only peak unaccounted for in fig. 3 is the 900 cm-’ feature, probably due to a small amount of CH groups which may be also formed in the process.

4. Surface phonon dispersion In our discussion of chemical applications of surface vibration spectroscopy, we have treated the vibration spectrum of adsorbed molecules as if the molecules were vibrating completely independently of each other. While this is frequently a reasonable approximation, it neglects lateral interactions between the adsorbed molecules. Such interaction may come about (i) via direct chemical or physical (dipolar forces) interaction between the adsorbed species, (ii) electronic coupling through the substrate, and (iii) mechanical coupling through the substrate. An example of direct bonding within the adsorbate layer would be a graphitic overlayer

H. Ihach / Elementary

184

q,, (A-‘] Fig. 4. Dispersion of the Rayleigh surface phonon (“S, phonon” [18]) on Ni(lOO) measured along the [l lo] direction (rx) [20]. Data points have been taken at two different impact energies with a scattering geometry as depicted in the insert. The resolution was - 50 cm-’ (FWHM). As both phonon loss and phonon gain are observed in this experiment as sharp features the phonon frequencies can be determined with an accuracy of f 2 cm ’

of carbon. Dipole coupling [ 161 between adsorbed molecules has been observed with molecules which have strong dipole moments associated with certain vibrational modes (CO, NO). Through-substrate electronic coupling is largely held responsible for the formation of ordered overlayers of adsorbates [ 171. Mechanical through-substrate coupling occurs when the substrate atoms mediate the vibrational motions between adsorbed molecules or atoms by participating in the vibrational motion. This effect will be large for molecular vibrations which are comparable in frequency with the substrate phonon spectrum. For the examples discussed in the previous section the maximum phonon frequencies were 190 cm ~ ’ and 295 cm- ’ for Pt and Ni respectively. Most molecular frequencies in figs. l-3 are well above that. Substantial through-substrate mechanical coupling is expected only for the lower end of the molecular spectrum. The various forms of lateral coupling will lead to frequency shifts and broadening of the vibra-

surface excitations and EELS tional features in a spectrum when the surface is disordered. For a two-dimensionally-ordered surface with an ordered adsorbate layer the vibrational frequencies o become a function of the wavevector parallel to the surface, q,, . The dispersion relation w(q,,) is the two-dimensional equivalent of the bulk phonon dispersion relation in ordered solids, and the excitations are therefore called surface phonons. Surface phonons exist not only when the surface is covered with an adsorbate; they also exist on clean surfaces. The theory of these surface phonons was developed in the sixties. A comprehensive study was performed by Allen et al. [ 181. Until recently no method was available to study the dispersion of surface phonons experimentally. In the first investigation the inelastic scattering of He atoms was used and the dispersion of surface phonons on LiF and other alkali-halide surfaces was measured [ 191. In fig. 4, a similar result for the Ni(lOO) surface is shown except now measured with electron energy loss spectroscopy using impact scattering. The q,, vector is chosen by arranging the angles between the incoming and outgoing beam appropriately. The data have been fitted to a force constant model [20] and it has been suggested that the details of the dispersion curve near x can be understood by assuming that the atoms between the first and the second layer of the surface are more strongly bonded than bulk atoms. Thus one sees microscopic information is obtained from such measurements. The importance of the result in fig. 4, however, rests not so much with the particular surface phonon as with the fact that one now has a method whereby complete dispersion curves of adsorbate vibrations can be measured which will yield quantitative information on the lateral interactions as described above. Here, electron energy loss spectroscopy does not suffer from the same restrictions as He scattering, where the accessible frequency range has a (rather low) upper bound and also vacuum conditions seem to be a problem. At the time this paper is being written a complete dispersion measurement of the modes of oxygen layers on nickel is underway. It is hoped that this and other studies of the same kind will lead to a more quantitative picture of surface physics.

H. Ibach / Elemenmy

References [ 1] H. Froitzheim,

H. Ibach and D.L. Mills, Phys. Rev. Bl 1 (1975) 4980. [2] R. Matz and H. Liith, Phys. Rev. Letters 46 (1981) 500. [3] J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. [4] H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press. 1982). [5] H. Ibach, Phys. Rev. Letters 24 (1970) 1416. [6] C.H. Li, S.Y. Tong and D.L. Mills. Phys. Rev. B21 (1980) 3057. [7] S.Y. Tong, C.H. Li and D.L. Mills, Phys. Rev. B24 (1981) 806. [8] I.C. Walker, A. Stamatovic and S.F. Wong, J. Chem. Phys. 69 (1978) 5532; G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. [9] J.E. Demuth, D.S. Schmeisser and Ph. Avouris, Phys. Rev. Letters 47 (1981) 1166. [lo] H. Ibach, J. Mol. Struct. 79 (1982) 129. [ll] S. Lehwald, H. Ibach and J.E. Demuth, Surface Sci. 78 (1978) 577. [ 121 L.H. Dubois, D.G. Castner and G.A. Somorjai, J. Chem. Phys. 72 (1980) 5234.

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185

[13] See ref. [4], pp. 326 ff. [14] P. Skinner, M.W. Howard, I.A. Oxton, S.F.A. Kettle, D.B. Powell and N. Sheppard, J. Chem. Sot. Faraday Trans. II, 77 (1981) 1203. [ 151 H. Steininger, H. Ibach and S. Lehwald. Surface Sci. 117 (1982) 685. [16] H. Ibach and D.L. Mills, ref. [4], p. 98; S. Andersson and B.N.J. Persson, Phys. Rev. Letters 45 (1980) 1421. [ 171 T.L. Einstein and J.R. Schrieffer, Phys. Rev. B7 (1973) 3629. [18] R.E. Allen, G.P. Alldredge and F.W. de Wette, Phys. Rev. B4 (1971) 1648; see also: J.E. Black, D.A. Cambell and R. Wallis, Surface Sci. 105 (1981) 629; T.S. Rahman, J.E. Black and D.L. Mills Phys. Rev. B25 (1982) 883. [19] G. Brusdeylins, R. Doak and J.P. Toennis, Phys. Rev. Letters 46 (1981) 437. [20] S. Lehwald, J.M. Szeftel, H. Ibach, T.S. Rahman and D.L. Mills, Phys. Rev. Letters 50 (1983) 518.