Electrochemical Surface Science - American Chemical Society

Chapter 8

Molecules at

Surfaces in Ultrahigh Vacuum

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 27, 2018 | https://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch008

Structure and Bonding J. Somers, T. Lindner, and A. M . Bradshaw Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, Berlin 33, Federal Republic of Germany

D-1000

Using spectroscopic techniques based on photoabsorption and photoelectron emission it i s possible to obtain information on the geometry and electronic structure of adsorbed molecules as well as of adsorbed molecular fragments resulting from simple heterogeneous reactions. In fact, the correct assignment of the electronic energy levels of such species using photoemission i s often only possible once the molecular orientation i s known.

Our understanding of the energetics and dynamics of molecular adsorption on single crystal metal surfaces under ultra-high vacuum (uhv) conditions i s presently limited by the meagre amount of i n formation available on the geometric structure and electronic energy levels of surface molecules. For example, only in the case of a few simple systems such as diatomics - and then usually CO are the adsorption s i t e and molecular orientation known with any accuracy. Structural analysis via low energy electron d i f f r a c t i o n (LEED) i s restricted to ordered overlayers. Whilst many adsorbed molecules form regular two-dimensional arrays, i t i s by no means the rule. For molecular fragments formed as intermediates in heterogeneous reactions (e.g. formate, methoxy, cyano) i t i s certainly the exception. Moreover, LEED theory at i t s present state of development has difficulties in coping with polyatomic molecules. There are, however, a number of techniques which do not require the existence of an ordered array and which s t i l l give structural information in a direct or indirect way. One p o s s i b i l i t y i s to measure the angular distribution of ions desorbed as a result of electronor photon-induced electronic transitions. The technique i s normally referred to as ESDIAD and i s described by Madey in another chapter of the volume. Another p o s s i b i l i t y i s to apply selection rules in photoemission to establish molecular orientation [1] but, as we shall see below, i t i s not possible to simultaneously determine structure and assign l e v e l s . A new development in recent years has been the application of structural tools based on photoabsorption and photoionisation: x-ray absorp0097-6156/88/0378-0111$06.00/0 ° 1988 American Chemical Society

Soriaga; Electrochemical Surface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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tion fine structure [2] and photoelectron d i f f r a c t i o n [3]. Since they both require a continuously tunable source of soft x-rays, usually in the region 200 - 1000 eV photon energy, their application i s dependent on the a v a i l a b i l i t y of synchrotron radiation and suitable grazing-incidence grating monochromators [4]. (This spectral range covers the core levels of C, N and 0; 1s (or higher) levels of heavier elements are accessible at higher photon energy with crystal monochromators.) The measurement of the polarisation dependence of the near edge x-ray absorption fine structure (NEXAFS) i s a particularly straightforward technique, i f applied correctly, and figures prominently in this a r t i c l e . The most direct technique for probing surface electronic structure i s undoubtedly photoemission, or photoelectron spectroscopy. In a reasonable approximation, referred to as Koopmans' theorem, the measured ionisation potentials can be equated with the ground state o r b i t a l energies. When the molecule i s only weakly adsorbed a comparison with the photoelectron spectrum of the corresponding gas phase species normally suffices to assign the various adsorbate-induced features. For this purpose, laboratory l i n e sources (Hel, etc.) are usually adequate. Where a strong perturbation of the energy levels of the molecule takes place, assignment can only be carried out using selection rules preferably in conjunction with polarised l i g h t . This i s p a r t i c u l a r l y true of molecular fragments for which no comparison with gas phase species is possible. For the application of photoemission selection rules prior knowledge of the orientation of the molecule i s necessary. Synchrotron radiation in the photon energy range 10 - 50 eV (the so-called normal-incidence region) i s the obvious choice for such investigations because of i t s tunability and i n t r i n s i c a l l y high polarisation. In the investigations of molecular adsorption reported here our philosophy has been to f i r s t determine the orientation of the adsorbed molecule or molecular fragment using NEXAFS and/or photoelectron d i f f r a c t i o n . Using photoemission selection rules we then assign the observed spectral features in the photoelectron spectrum. On the basis of Koopmans' theorem a comparison with a quantum chemical cluster calculation i s then possible, should this be available. A l l three types of measurement can be performed with the same angle-resolving photoelectron spectrometer, but on different monochromators. In the next Section we b r i e f l y discuss the techniques. The third Section i s devoted to three examples of the combined application of NEXAFS and photoemission, whereby the f i r s t - C0/Ni(100) - i s chosen mainly for didactic reasons. The results for the systems CN/Pd(111) and HC00/Cu(110) show, however, the power of this approach in situations where no a priori predictions of structure are possible. Photoionisation and photoelectron emission Below the photoionisation threshold a core electron in a free molecule can be excited into empty anti-bonding molecular o r b i t a l s (m.o.'s) as well as into Rydberg states. These transitions are observable as sharp features directly below the corresponding absorption edge (carbon K, oxygen K etc.). Above the



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photoionisation threshold further transitions into higher-lying anti-bonding v i r t u a l m.o.'s w i l l occur, their broad spectral features being superimposed on the background continuum absorption. A l l these features associated with the existence of the molecular potential constitute the near edge x-ray absorption fine structure (NEXAFS) or x-ray absorption near edge structure (XANES). At higher energies above the edge further very weak structure may be v i s i b l e due to a scattering phenomenon. This i s the extended x-ray absorption fine structure (EXAFS) resulting from interference between the emitted photoelectron wave and waves backscattered from other atoms in the molecule. (The excitation into anti-bonding m.o.'s above the threshold may also be viewed as EXAFS-type scattering resonances. Hence the term "shape" resonance: the energy and width of the features depends on the shape of the molecular potential.) When the molecule i s placed on the surface, several factors come into play. F i r s t l y , the experiment has to be performed quite d i f f e r e n t l y . Since the substrate i s usually a compact s o l i d surface, a measurement of the absorption spectrum in transmission i s no longer feasible. As shown schematically in Figure 1a, the y i e l d of Auger electrons, or the high energy portion of the secondary electrons (partial electron yield) are measured, both signals being proportional to the number of excited core electrons in the immediate surface region. Upon adsorption the near edge resonances may be shifted and their degeneracy l i f t e d or they may even disappear as a result of the formation of the bond to the substrate. New resonances (so-called substrate resonances) may be observed. Since the chemisorption process invariably results in a molecule with a single, fixed orientation and since the excitations are subject to dipole selection rules, the transitions in NEXAFS are polarised. This forms the basis for the determination of molecular orientation. Since the surface atoms are l i k e l y to be r e l a t i v e l y strong backscatterers the extended fine structure w i l l be dominated by the scattering from the substrate. This r e l a t i v e l y weak modulation at higher energy i s referred to as the surface EXAFS, or SEXAFS. An analysis of the SEXAFS can give further structural information, most valuably, on the adsorption s i t e (for further d e t a i l s , see Ref. [2]). In photoelectron spectroscopy, shown schematically in Figure 1b, the electrons emitted as a result of their excitation into states above the photoionisation threshold are analysed according to their kinetic energy. The energy balance i s given simply by hv = EB + EK, where EB i s the energy of a bound level and EK the k i n e t i c energy of the photoemitted electron. The technique i s often referred to as XPS (x-ray photoelectron spectroscopy) or ESCA (electron spectroscopy for chemical analysis), the l a t t e r being the o r i g i n a l acronym proposed by K. Siegbahn. If the photon energy i s low (hv < 50 eV), such that essentially only valence electrons are excited, then we normally refer to UPS ( u l t r a - v i o l e t photoelectron spectroscopy) or simply photoemission. At these low photon energies the photoionisation cross-section for valence (or outer s h e l l ) electrons i s also highest. For a molecule on a surface the primary excitation i s a dipole transition from a molecular orbital, modified by the interaction with the substrate, into a continuum



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c) Photoelectron diffraction

Figure 1: Schematic representation of the three techniques (a) x-ray photoabsorption (NEXAFS/SEXAFS), (b) photoelectron spectroscopy (photoemission) and (c) photoelectron d i f f r a c t i o n .



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l e v e l . The same photoabsorption selection rules thus apply as in NEXAFS. Whereas in NEXAFS, however, the f i n a l state i s a bound or continuum level associated with the molecule i t s e l f and thus of (potentially) known symmetry, the final state continuum wavefunction in photoemission must be selected in an angle-resolved experiment. When the molecular orientation i s also known, the symmetry of the i n i t i a l state can be determined. How this procedure can be used in assignment w i l l become clear in the examples below. In photoemission there i s an interesting experimental variation known as constant i n i t i a l state (CIS) spectroscopy, in which the photon energy and the analysed kinetic energy are scanned simultaneously. For the excitation out of a given occupied l e v e l , a whole range of f i n a l states i s then sampled. This approach i s p a r t i c u l a r l y useful in band structure studies of s o l i d s when the momentum, or wavevector, of the f i n a l state i s also know, i . e . when the emission direction i s also defined. By the same token, a d i f f e r e n t i a l p a r t i a l cross-section i s measured in such an experiment on a free molecule. If the photoelectron current i s angleintegrated, meaning that a l l the photoemitted electrons are collected, then the photoabsorption spectrum, or p a r t i a l photoionisation cross-section, should be obtained. Putting the molecule (or an atom) on a surface and taking a CIS spectrum of a core l e v e l introduces, in the same way as in SEXAFS, scattering from the substrate (Figure 1c). The resulting interferences, which depend on the adsorption s i t e and orientation modulate the differential cross-section; the phonomenon is normally referred to as photoelectron d i f f r a c t i o n (PED). It has recently been shown that the modulations in intensity are of the order of 50% of the background signal in experiments on the C and 0 1s levels [5]. (Photoelectron d i f f r a c t i o n effects are also observed when the photoelectron emission angle i s varied at fixed photon energy [6]. It i s thus sensible to distinguish between energy-scanned and anglescanned PED.) By performing model calculations for a given structure and emission direction and comparing with experiment, information on adsorption s i t e and orientation i s obtained. How does the technique relate to SEXAFS? By varying the photon energy in PED the scattered electron intensity i s distributed between the various f i n a l state manifolds corresponding to different emission directions. If i t were possible to angle-integrate over the whole photoelectron current, then we would recover the SEXAFS s i g n a l . Again, i t i s essentially the difference between a p a r t i a l crosssection and a d i f f e r e n t i a l p a r t i a l cross-section. Before concluding this section i t should perhaps be noted that SEXAFS has so far been more successful in surface structural studies than photoelectron d i f f r a c t i o n , largely because multiple scattering effects can be neglected. On the other hand, the potential of PED has not yet been f u l l y realised; we show some PED data below which i l l u s t r a t e t h i s point. Some examples Nif 1001-C0. Figure 2 shows the NEXAFS at the C and 0 1s edges for CO adsorbed on a Ni{100} surface at half-monolayer coverage [7]. This surface concentration corresponds to the formation of an



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a)

C1s

b) 0 1 s V///7///////,

e =20° E

e 20° E S

Xi

e =90

c

c

1

280

1

290

1

300

— I

310

—i

530

1

1

540

550

1—

560

Photon energy (eV)

Figure 2: NEXAFS at the (a) carbon and (b) oxygen Ni{100}(/2x/2)R45°-C0.

1s edges for



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ordered overlayer designated as (/2x/2)R45° in the standard notation. In each case spectra for two angles of the e l e c t r i c vector relative to the surface normal, 8 E = 20° and 90°, are shown. Two d i s t i n c t resonances can be seen, corresponding to transitions into the bound 2TT level and into the 6a-derived level in the continuum. We note the difference in linewidth between these two features. Equally pronounced i s the different polarisation dependence in each case. At 8 E = 90° the a resonance i s v i r t u a l l y absent and the TT resonance strong. At 8 E = 20° the integrated intensity of the a resonance i s larger than that of the TT. Starting from the dipole matrix element for an electronic transition, i t i s easy to show [8] that for an isolated, oriented diatomic the a resonance i s polarised along the molecular axis and the TT resonance in a plane perpendicular to that axis. The corresponding angular dependences are given by I (a) — P cos^a and I ( TT ) ~- 1 - P cos^a, where a i s the angle between the C-0 axis and the E vector and P the degree of polarisation of the incident l i g h t . I f the molecule i s adsorbed with i t s axis perpendicular to the surface, which we might expect from a l l the previous studies on this system (and there have been many!), the effective symmetry either remains C

e =20 ,'~"7 o

E

a)

ii

azimuth

Electrochemical Surface Science - American Chemical Society

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