Chapter 5: Photoelectron Spectroscopy

Chapter 5: Photoelectron Spectroscopy

160 CHAPTER 5 PHOTOELECTRON SPECTROSCOPY A. Goldmann 1. INTRODUCTION Considerable experimental and theoretical progress made within the last decad...

1MB Sizes 1 Downloads 147 Views

160

CHAPTER 5

PHOTOELECTRON SPECTROSCOPY A. Goldmann

1.

INTRODUCTION Considerable experimental and theoretical progress made within the last decade has promoted photoelectron spectroscopy (PES) - and recently also its timereversed counterpart, inverse photoelectron spectroscopy (IPES) - to the most important techniques to study the electronic states of metal surfaces - both clean and after interaction with adsorbates. Two features of PES and IPES are of particular interest: First, initial and final state energies of radiative transitions are directly determined by the experiment. Other methods, e.g. light absorption or reflection, can in general only determine the energy difference between initial and final state. Second, the electron momentum fi~ may be determined in angle-resolved experiments using single-crystal samples. Thus PES and IPES can supply the full information on the electron energy eigenvalues E(~) and their dependence on the electron wave vector k. In the context of the present book, where the use of polycristalline films it is mainly the first aspect averages out much of the detailed ~-information, which yields the useful spectroscopic information, in particular on occupied and empty level energies or the electronic densities of valence states. However, also the possibility to measure work functions and their adsorption-induced changes, or to exploit cross-section effects to deduce detailed information on orbital character, are of particular interest. Furthermore, in many cases PES and IPES peak intensities allow direct determination of surface concentrations or adsorbate coverages, make possible depth profiling and related techniques, and provide a convenient monitor for adsorption kinetics. It is the aim of the present contribution to illustrate some typical applications of PES and IPES to chemisorption studies on metals. We do not intend to describe the techniques in an exhaustive way, since several reviewing articles and detailed monographs are available (ref. 1-8). Therefore, only a very basic overview of the methods will be given in section 2. Examples of clean surface investigations are presented in section 3.1. In section 3.2 we discuss several selected adsorption studies to illustrate the use and versatility of PES and IPES. Finally, section 4 summarizes some future possibilities.

161 THE METHODS Principles of Photoemission and Inverse Photoemission The typical PES experiment is illustrated in the upper half of Fig. 1. The sample is irradiated by photons of energy nw. If a photon is absorbed in an occupied initial state Ii>, at energy Ei below the Fermi level EF (E i ~ 0 at E F), 2

2.1

E

PES

IPES

Fig. 1. Schematics of photoemission (upper half) and inverse photoemission (lower half). The shaded area of the energy diagram is accessible to the respective technique. an electron is excited into the empty final state If> at Ef. Energy conservation gives Ef - Ei ~ hw, using the convention that Ef > 0 and Ei ~ O. If Ef > Ev' the energy of the vacuum level, the electron in the excited state may leave the sample. The emitted photoelectrons are then analyzed with respect to their intensity, kinetic energy Eki n and other variables of interest (e.g. emission direction, spin-polarization). PES thus gives information on the occupied states below EF and empty states at Ef > Ev' From energy conservation we find fiw ~ E - Ei ~ Eki n +


162

is evident that both Ei and Ef(>E v) are obtained if ¢ is known. The determination of ¢ by PES is discussed below. Note that the PES initial state energy Ei(;O) is often also named binding energy EB, with the convention that EB = IEil. Inverse photoemission is illustrated in the lower half of Fig. 1. An electron at Ei = E + ¢ impinges on the sample, penetrates the surface and enters the ki n previously empty state Ii> at Ei > Ev' By a radiative transition of energy nw this state is connected with the empty state If> at Ef > EF. The emitted photon is registered in a detector. Again by energy conservation Ei and Ef are determined once Eki n, nw and ¢ are known. IPES can probe all states above EF, while states below EF and above Ev are accessible to PES. A suitable combination of PES and IPES can thus investigate all electronic states. Most PES experiments measure an electron distribution curve (EOC), i.e. the number N(E ki n) of emitted electrons, see Fig. 2. If nw is sufficiently large, E

Ej

I----+-

Fig. 2. Illustration of the fact that in PES the density of occupied states N(E.) is often approximately reflected in the emitted electron energy distributio~ curve N(E ki n). emission out of core levels is observable. The area of the corresponding peak (shaded in Fig. 2, and superimposed to a continuous background of inelastically scattered electrons) is proportional to the number of emitting atoms. Its binding energy Ei identifies the emitting element and very often ("chemical shift") also the chemical environment. Emission from occupied valence states in PES or into empty valence states in IPES yields information on the density of states (DOS). In general, however, the EOC does not directly reflect the density of states N(Ei), as idealized in Fig. 2. In the following we will discuss

163

this point for PES in some detail. PES of bulk states can transparently be described by a three-step model (for more refined treatments we refer to Ref. 1-4): photoexcitation of an electron, travelling of that electron to the surface, and escape through the surface into the vacuum. Beyond the low-energy cutoff at Ev travelling through the solid and escape are described by smooth functions of E and will not give rise to structure in N(E ki n). Therefore primarily the photoexcitation process determines the shape of the EDC. For bulk states, where crystal momentum n~ is a quantum number conserved in the reduced zone scheme ("vertical transitions" in Fig. 1) we then find for the distribution of photoexcited electrons 3k 2 (1) N(E k" , Fiw) 'V I f d l1 0,° 2 1n i ,f ... where 01 = s {Ef(Js.) - Ei (~) - nw} and 02 o{Ef(~) - 1> - Eki n} , and the ~ space integral is to be extended only over occupied states Ii>. The 01- function assures energy conservation, while 02 selects from all transitions possible with photons of energy nw only those that are registered by the electron energy analyzer. If we take for the moment the transition matrix element to be constant, equ. (1) reduces to the so-called energy distriMf i = bution of the joint density of states N(E k" , nw) 1n

'V

I f i, f

3

d k 01 02

(2)

We will then expect that at low photon energies (typically nw < 20 eV) the EDC does generally not reflect the density of occupied states, since only few final states for photoexcitation are available. However, at increasing Nw , the number of accessible final states tncreases and the intensity modulation through these If> states becomes less important. The EDC will then progressively become a replica of the initial density of states (DOS), as long as Mf i = constant. We will discuss below experimental results under this aspect. Similar considerations are applicable to IPES (Ref. 5-8). Things get much easier for the investigation of 2-dimensional adsorbate states: these can couple directly to freeelectron states in the vacuum. The EDC will then replicate the density of occupied (PES) initial and of empty (IPES) final states, modulated in intensity by the corresponding matrix elements Mf i. A useful byproduct of PES is the determination of 1>. The underlying energetics are explained in Fig. 3. Fig. 3a shows how an electron is excited from the sample (work function 1>s' Ei = EF = 0) to a maximum kinetic energy Em = Nw - 1>s' The EDC, however, is measured in the analyzer (A). If 1>A < 1>s' the photoelectron will be accelerated towards the analyzer and gain kinetic energy 61> , compare Fig. 3b. Then the width of the EDC, as measured from the experimentally observed Fermi level at Em to the low-energy cutoff at Eki n = 0

164

E /

Em

/

Em

I

nw-s nw

s

Ev s (0)

nw-s

K

___ J

A (b)

/

/

/

/

s 'U o

E~

EF

(c)

Fig. 3. Schematic energy diagram for the determination of the sample work function ~s from PES. will be liw - ¢s and ~s is measured. If however ~A> ~s' the width of the EDC would be given by liw - ~A' To obtain ~s' a negative voltage Vs is therefore applied to the sample, compare Fig. 3c. This vo' typically 5.. 10 V, garantees that the width of the EDC reflects nw - ~s' 2.2

Experimental aspects The essential components for PES are a monochromatic photon source and a high-resolution electron energy detector. Inversely, IPES requires a monochromatic electron source and a high-resolution photon energy detector. In PES work it has become customary, to distinguish between ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). They differ only in the energy of the primary radiation (nw = 20-40 eV in UPS, fiw> 1 keV in XPS) and generally in the information depth 1(E ki n), due to the dependence of the electron mean free path on Eki n (typically 1 < 10 ~ in UPS, 20-40 ft in XPS). The most popular sources for UPS are gas discharge lamps, using very monochromatic (lIfiw < 17 meV) resonance lines of He (HeI: fiw = 21.2 eV, Hell: fi w = 40.8 eV). State-of-the-art electron detectors, mostly electrostatic concentric hemisphere analyzers, allow an energy resolution lIE ki n < 50 meV. Often, however, a compromise due to intensity problems is required, and UPS spectra are typically recorded at a total (photon + electron) resolution between 50 and 250 meV. In most XPS experiments characteristic soft X-rays (A1Ka : nw = 1487 eV, MgKa : nw= 1254 eV) are employed, which allow a total resolution between about 1 eV and 0.22 eV (if a photon monochromator is used). For further details and other line sources we refer to ref. 2. In recent years the gap between UPS and XPS has been closed by the increasing availability of

165

synchrotron radiation facilities (ref. 9). Of course, the use of synchrotron radiation is much more inconvenient than that of the (much cheaper!) laboratory light sources. It offers, however, the challenging possibility to vary nw continuously, thereby to study the influence of nw on the matrix element Mf i, and to tune the surface sensitivity according to the energy dependence of 1(Eki n)'

For IPES work the electron source is a fairly standard item. Its present resolution is limited by the thermal energy spread of the cathode (about 0.26 eV for BaO). Two different photon detection schemes are most often employed: In the "isochromat" mode electrons with Eki n impinge on the sample. The isochromat spectrum registers the emitted photon intensity at a fixed hw during a scan of E . A typical arrangement useful for adsorption studies is displayed in Fig. 4. k"ln

Fig. 4. Experimental arrangement for inverse photoemission in the isochromat mode: G = electron gun, S = sample, M = mirror focussing photons emitted from the sample onto the counter C which operates at fixed nw = 9.7 eV. It is very efficient since the focussing mirror accepts a large solid angle. The photon detector is a Geiger-MUller counter which is iodine filled and has a CaF 2 (or SrF2, see ref. 10) window. This combination of counting gas and window acts as a band pass filter which detects photons at nw = 9.7 eV (or 9.5 eV) with an overall FWHM resolution of 0.8 eV (or 0.4 eV). For details see ref. 5. Clearly the arrangement of Fig. 4 is the IPES counterpart of UPS. IPES can also be performed at higher photon energies (e.g. at the XPS energy of nw = 1487 eV, see ref. 11). At these energies, however, severe radiation damage generally prevents the study of adsorbate systems. Besides in the isochromat mode, IPES data can also be recorded using a photon spectrograph

166

(ref. 12) which allows to tune the photon energy (8 eV < nw < 30 eV) at good resolution (~nw = 0.3 eV). Due to the low efficiency of the photon monochromator, the arrangement shown in Fig. 4 is generally to be preferred for the study of radiation-sensitive adsorbate systems. 3 SELECTED EXPERIMENTAL RESULTS 3.1 Clean surfaces 3.1.1 From atoms to energy bands It is obvious that PES can be applied also to free atoms and molecules in the gas phase. Typical results are summarized in the literature, both from UPS (ref. 13, 14) and XPS (ref. 15). These data are extremely useful for comparison with PES results from chemisorbed molecules. XPS offers the possibility to measure the approximate initial state DOS, as discussed further below. It is thus possible to study in detail the transition from atomic orbitals to bulk valence bands, i.e. from atomic to metallic behaviour. A typical example is reproduced in Fig. 5. Clusters of Pd with (from bottom to top) increasing diameter have been prepared by deposition of evapora-

.....>.iii c

.....QlC

.......

8

4

0: EF

Binding energy (eV) Fig. 5. XPS valence band sp.ectra of Pd clusters, with cluster volume increasing by factors of 2 between adjacent spectra. The top spectrum corresponds closely to that of bulk Pd. Data from ref. 16.

167

ted atoms onto an amorphous carbon substrate. XPS spectra were then taken with monochromatized A1K a radiation, corrected by subtraction of carbon background and inelastic-scattering tail, and plotted as a function of cluster volume. These data cover the range from mostly isolated atoms to bulk like clusters. With increasing cluster size, the peak observed near 3.5 eV for the isolated atoms broadens until at about 3.10 15 atoms/cm 2 (ref. 17) the observed spectrum is virtually identical to the valence band spectrum of Pd. The obvious effect of cluster size on electronic structure originates from two sources. First, with the bUildup of a cubic environment the crystal field splits the atomic d levels into a doublet with about 1.5 eV spacing. Second, mixing of the atomic levels throughout the progressively better defined k-space will cause these levels to broaden into bands, with a final bandwidth of 5-6 eV for Pd metal. Several experimental as well as theoretical studies suggest that metallic bulk properties are approached for cluster sizes of 100-200 atoms (ref. 16). Another example for size-dependent valence states is shown in Fig. 6 for small Au clusters (ref. 18). The average coverage of Au on the vitreous carbon substrate

>.

'iii c

CIJ

'E

'0 CIJ N

o

...

E o

Z

6

4

2

0

Binding energy (eV)

Fig. 6. Valence-band spectra of Au clusters on a vitreous carbon substrate. The substrate contribution to each spectrum has been subtracted. Data from ref. 18. was (a) 1.2 x 1014, (b) 2.4 x 1014, (c) 3.6 x 1014 and (d) 11.4 x 1015 atoms/ cm 2. From Fig. 6 the increasing spin-orbit splitting of the Au 5d levels with increasing cluster size, from about 1.6 eV (trace a) to 2.6 eV (trace d), is

168

clearly observed. Two experimental aspects should be mentioned here. While there is ample evidence for the correlation between cluster size and XPS density of states, the position of its center of gravity with respect to EF is often strongly influenced by experimental conditions. If the cluster is supported on poorly conducting substrates, a positive charge remains on the cluster during the escape of the photoelectron. In consequence size-dependent binding-energy shifts are observed that are not related to the electronic initial state structure. For details see ref. 16, 18. In an other recent study Au, Ag and Cu were deposited on Al(100). From the non-convergence of the DOS to their bulk value, even at an average overlayer thickness between 10 and 20 ~ , the growth of tightly packed clusters with diameters between 10 and 20 ~ could be inferred (ref. 19). Small clusters show an increased ratio of surface to bulk atoms as compared to flat surfaces. Therefore the study of chemisorption phenomena on clusters will considerably extend the more conventional thin film studies. 3.1.2 Surface versus bulk effects Surface atoms experience a different local environment relative to the bulk atoms which is reflected in a changed local electronic structure. However, screening lengths of unreconstructed metals are characteristically so small that the charge distributions are more or less bulk like already for the second and deeper layers. This is seen very clearly from Fig. 7 which reproduces contours

Fig. 7. Contours of equal charge density calculated for Cu(100). Contours differ by 12. The crystal is cut perpendicular to the surface at the top of the figure. From ref. 20.

169

of equal charge density calculated for a Cu(100) surface (ref. 20). The surface electronic structure is thus located essentially in the topmost one or two layers. Since the typical sampling depths are 15-40 ~ for XPS, and generally 5-15 ~ for UPS and IPES, these techniques yield information about bulk and surface properties simultaneously. In fact the angle-integrated spectra are generally dominated in intensity by contributions from bulk bands, and it is not trivial at all to identify surface contributions. We must therefore discuss which information may be obtained in spectra collected from polycrystalline samples. Let us first consider two examples which yield almost exclusively bulk information. Fig. 8 contains spectra taken with synchrotron radiation between nw = 15 and 90 eV from polycrystalline Au films (ref. 21). It is obvious that,

GOLD

"hw(eV)

I

90

-

:>..

"iii

cQ)

. f;

40 30 20

15 -10

-8

-6

-4

-2

O=EF

Initial energy leV)

Fig. 8. Photoemission spectra taken from polycrystalline Au films at different photon energies nw. From ref. 21. although the fine structural detffi1s depend on photon energy, the data taken with nw > 20 eV clearly represent the approximate shape of the d-bands of Au

170

(2-8 eV below EF) and part of the s,p-band (0-2 eV). For comparison, see also the XPS result for Au in Fig. 6d, the high-resolution spectra reproduced below in Fig. 10a,b and the calculated DOS in Fig. 10e. We see that even in the UPS regime with sufficient care an approximate shape of the valence band DOS can be deduced. A similar result (ref. 22) was obtained for Ag films, see Fig. 9.

Ag

XPS XPS

>......

"iii

c 2c

c Hell

5

I

Hel

Theory

e B

6

4

2

Binding energy (eV) Fig. 9. Photoelectron spectra obtained from Ag films at various photon energies: nw = 1287 eV (b), 40.8 eV (c) and 21.2 eV (d). Spectrum a results from deconvolution of (b) (ref. 23). Trace e: calculated bulk density of states. From ref. 22. Very high resolution (~E) spectra were taken with HeI (~E = 0.03 eV) and Hell (~E = 0.08 eV) radiation. These results are compared to a deconvoluted XPS result and the calculated bulk DOS. It is evident from Fig. 9 that the main structures observed over this wide ~w interval are remarkably stable in binding energy and correlate rather well with theory. This demonstrates that they result from structure in the initial bulk DOS, although in the UPS range the relative intensities are strongly modulated (due to limited number of final states and matrix element variations) as discussed in section 2.1.

171

Nevertheless, surface effects may be clearly identified. This will be demonstrated by the next two studies. Fig. 10 reproduces valence band results taken with monochromatized A1Ka radiation from evaporated Au films (ref. 24). While

8

6

2

O=EF

Binding energy (eV)

Fig. 10. separation of surface (c) and bulk (d) contribution to the valence density of states of polycrystalline Au. From ref. 24. spectrum a was measured at an electron take-off angle of e = 700 , spectrum b corresponds to 0 = 00 (along surface normal). Observations under different take-off directions sample, due to the small electron escape depth, the contribution of the surface ( = first layer, see Fig. 7) and those of the bulk (= second and deeper layers) with different weight. In fact, small differences between traces a and b in Fig. 10 are already observable in the raw data. By appropriate experimental techniques (ref. 24) the weighting factors may be determined from a line shape analysis of the 4f core level spectra, and a decomposition into both contributions is possible. The result is shown in Fig. 10: spectra c and d represent the experimental surface DOS and bulk DOS, respectively. For comparison, a calculated bulk DOS (trace e) is included. The conclusions of this investigation are: The surface electronic structure is confined to the first atomic layer. The width of the surface DOS is narrowed (8~2)% with respect to the bulk DOS and its center of gravity is shifted by

172

0.5 + 0.1 eV to EF. This result is also in qualitative agreement with the observations made on small Au clusters, compare Fig. 6. Surface induced local d-band states on polycrystalline Ag have also been identified in PES experiments using linearly polarized synchrotron radiation at fiw < 50 eV (ref. 25). Typical results for nw = 40 eV are summarized in Fig. 11. Spectrum a shows a strong peak around Ei = -4.2 eV. Several experimental observations support the conclusion that a significant part of this structure is due to surface emission: a 10 ~ overcoat of Al film leads (ref. 25,

Ag

I

tlw=40eV /

/

/

/~

.

..

, ".

/

(e)

o (e)

-8

-7

-6

-5

-4

-3

Initial energy leV) . EF =0

Fig. 11. Photoemission results obtained for Ag under different experimental conditions (a). For details see text. Difference curves show contribution of d-like surface density of states (b) and from adatoms at steps (d) after cryodeposition. Data from ref. 25. not shown in Fig. 11) to about 20% attenuation at -5 < Ei < -4 eV while the remainder of the d-band region remains essentially unchanged. Recent experiments have demonstrated (for details see ref. 3) that excitation of photoemission with p-polarized light may enhance surface contributions. This effect was exploited in the present case. A spectrum obtained with s-polarized light (light vector E parallel to surface) is reproduced by the solid line in Fig. 11a. A change

173

to p-polarization (E normal to surface) enhances the intensity in the range between Ei = -4 and -5.2 eV by about 7% (dashed line). Curve b in Fig. 11 shows the difference curve, which indicates enhanced emission in the region, where the Al overcoat perturbed a high density of surface resonances. These observations correspond closely to local-orbital calculations (ref. 26) for a Ag(100) surface, that predict a large surface DOS near the top of the 4d bands. These states contribute (ref. 26) about 22% to the charge density in the surface plane. We observe a remarkable similarity between curve b and the calculated surface DOS, curve c. Note that the shaded area in Fig. 11c corresponds to a surface band existing in only a very limited k-space region which therefore cannot be detected on polycrystalline films. While the results of Fig. lIb were obtained for films prepared at RT, a different (additional) effect is observed when thin Ag films (3 ~) are deposited at 1200K on thicker films evaporated before at RT. The result (s-polarized light) is displayed by the dotted line in Fig. 11a. Again we observe an increase in intensity, but only in the narrow range between -4 and -4.5 eV. The corresponding difference curve is shown in trace d and shows a rather narrow peak at Ei = -4.2 eV. Experiments at several other photon energies confirm this result and thus prove that it is not due to direct transitions or final-state effects. The observed peak at -4.2 eV may be associated with defect sites at the surface, e.g. adatoms at steps, the number of which is increased by the cryodeposition. Two observations support this interpretation: The sharp peak disappears irreversibly after a few hours annealing at RT and, secondly, the difference curve resembles closely the PES spectrum from Ag atoms, see panel e. The examples discussed in this section clearly demonstrate: While both XPS and UPS (and also IPES) spectra taken from clean polycrystalline films are dominated by the bulk DOS, the surface DOS associated with the metal d-bands may also be derived by appropriate experimental techniques. 3.1.3 Alloy systems The next example discusses an application of PES to the alloy system Cu xA9 1_x' where information can be gained about the electronic structure as a function of x and its change with geometrical structure (ref. 27). The idea was to study the effect of random disorder in a binary metal alloy on the electronic DOS. The experimental difficulty is that both Cu and Ag have only a very small « 1.5 At.%) solid solubility in each other. This problem was overcome using a co-sputtering technique which permits the two types of atoms to mix on an atomic scale and to form a metastable solid solution over the whole composition range. UPS spectra taken for the metastable films at nw = 40.8 eV are reproduced

174

in Fig. 12. As discussed above such data should replicate the essential features of the d-like DOS, albeit modulated in intensity by final state effects. The dashed lines in Fig. 12 clearly demonstrate how the 2.15 eV

CU x Ag,-x 1iw=40.8eV

x=

o

004 0.20

0.56 0.72

0.89

o

1.00

2

4

6

8

Binding energy leV)

Fig. 12. Photoelectron spectra for metastable CUxAg _ alloys taken at nw 40.8 eV. The composition was determined from core l~v~l intensity ratios (ref. 27).

=

splitting observed in pure Ag (x = 0) reduces to about 0.65 eV in the limit x + 1, i.e. to just the value of the Ag atomic spin-orbit splitting. This is in accord with the virtual-bound-state model, according to which the Ag levels should form a localized state below the Cu d-bands and therefore exhibit nearly atomic character. A spectrum taken at RT with nw = 21.2 eV for the composition x = 0.72 is reproduced in Fig. 13, bottom. There is substantial overall agreement between the data taken at different nw. This can be interpreted to be due to the destruction of the final state structure, induced by the random disorder. Figure 13 gives further evidence that the results of Fig. 12 are due to a randomly disordered alloy. To prove this, films were prepared with the substrate held at T = 2250C, so that Cu and Ag separated and a two phase structure was obtained. In this case the spectrum can be described by a linear superposition of the pure metal results, see Fig. 13. This example demonstrates that structural information may often also be obtained from PES (and IPES) data. The fact that the Ag (and Cu, see Fig. 12)

175

d-bands in the random alloy become narrow in the dilute limits shows that clustering effects are not significant for these films co-sputtered onto SUbstrates held at RT.

>'iii c

x = 0.72

Q)

225°C

C

t-<

x = 0.72

RT

Tlw = 21.2 eV 4

8

Binding energy leV I Fig. 13. Photoelectron spectra for metastable CUn 7?Ag o ?8 random alloy (bottom) and two separated phases (middle) of sam~ nomTnaT composition. Top: spectra obtained for the pure components.(ref. 27). 3.2 Adsorption studies 3.2.1 Oxidation of nickel - an IPES investigation As a typical example of the application of inverse photoemission spectroscopy (IPES) to a chemisorption problem we will discuss the oxidation of polycrystalline nickel (ref. 28). Oxygen on nickel chemisorbs dissociatively for exposures
176

Ni (potyl-O, 700K. 2 ,165 Torr

.....>'iii

c

.....COJ

d

4

8

12

above EF (eV) Fig. 14. Isochromat spectra taken at hw = 9.7 eV for various stages of oxidation of a polycrystalline Ni sample. Note the disappearance of the metallic Ni d-band peak just above EF and the development of a gap above EF which is characteristic of NiO. Data taken from ref. 28. (ref. 28). Oxidation was carried out at 700 K in 2'10- 5 Torr O2, under which conditions formation of predominantly stochiometric NiO is favoured. Increasing oxygen dose, compare traces band c, decreases the metallic Ni d-band peak. Simultaneously a new feature at 4 eV above E develops. The final oxide isoF chromat is characterized by a very low intensity just above EF (corresponding to an energy gap), a steep rise around 3 eV (the lower edge of the NiO conduc~ tion band) and another less pronounced rise above 10 eV. Inspection of Fig. 14 suggests that the intermediate stages of oxidation may be explained by a linear superposition of spectra corresponding to clean Ni and the final oxide state. A resulting decomposition of trace c (2500 L) is displayed in Fig. 15. The data points reproduce trace c of Fig. 14 and indicate typical statistical quality of isochromat spectra. The two solid curves in Fig. 15 show the contribution of Ni and NiO. Such decompositions performed for different oxygen exposures may be used to study the growth of the oxide layer. The authors of ref. 28 assumed a logarithmic law. This means that the oxide layer thickness d corresponding to exposure l,at constant partial pressure is given by d = do In(l + L/L o)

177

Ni(poly)+02 700K, 2500L

.....>'Vi C

.....CQl

NiO

EF=O

4

8

12

Energy above EF (eV) Fig. 15. An isochromat spectrum corresponding to an intermediate state of oxidation (dotted curve, equals trace c in Fig. 14) is decomposed into contributions from metallic Ni and pure NiO, using least squares methods. Data taken from ref. 28. 10,,....---.---~--.-----;r----..

~

8

V)

NiO

08

:f' VI

06

-.~-

0.4

700K 2·1(j'STorr 7S00l

C

2S00l 800l

aIL-_----'---_ _L-_----'---_-----.J'-----------.J a 02 0.4 0.6 0.8 10 Calculated intensity (S/Soo )

Fig. 16. Measured intensity of the NiO isochromat signal compared to values derived from a logarithmic growth law. Data from ref. 28. where do and Lo are free parameters which depend sensitively on the experimental oxidation conditions. If we assume a homogeneous film growth, and if we further assume an exponential damping of the incident electron current due to the finite elastic mean free path in the nickeloxide film, the intensity Sid) of the nickeloxide signal in its dependence on d is given by Sid) = S(~){1 - exp(-d/A)} where S(~) = S~ in Fig. 16 represents the intensity from an infinitely thick

178

oxide sample and where A is the electron mean free path in the oxide. Combining both expressions gives S(d)/S(oo} = {I - (1 + L/Lo)-S} with S = do/A. Figure 16 shows values of S(d}/S(oo) as obtained from the experimental decompositions (Fig. 15) and values predicted from the growth model using Lo = 1088 Land S = 0.4. An almost perfect confirmation of the logarith~ mic growth law is obvious from Fig. 16. This example shows clearly how kinetical studies are possible by monitoring changes in IPES (or PES!) spectra. Moreover, of course, the adsorbate induced changes of the electronic density of states above (or below) EF can be studied in considerable detail. For further discussion of that aspect the reader is referred to ref. 28. One interesting spectroscopical possibility should be mentioned here: No empty bulk states are expected within the energy gap region, below about 2.7 eV above EF. The rather weak shoulder observed experimentally around 1 eV in Fig. 14 (arrow on trace d) was therefore assumed to result from radiative transitions into localized empty states in NiO. 3.2.2 Adsorption on smooth and porous Ag films The phenomenon of surface enhanced raman scattering (SERS, ref. 29) has also been observed on Ag films deposited at low substrate temperatures Ts' These films lose most of their SERS activity by annealing at RT. Their properties have therefore attracted considerable attention in recent years. By combination of PES and thermodesorption spectroscopy (TDS) it could be demonstrated that the coldly deposited Ag films are porous (ref. 30-32). In the following we will demonstrate, how temperature dependent structural changes of these films can be monitored by PES and how their different adsorption properties are revealed by UPS and XPS investigations. Temperature dependent UPS results are reproduced in Fig. 17, which shows HeI spectra of clean films deposited at 30 K, 140 K and RT, respectively. The results for Ts = RT are typical for polycrystalline Ag, compare Fig. 9. The spectrum of Ag deposited at Ts = 140 K shows an additional emission at about 4.2 eV below EF. This is ascribed to localized 4d states at defects, as discussed in the context of Fig. 11 (trace d). The spectrum observed at Ts = TM= 30 K obviously has less structure within the d-band region. As mentioned above, this film has the highest porosity and the highest degree of structural disorder. It is therefore plausible to assume that the electron k-vector is no longer a good quantum number. In consequence, structures caused by bulk critical points in direct (k-conserving) transitions are partly quenched. Annealing the films to temperatures TA changes the UPS spectra irreversibly. A characteristic measure for these changes is the corresponding change in work

179

Ag

TM = Ts = RT

:f' VI

TM =30 K Ts=RT

c

Q) ...... c ......

TM = Ts = 140 K

TM=T s=30K 4

8

Binding energy leV) Fig. 17. UPS spectra (nw = 21.2 eV) of three different Ag films, about 200 nm thick, which were deposited at temperature T and measured at T Experimental resolution 60 meV. Note that the intensities sare not normalizedM.to each others. From ref. 32. function 6~; 6~ can be measured, with a typical accuracy of ~ 15 meV, from the shift of the low-energy cut-off as explained earlier. This energy region of the HeI spectra is reproduced in Fig. 18 for different TA. The results are summarized in Fig. 19, which indicates saturation of 6~ for TA > 240 K. In fact, annealing beyond this temperature irreversibly annihilates the porosity, as verified by adsorption studies (ref. 30 - 32) now to be discussed. Porous films deposited at T = 30... 120 K and then exposed to oxygen at s TE = 30 K show the 01s core level emission at a binding energy of 536.7 ~ 0.3 eV, see Fig. 20. Smooth films, produced at Ts = RT and then exposed to O2 at TE = 30 K show the same binding energy of'536.7 eV, but different exposures are needed to achieve comparable emission intensities. This is easily interpreted below. At TE = TM = 30 K molecular oxygen is physisorbed, as verified by the UPS spectrum reproduced in Fig. 21. These O2 valence peak positions also do not depend on Ts of the Ag film nor on the oxygen exposure. They show a clear correspondence to the PES spectrum obtained for gaseous O2 and shifted rigidly by

180 4.---r----,.--,----,-----,,--...,

TA (K) 44 62 100 130 160 190 220 16.8

17.0

17.2

Binding energy (eV) Fig. 18. Low-energy cut-off observed in HeI spectra from Ag films deposited at Ts = 44 K and annealed for 1 min to TA. A bias potential of 8 V was applied between sample and analyzer t. Data from ref. 32. 200

?--?-?

Ag

>Ql

E

?/A

100

-&

<]

0 40

A/

?/

120

/

05

>Ql

?/

-&

c

4.25

0

-... u

c

:::J

oX

Ts= TM= 44 K

200

0

280

4.15

~

TA (K)

Fig. 19. Change in work function ~~ (left scale) and absolute value of from Fig. 18. Error bars refer to ~~. (ref. 32).

~

derived

tAs is evident from these results, there is an experimental problem to define the width nw - ~s of the electron distribution curves: The high energy end at Em (compare also Fig. 3) is clearly determined by the inflection point of the step generally observed at EF, compare for example the results obtained at TM=TS=RT in Fig. 17 or the well defined steps at EF in Fig. 13. However, the shape of the low energy cut-off at Ev is not well understood. In particular, in the present example its experimental width of about 0.1 eV does not reflect the experimental resolution of ~E = 30 meV, but rather the probability of electrons for transmission through the rough Ag surface. There is then no a priori argument whether Ev is determined by the inflection point in these curves or whether the quasi llnear intensity decrease should be extrapolated to zero intensity. This discussion explains, why absolute ~ values obtained from UPS spectra are generally less accurate (typically ~ 0.1 eV) than ~~ results (~ 15 meV).

181

>."iii

c:

(\)

.-

c ......

!k

TA RT

120K

~ ~ ~

(5L) (nOll

80K (25L) 60K (30 Ll 40 K 130 Ll

:;fit

520

:

30 K

60L

30 K

30L

540

560

Binding energy leV) Fig. 20. XPS spectra (A1K ) measured at TM = 30 K showing the 01s core level emission. The films were ~eposited at Ts = 120 Kand then exposed at TE = 30 K to 30 and 60 L 02' respectively (bottom curves). Spectra 2 to 7, counted from bottom, show the changes with annealing temperature TA after 60 L 02 initial exposure. (Additional incremental 02 exposure during warm up is indicated in brackets). From ref. 33.

lng I

30g

1nu / I

"

20u I

....>-

"iii

c

....C

Ql

OL.-f---.L......--...l---....L....---'----' 16 8 EF = 0

Binding energy leV)

Fig. 21. Hell spectrum of a Ag film exposed to 200 L 02 at TE = 30 K and recorded at TM = 30 K, showing emission from the orbitals of physisorbed 02' (ref. 33) .

182

2.2 ~ 0.2 eV with respect to Ev (vertical bars in Fig. 21). However, again different exposures are necessary to observe equal emission intensity from Ag films prepared at T = 30 K(porous) or Ts = RT (smooth). This is demonstrated s in Fig. 22 where we have plotted the intensity of the 1rr u peak in Fig. 21 as a

He! (l1t u)

3

Ts = RT

en

c

.... CIJ C

-

0.1

Exposure

(Ll

Fig. 22. Peak intensity of the 1rru orbital of physisorbed 02 versus 02 exposure for smooth (Ts=RT) and porous (Ts=30K) Ag films (ref. 33). function of O2 exposure. Below 10 L, the observed signal is low for the porous film, because the molecules reside deeper in the pores (estimated depth up to 102 ~, ref. 30) than the escape depth of the photoelectrons. The O2 can then not be seen by PES. In contrast, the smooth film shows a signal from the very beginning, until saturation at about monolayer coverage is attained (substrate temperature too high for multilayer adsorption). However, for exposures above 50 L, the signal from the porous films exceeds that from smooth films. Obviously the overall roughness of the cold-deposited films increases the observable surface area considerably. Qualitatively the same trend is found for the XPS intensities measured for the 3d5/ 2 core level of Xe, see Fig. 23. Adsorption and measurement were performed at TE = TM = 30 K. Again the porous film prepared at T = 100 K shows the delayed observability of photoemission: No signal is s observed before the pores are filled. In contrast the same film shows results almost identical to those from the smooth film (Ts = RT) after a 15 min annealing at TA = RT. Obviously the adsorbed Xe (whose presence is proven by TDS!) was hidden in the pores before. An interesting behaviour is also observed for films prepared at or annealed to intermediate temperatures. This is seen from Fig. 20: at TA > 30 K we observe

183

3

-

>.

-III

c

o

Xe 3dS/2

0

2

0

Q)

c

l:1

= RT TS = lOa K {TS = lOOK TA = RT TS

AI

~o

0

--

~o

a

0

o-P 0.1

0

,d

/

10

100

Exposure (L) Fig. 23. Xe (3d5/2) core level intensity as a function of Xe exposure for two Ag films deposited at room temperature (circles) and at 100 K (squares). The triangles indicate the exposure dependence after the film was annealed for 15 min at RT. Data taken from ref. 32.

>.

VI

c

cleon

Q)

C

I-l

400 L 02

EF

=0

4

8

Binding energy (eV)

Fig. 24. He! spectrum of a Ag film deposited at Ts = 155 K and kept at this temperature, before and after exposure to 400 L02' Bottom curve shows difference spectrum covered minus clean. Data taken from ref. 33.

184

O desorption. Between TA = 40 - 70 K, the 536.7 eV line disappears and a 2 "chemically shifted" new DIs line emerges at 529.6.:':. 0.5 eV. This line has been attributed (ref. 33, 34) to a molecular oxygen species, that is chemisorbed only in the presence of the surface defects discussed above (at 4.2 eV below EF; compare Figs. 11, 17). Raman vibrational spectroscopy shows that this oxygen is aasorbed nominally as O and O~- (ref. 33, 34). Of course this particular species can also identified by its UPS (HeI) spectrum, see Fig. 24. It is characterized by emission features at 2.2, 3.3 and 8.8 eV (arrows). A detailed interpretation of these peaks in terms of emitting orbitals is still missing. Finally, when the Ts = 155 K film is warmed up to room temperature, no further shift is observed in the DIs spectrum. However, the 8.7 eV peak in Fig. 24 is lost and the structure at 3.3 and 2.2 eV changes into a broad band between 2 and 4 eV (ref. 33). The investigation of differently prepared Ag films and the study of their different adsorption properties (ref. 30-34) gives a convincing example for the wealth of information to be gained by PES. Let us finally discuss two other chemisorption studies on Ag films and Ag clusters, respectively. Fig. 25 reproduces a study of pyridine chemisorption on

2

>.

.iii

c


J;

15

10

5

Binding energy (eV)

Fig. 25. UPS spectra (nw = 40 eV) from (b) polycrystalline Ag film, (a) 6 L pyridine adsorbed and measured at 120 K, (a-b) difference spectrum adsorbate minus Ag. Lowest panel: HeI-spectrum of pyridine gas. From ref. 35. a Ag film (ref. 35). Comparison of the difference spectrum with that of gaseous pyridine (aligned in energy to the difference spectrum) reveals the following results: except some broadening the features at binding energies> 7 eV closely resemble the gas-phase result. In contrast, drastic changes occur near the upper

185

edge of the Ag 4d band. A peak shows up at 3.7 eV below EF only ~ 0.4 eV wide (arrow). Also the emission in the top part of the 4d bands is attenuated, causing the minimum in the difference curve around 4.2 eV. This is just the energy where Ag defect states and surface resonances were observed, compare Fig. 11. The authors assign the sharp peak at 3.7 eV to the energetically shifted nitrogen lone-pair orbital 7a1(n) which takes part in a weak chemical bond of pyridine with the Ag surface. This bond apparently involves the local Ag surface bands since they are heavily quenched, whereas the remainder of the 4d band is only little affected. We mention in passing, that pyridine on Ag was recently also investigated (ref. 36) by IPES. An adsorbate-induced peak just at the upper d-band edge has been reported also in many other chemisorption studies. An example is shown in Fig. 26, where the chemisorption of chlorine on Ag was studied (ref. 37). Gaseous CHC1 3 was 12

..... ~

"in

cQ)

.....c bulk 8

4

O=EF

Binding energy leV)

12

8

4

O=EF

Binding energy leV)

Fig. 26. Left: HeI photoemission spectra from Ag clusters of different sizes. Right: HeI difference spectra obtained after chemisorption of Cl on Ag clusters. The numbers give the mean size (number of Ag atoms). Data from ref. 37. admitted, which decomposes on Ag leading to Cl overlayer formation. The bottom curve of Fig. 26 (right) corresponds to saturated Cl adsorption on bulk Ag. The peak at 3.9 eV is explained by a hybridization of C13p and Ag4d orbitals. It is now interesting to study size-dependent effects if Cl is chemisorbed on small Ag clusters (ref. 37). In fact, the difference spectra reproduced in Fig. 26 (right) indicate that such effects persist up to a mean cluster size of 30-40

186

atoms. In particular the broad Cl-induced peak around 7.5 eV observed for the smallest clusters disappears with increasing number of Ag atoms. Simultaneously the peak at about 4.4 eV gains intensity and shifts to the "bulk" value of 3.9 eV. Also an attenuation of the Ag4d state emission around 5 eV, as resolved by the negative component in the difference curves, is observed only for the larger particles. These observations have been explained (ref. 37) on the basis of computed DOS curves as follows: For the smallest clusters the C13p component of the DOS splits into two roughly equal parts located on either side of the Ag d states, compare also Fig. 26 (left). The two parts consist of bonding or antibonding combinations of atomic orbitals. With increasing cluster size the peak at low binding energy begins to dominate the DOS intensity, obviously correlated with the increased width of the Ag d states. This study demonstrates the sizedependence of Cl chemisorption on small Ag clusters. At least 30 - 40 atoms are required for the difference spectrum to reach a limiting shape. An important observation was that the small particles are more reactive than the larger particles or smooth films of Ag for the CHC1 3 decomposition. 3.2.3 Halogen-induced corrosion processes The interaction of metal surfaces with halogens has received considerable attention in the last few years. One practical aspect of these studies is the fact that halogen-interaction with metal, semiconductor and insulator surfaces is an elementary step in dry etching processes used in microelectronic fabrication. From a more basic point of view we mention that halogens exhibit a large electronegativity. In consequence halogen molecules react very strongly with most metal surfaces. For example, exposure of Cu or Ag to C1 2, Br2 or 12 at room temperature leads to dissociative adsorption with an initial sticking probability of about 0.5. Above a monolayer surface coverage this value drops to < 0.01 but halogen uptake continues and leads to the corrosion of deeper layers by formation of metal halides. It is of great interest to investigate the underlying corrosion mechanisms. In the following we discuss some PES results which study the corrosion of Cu and Ag by chlorine. In a recent study (ref. 38) 1000 A to 5~m thick Cu films were prepared under UHV conditions and exposed to C1 2 gas at RT. XPS intensities of the Cu2P1/2 and CU2~~/2 core levels have then been measured as a function of gas pressure (10 Torr ~ p ~ 10 Torr) and exposure time (10 s ~ t ~ 3000 s). A typical result is reproduced in Fig. 27. Trace a shows the Cu2p doublet after 10 sec at 0.5 Torr C1 2. Exposures at the same pressure for 100 sec and 1000 sec lead to the spectra labeled band c, respectively. Obvious changes with increasing exposure are observed. How can we understand them? It is well established that C1 2 adsorbs dissociatively on copper (ref. 39).

187

Cu Film (1000Al +Cl (e)

.....>'iii

(b)

C

.....Q)c

(0)

975

925

950

Binding energy (eV) Fig. 27. XPS spectra (nw = 1487 eV) showing the Cu2p doublet after exposure of a 1000 ~ Cu film to C12 at 0.5 Torr for (a) 10 sec, (b) 100 sec, (c) 1000 sec. Data from ref. 38. On chemical grounds we may then expect the atomic Cl to react with Cu to form CuCl and/or CuC1 2. Therefore the XPS spectra of these materials were measured for comparison and they are displayed in Fig. 28, together with the result

>-

'iii

cQ)

c

CuCI

Cu

975

950

925

Binding energy (eV) Fig. 28. XPS spectra (nw=1487 eV) showing the Cu2p doublet for a clean Cu film and powder CuCl and CUC1 2 compounds. Data from ref. 38.

188

obtained for clean Cu. Three observations are of interest: Cu and CuCl differ in intensity, but no chemical shift is observed. In contrast such a shift of about 2.7 eV is clearly resolved between CuCl and CuC1 2. Also the CUC1 2 spectrum exhi84s1 bits pronounced satellite lines (most likely related to Cu3d 9 - 3d "shakeup" transitions, for details see ref. 40, 41. Such processes can occur during the photoelectron emission. They are, however, forbidden in the closed-3d shell systems Cu and CuCl) on the high binding energy side of the "main" lines. Therefore formation of CUC1 2 can be clearly distinguished from CuCl by these characteristic fingerprints. Finally we mention the small shoulder observed in the CUC1 2 spectrum on the lower binding energy side of the main 2P3/2 peak: This indicates partial decomposition of CUC1 2 under X-ray irradiation. The intensity of this shoulder increases under prolonged photon bombardment. Our example demonstrates that X-ray-induced beam damage may be significant, but can often be monitored by the PES experiment itself. Returning back to the results presented in Fig. 27, we clearly see the coexistence of CuCl and CUC1 2 in Fig. 27 band c, with increasing amount of CUC1 2 for the larger exposure. From a careful analysis of the core level intensities based on a calibration obtained from Fig. 28 - the authors were able to determine an "average' stochiometry x corresponding to CUCl x. The results found for Fig. 27 are x = 1.4 and x = 1.6, respectively, for traces band c. The XPS core level results presented in Fig. 28 do not allow, however, to distinguish CuCl from Cu. This is possible nevertheless, if the corrosion is monitored by the additional observation of X-ray induced Auger electron emission. These Auger electrons are always included once a XPS spectrum is registered over a sufficiently wide energy Auger spectra, taken from scan. Fig. 29 reproduces the region of the CUL 3M4,SM 4,S a 1000 A thick Cu film exposed to C1 2 under various conditions which are summarized in the figure caption. Clearly CuCl (x = 1, trace e) differs drastically in its Auger fingerprint from clean Cu (x = 0). In the intermediate region, besides the CuCl peak the characteristic Cu peak is still visible as a shoulder that finally disappears at x = 1. The absence of CUC1 2 at this composition can be verified by the XPS core level spectra, compare Fig. 28. Finally at x = 1.8 the shape of the Auger line has changed again, and the corresponding XPS core level spectrum indicates predominant presence of CuC1 2. It turns out that in general x is a nontrivial function of p and t, which in addition is strongly dependent on the film thickness. For further details we refer to ref. 38. This example was chosen to demonstrate how detailed chemical information on reaction products can be obtained within the information depth of XPS, typically probing about 20 - 30 ~ in the present case: The combination of core level intensities, core level chemical shifts and Auger spectra allows the quantitative identification of Cu, CuCl and CUC1 2 in the Cu + C1 2 interaction. Of course, XPS core level intensities may also be used to monitor depth pro-

189

Cu-Auger

>..'iii

c

Ql ..-

~

18 -

~

'----tel '---(f)

906

924

Kinetic energy (ev)

Fig. 29. X-ray induced Cu LMM Auger spectra of 1000 ~ t~ick Cu films: clean (a), exposed to C1 2 at RT for 10 sec at 10-3 Torr (b), 5·10- Torr (c), 10-2 Torr (d), 0.5 Torr (e), and exposed for 1000 sec at 1 Torr (f). The corresponding values of x as in CUCl x determined by XPS are indicated. Data from ref. 38. files. However, it is well known that the ion bombardment can cause severe chemical changes. These may then often be identified by their XPS fingerprint, as demonstrated in Fig. 30. A sample of stochiometric CUC1 2 was bombarded by 2 keY Ar+ ions for 1 min and 30 min, respectively. It is easily seen how the CUC1 2 spectrum, with its prominent Cu2P3/2 peak at 935.7 ~ 0.03 eV and its characteristic satellite structure, is changed into that of Cu and CuCl (Cu2P3/2 at 932.8 ~ 0.03 eV) thereby demonstrating the corresponding decomposition. Our last example discusses the application of UPS in a study of the Ag + C1 2 interaction. As discussed above UPS spectra often reveal very specific information on the particular bonding properties and are then most promising for "fingerprint" studies. After the chlorine-induced corrosion of Ag we expect AgCl as a reaction product. In close analogy to the Cu + C1 2 system discussed before, the Ag3d 3/2, 5/2 core levels show no chemical shift when going from clean Ag to atomically chemisorbed overlayers to solid AgCl. In contrast these three distinctly different phases can be uniquely characterized by UPS (compare also Fig. 26). Normal emission spectra taken as a function of C1 2 exposure from Ag(lOO)

190

CuCI 2 :Ar+ - Bombardment

>,:

~Ic

Q)

30 min

r----~

1 min

c

(02)

Omin

975

950

925

Binding energy leV) Fig. 30 XPS spectra (~w = 1487 eV) of the Cu2p doublet observed for stochiometric CUC12 (bottom) and after subsequent bombardment (1 min, 30 min) by 2 keV Ar+ ions. clearly distinguish the clean surface from that covered by an ordered c(2x2}-Cl overlayer (ref. 42). Also, normal emission results from Ag(llO} are markedly different when the surface is covered by either a p(2xl}-Cl or a c(4x2}-Cl overlayer (ref. 43). If, however, these ordered overlayer systems are exposed to further C1 2 doses at T = 90 K, one final "saturation" spectrum is obtained which is observed to be identical on Ag(100}, Ag(llO) and Ag(111} and which no longer shows any dependence on the electron emission angle 0. Obviously one and the same disordered surface has been produced on all three single crystals. The corresponding HeI spectrum (~w = 21.2 eV) is reproduced in Fig. 31a (solid line). If we now measure an EDC at nw = 40.8 eV (Hell) the result indicated by the dashed line in Fig. 31a is obtained. It turns out that the latter curve is identical to the Hell spectrum observed earlier for polycrystalline AgCl (ref. 44). This means that at ~w = 40.8 eV we detect only AgCl on the three corroded crystal surfaces. However, the HeI spectra (Fig. 31a, solid line) are not identical to the HeI spectrum of polycrystalline AgCl, which is reproduced by the from an additional dashed curve in Fig. 31b. Obviously there is a cont~ibution component. In fact, if after suitable background correction (dash-dotted line) we subtract the HeI spectrum of pure AgCl from the "saturation" spectra of the corroded surfaces, we end up with the solid line shown in Fig. 31b. Comparison with the HeI spectrum of gaseous C1 2 in Fig. 31c clearly shows that condensed molecular chlorine was observed. From such studies, ;n combination with XPS measurements, the corrosion mechanism could be inferred in considerable detail

191

Ag (110)+30L CI 2

>...... U1 C

Q) ......

C ......

leI

-4

-8

-12

Initial energy (eV) Fig. 31 (a) Hel (solid line) and Hell (dashed line) spectra measured at 90 K from Ag(110) exposed to about 30LC12 at 90 K. (b) decomposition of the Hel spectrum into contributions from stochiometric AgCl (dashed line) and condensed C12 (solid line). (c) Hel spectrum of gaseous C12 for comparison, shifted to adjust energy scales. Data from ref. 43. (ref. 43): In the first step, which is temperature independent, dissociative chemisorption at submonolayer coverage takes place. At RT the sticking coefficient for further chlorine uptake is very small, and extremely high C1 2 exposures are necessary to corrode the surfaces, in qualitative agreement with the results reported above for Cu + C1 2. However, with the sample at 90 K, C1 2 condensation occurs on top of the Ag/Cl overlayer and C1 2 is always present on the surface (until the C1 2 supply is stopped). Ag+ ions diffuse to the surface, dissociate the C1 2 and form AgCl. The results presented in Fig. 31 demonstrate how the variation of the photoionization matrix element Mf i with photon energy may be exploited. The idea is explained in some more detail by Fig. 32, which shows (solid lines) the cross section ad for emission from Ag4d and Cu3d in their dependence on nw. Also the cross section ap is given for the 3p shell of Ar, which is isoelectronic to Cl-. As is evident, at hw = 21.2 eV both p-electrons and d-electrons will contribute

192

to the observed spectra. In contrast, a p « ad at fiw " 40.8 eV. This is the reason why condensed C1 2 could not be detected at all in the Hell results. In fact, the observed Hell spectrum represents - in the limit that the EDC replicates the DOS - the partial d-like DOS of AgCl (ref. 44). Both p-like and dlike DOS contributions from AgCl are observed at nw " 21.2 eV and therefore the Hel spectrum of AgCl differs drastically from the Hell result, compare Fig. 31.

-E

N

40

u

';2,

Ag

0

~

c 0

20

u

Q)

11l

Cu

11l 11l

....

0

U

0

10

30

SO

Photon energy (eV) Fig. 32 Photoionization cross sections of Ar (ref. 45) and Cu and Ag (ref. 46). These curves represent basically a(3p) for Ar and a(3d), a(4d) for the metals. Clearly the strong dependence of a on nw can be exploited to measure approximatel partial densities of states. The same method can also be used to separate e.g. p and f densities of states in lanthanide and actinide compounds (ref. 47). In our context it is also important to point out, that by appropriate variation of "w the relative contribution of an adsorbate to the total EDC (which includes all substrate bulk features from several atomic layers) may be considerably enhanced in intensity. In particular the use of tunable synchrotron radiation can very much enhance the sensitivity for chemisorbed species at submonolayer coverages. If the variation of the partial cross sections with photon energy is known, as in the present example, the angular momenta of the emitting orbitals may then be deduced by measuring EDC's at several photon energies. This technique is of particular use, if hybridization effects between substrate and adsorbate states are to be identified.

193

4.

SUMMARY AND OUTLOOK

We have discussed some typical applications of PES and IPES to polycrystalline samples. A wealth of spectroscopic information could be obtained, e.g. on condensed molecules, small clusters, porous and smooth films, atomic chemisorption states and corroded surfaces. Binding energies below and above EF, bulk and surface densities of states, work function changes, alloy concentrations and adsorbate coverages, specific data on chemical bonds and their orbital character, and parameters of adsorption kinetics were derived. Although space limitation did not allow to discuss all the surface science behind the examples, I hope that the potential of the various photoelectron spectroscopies became evident. Which developments and improvements can we expect in the near future? Clearly one desirable goal is to obtain spatial resolution in XPS investigations ("submicron resolution for sub-micron structures"). This would give the opportunity to study e.g. the local electronic structure at extended defects. As compared to scanning Auger techniques, scanning XPS will be a much less damaging technique, in particular with respect to adsorbate systems. The feasibility of a scanning photoelectron microscope was studied recently (ref. 48): On the basis of a synchrotron light source and zone plate X-ray optics a spatial resolution of 50 nm seems possible, at an energy resolution of 1 eV and count rates of the order of 104 cps. Another point of development concerns "real-time" PES. This allows e.g. to study reaction dynamics as a function of time and temperature or to investigate diffusion processes in an interface region. Two approaches have been attempted: Time-modulated reactive beam techniques may be combined with UPS, giving a time-resolved photoelectron signal from adsorbates and intermediate species (ref. 49). Otherwise a time scale down to picoseconds can be defined in experiments using the pulse-structure of synchrotron light from storage rings (ref. 9), or in two-photon absorption experiments where one of the photon sources is a pulsed laser. Once spatial resolution and time resolution will be routinely available in combination with the various electron spectroscopies discussed above, qualitatively new insights into the mechanisms of chemisorption may be expected. Acknowledgement The continuous financial support of my group by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. REFERENCES 1 B. Feuerbacher, B. Fitton and R.F. Willis (Eds.), Photoemission and the Electronic Properties of Surfaces, Wiley-Interscience, New York, 1978. 2 M. Cardona and L. Ley (Eds.), P~toemission in Solids I;L. Ley and M. Cardona (Eds.), Photoemission in Solids II, Springer, Berlin, 1978. 3 E.W. Plummer and W. Eberhardt, Adv. Chem. Phys., 49 (1982) 533.

194

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

F.J. Himpsel, Adv. Phys., 32 (1982) 1. V. Dose, Progr. Surface Sci., 13 (1983) 225. N.V. Smith, Appl. Surface Sci., 22/23 (1985) 349. F.J. Himpsel, Comments Condo Mat. Phys., 12 (1986) 199. V. Dose, Surface Sci. Reports, 5 (1985) 337. C. Kunz, in L. Ley and M. Cardona (Eds.), Photoemission in Solids II, Springer, Berlin 1978, p. 299. A. Goldmann, M. Donath, W. Altmann and V. Dose, Phys. Rev., B32 (1985) 837. J.K. Lang and Y. Baer, Rev. Sci. Instrum., 50 (1979) 221; J.K. Lang, Y. Baer and P.A. Cox, Phys. Rev. Lett., 42 (1979) 74. Th. Fauster, D. Straub, J.J. Donelan, D. Grimm, A. Marx and F.J. Himpsel, Rev. Sci. Instrum., 56 (1985) 1212. D.W. Turner, C. Baker, D.A. Baker and C.R. Brundle, Molecular Photoelectron Spectroscopy, Wiley, New York, 1970 K. Kimura, J. Katsumata, Y. Achiba, T. Ymmasaki and S. Iwata, Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules, Japan Socc. Press, Tokyo, 1981. K. Siegbahn, C. Nordling, G. Johannson, J. Hedman, P.F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L.O. Werne, R. Manne and Y. Baer, ESCA Applied to Free Molecules, North-Holland, Amsterdam, 1969. S.B. DiCenzo and G.K. Wertheim, Comments Solid State Phys., 11 (1985) 203. M.G. Mason, L.J. Gerenser and S.T. Lee, Phys. Rev. Lett., 39 (1977) 288. G.K. Wertheim, S.B. DiCenzo and S.E. Youngquist, Phys. Rev. Lett., 51 (1983) 2310. W.F. Egelhoff, Jr., J. Vac. Sci. Technol. 20 (1982) 668. F.J. Arlinghaus, J.G. Gay and J.R. Smith, Phys. Rev., B21 (1980) 2055; B23 (1981) 5152. J. Freeouf, M. Erbudak and D.E. Eastman, Solid State Commun., 13 (1973) 771. F.L. Battye, A. Goldmann, L. Kasper and S. Hufner, Z. Phys., B27 (1977) 209. G.K. Wertheim, D.N.E. Buchanan, N.V. Smith, M.M. Traum, Phys. Rev. B10 (1974) 3197. P.H. Citrin, G.K. Wertheim and Y. Baer, Phys. Rev. Lett., 41 (1978) 1425; Phys. Rev., B27 (1983) 3160. E.E. Koch, J. Barth, J.-H Fock, A. Goldmann and A. Otto, Solid State Commun., 42 (1982) 897. J.R. Smith, F.J. Arlinghaus and J.G. Gay, Phys. Rev., B22 (1980) 4757. N.J. Shevchik and A. Goldmann, J. Electron Spectrosc. and Rel. Phenom., 5 (1974) 631; and unpublished results (Fig. 13). H. Scheidt, M. Globl and V. Dose, Surface Sci., 112 (1981) 97. R.K. Chang and T.E. Furtak (Eds.), Surface Enhanced Raman Scattering, Plenum, New York, 1983. E.V. Albano, S. Daiser, R. Miranda and K. Wandelt, Surface Sci., 150 (1985) 367; 150 (1985) 386. A. Otto, J. Billmann, J. Eickmans, O. ErtUrk and C. Pettenkofer, Surface Sci. 138 (1984) 319. J. Eickmans, A. Otto and A. Goldmann, Surface Sci., 171 (1986) 415. J. Eickmans, A. Otto and A. Goldmann, Surface Sci., 149 (1985) 293. J. Eickmans, A. Goldmann and A. Otto, Surface Sci., 127 (1983) 153. J.H. Fock, J. Schmidt -May and E.E. Koch, J. Electron Spectrosc. Relat. Phenom., 34 (1984) 225; and Hasylab annual report, 1981. A. Otto, H. Frank and B. Reihl, Surface Sci., 163 (1985) 140. R.C. Baetzold, J. Am. Chem. Soc., 103 (1981) 6116. W. Sesselmann and T.J. Chuang, Surface Sci., in press (1986) D. Westphal and A. Goldmann, Surface Sci., 131 (1983) 113; and references therein. G. van der Laan, C. Westra, C. Haas and G.A. Sawatzky, Phys. Rev., B23 (1981) 4369. S. HUfner, Solid State Commun., 47 (1983) 943. E. Bartels and A. Goldmann, Solid State Commun., 44 (1982) 1419. K.K. Kleinherbers, Doctoral Thesis, Duisburg 1986, unpublished; K.K. Klein-

195

herbers and A. Goldmann, to be published. 44 J. Tejeda, N.J. Shevchik, W. Braun, A. Goldmann and M. Cardona, Phys. Rev. B12 (1975) 1557. 45 J.A.R. Samson, Adv. At. Mol. Phys., 2 (1976) 178. 46 H.J. Hagenau, W. Gudat and C. Kunz, J. Opt. Soc. Am., 65 (1975) 742. 47 D.E. Eastman and M. Kutznietz, Phys. Rev. Lett., 26 (1971) 846; D.E. Eastman and J.L. Freeouf, Phys. Rev. Lett., 34 (1975) 395. 48 J. Kirschner, in G. Schmahl and D. Rudolph (Eds.), Springer Series in Optical Sciences Vol. 43: X-Ray Microscopy, Berlin 1984, p. 308. 49 F. Steinbach and J. SchUtte, Rev. Sci. Instrum., 54 (1983) 1169; Surface Sci. 146 (1984) 551.